Merge remote-tracking branch 'nixos/master'

.github/workflows/backport.yml

@@ -21,7 +21,7 @@ jobs:
           fetch-depth: 0
       - name: Create backport PRs
         # should be kept in sync with `version`
-        uses: zeebe-io/backport-action@v0.0.9
+        uses: zeebe-io/backport-action@v1.0.1
         with:
           # Config README: https://github.com/zeebe-io/backport-action#backport-action
           github_token: ${{ secrets.GITHUB_TOKEN }}

configure.ac

@@ -41,8 +41,6 @@ AC_DEFINE_UNQUOTED(SYSTEM, ["$system"], [platform identifier ('cpu-os')])
 test "$localstatedir" = '${prefix}/var' && localstatedir=/nix/var
 
 
-CFLAGS=
-CXXFLAGS=
 AC_PROG_CC
 AC_PROG_CXX
 AC_PROG_CPP

doc/manual/src/SUMMARY.md.in

@@ -29,6 +29,7 @@
 - [Nix Language](language/index.md)
   - [Data Types](language/values.md)
   - [Language Constructs](language/constructs.md)
+    - [String interpolation](language/string-interpolation.md)
   - [Operators](language/operators.md)
   - [Derivations](language/derivations.md)
     - [Advanced Attributes](language/advanced-attributes.md)
@@ -59,6 +60,7 @@
 @manpages@
   - [Files](command-ref/files.md)
     - [nix.conf](command-ref/conf-file.md)
+- [Architecture](architecture/architecture.md)
 - [Glossary](glossary.md)
 - [Contributing](contributing/contributing.md)
   - [Hacking](contributing/hacking.md)

doc/manual/src/architecture/architecture.md

@@ -0,0 +1,115 @@
+# Architecture
+
+This chapter describes how Nix works.
+It should help users understand why Nix behaves as it does, and it should help developers understand how to modify Nix and how to write similar tools.
+
+## Overview
+
+Nix consists of [hierarchical layers].
+
+[hierarchical layers]: https://en.m.wikipedia.org/wiki/Multitier_architecture#Layers
+
+The following [concept map] shows its main components (rectangles), the objects they operate on (rounded rectangles), and their interactions (connecting phrases):
+
+[concept map]: https://en.m.wikipedia.org/wiki/Concept_map
+
+```
+
+   .----------------.
+   | Nix expression |----------.
+   '----------------'          |
+           |              passed to
+           |                   |
++----------|-------------------|--------------------------------+
+| Nix      |                   V                                |
+|          |      +-------------------------+                   |
+|          |      | commmand line interface |------.            |
+|          |      +-------------------------+      |            |
+|          |                   |                   |            |
+|    evaluated by            calls              manages         |
+|          |                   |                   |            |
+|          |                   V                   |            |
+|          |         +--------------------+        |            |
+|          '-------->| language evaluator |        |            |
+|                    +--------------------+        |            |
+|                              |                   |            |
+|                           produces               |            |
+|                              |                   V            |
+| +----------------------------|------------------------------+ |
+| | store                      |                              | |
+| |            referenced by   V       builds                 | |
+| | .-------------.      .------------.      .--------------. | |
+| | | build input |----->| build plan |----->| build result | | |
+| | '-------------'      '------------'      '--------------' | |
+| +-------------------------------------------------|---------+ |
++---------------------------------------------------|-----------+
+                                                    |
+                                              represented as
+                                                    |
+                                                    V
+                                            .---------------.
+                                            |     file      |
+                                            '---------------'
+```
+
+At the top is the [command line interface](../command-ref/command-ref.md) that drives the underlying layers.
+
+The [Nix language](../language/index.md) evaluator transforms Nix expressions into self-contained *build plans*, which are used to derive *build results* from referenced *build inputs*.
+
+The command line interface and Nix expressions are what users deal with most.
+
+> **Note**
+> The Nix language itself does not have a notion of *packages* or *configurations*.
+> As far as we are concerned here, the inputs and results of a build plan are just data.
+
+Underlying the command line interface and the Nix language evaluator is the [Nix store](../glossary.md#gloss-store), a mechanism to keep track of build plans, data, and references between them.
+It can also execute build plans to produce new data, which are made available to the operating system as files.
+
+A build plan itself is a series of *build tasks*, together with their build inputs.
+
+> **Important**
+> A build task in Nix is called [derivation](../glossary#gloss-derivation).
+
+Each build task has a special build input executed as *build instructions* in order to perform the build.
+The result of a build task can be input to another build task.
+
+The following [data flow diagram] shows a build plan for illustration.
+Build inputs used as instructions to a build task are marked accordingly:
+
+[data flow diagram]: https://en.m.wikipedia.org/wiki/Data-flow_diagram
+
+```
++--------------------------------------------------------------------+
+| build plan                                                         |
+|                                                                    |
+| .-------------.                                                    |
+| | build input |---------.                                          |
+| '-------------'         |                                          |
+|                    instructions                                    |
+|                         |                                          |
+|                         v                                          |
+| .-------------.    .----------.                                    |
+| | build input |-->( build task )-------.                           |
+| '-------------'    '----------'        |                           |
+|                                  instructions                      |
+|                                        |                           |
+|                                        v                           |
+| .-------------.                  .----------.     .--------------. |
+| | build input |---------.       ( build task )--->| build result | |
+| '-------------'         |        '----------'     '--------------' |
+|                    instructions        ^                           |
+|                         |              |                           |
+|                         v              |                           |
+| .-------------.    .----------.        |                           |
+| | build input |-->( build task )-------'                           |
+| '-------------'    '----------'                                    |
+|                         ^                                          |
+|                         |                                          |
+|                         |                                          |
+| .-------------.         |                                          |
+| | build input |---------'                                          |
+| '-------------'                                                    |
+|                                                                    |
++--------------------------------------------------------------------+
+```
+

doc/manual/src/command-ref/nix-build.md

@@ -37,10 +37,12 @@ directory containing at least a file named `default.nix`.
 
 `nix-build` is essentially a wrapper around
 [`nix-instantiate`](nix-instantiate.md) (to translate a high-level Nix
-expression to a low-level store derivation) and [`nix-store
+expression to a low-level [store derivation]) and [`nix-store
 --realise`](nix-store.md#operation---realise) (to build the store
 derivation).
 
+[store derivation]: ../glossary.md#gloss-store-derivation
+
 > **Warning**
 >
 > The result of the build is automatically registered as a root of the

doc/manual/src/command-ref/nix-copy-closure.md

@@ -47,7 +47,9 @@ authentication, you can avoid typing the passphrase with `ssh-agent`.
     Enable compression of the SSH connection.
 
   - `--include-outputs`\
-    Also copy the outputs of store derivations included in the closure.
+    Also copy the outputs of [store derivation]s included in the closure.
+
+    [store derivation]: ../../glossary.md#gloss-store-derivation
 
   - `--use-substitutes` / `-s`\
     Attempt to download missing paths on the target machine using Nix’s

doc/manual/src/command-ref/nix-env.md

@@ -205,10 +205,12 @@ a number of possible ways:
     unambiguous way, which is necessary if there are multiple
     derivations with the same name.
 
-  - If *args* are store derivations, then these are
+  - If *args* are [store derivation]s, then these are
     [realised](nix-store.md#operation---realise), and the resulting output paths
     are installed.
 
+    [store derivation]: ../glossary.md#gloss-store-derivation
+
   - If *args* are store paths that are not store derivations, then these
     are [realised](nix-store.md#operation---realise) and installed.
 
@@ -280,7 +282,7 @@ To copy the store path with symbolic name `gcc` from another profile:
 $ nix-env -i --from-profile /nix/var/nix/profiles/foo gcc
 ```
 
-To install a specific store derivation (typically created by
+To install a specific [store derivation] (typically created by
 `nix-instantiate`):
 
 ```console
@@ -665,7 +667,7 @@ derivation is shown unless `--no-name` is specified.
     Print the `system` attribute of the derivation.
 
   - `--drv-path`\
-    Print the path of the store derivation.
+    Print the path of the [store derivation].
 
   - `--out-path`\
     Print the output path of the derivation.

doc/manual/src/command-ref/nix-instantiate.md

@@ -17,13 +17,14 @@
 
 # Description
 
-The command `nix-instantiate` generates [store
-derivations](../glossary.md) from (high-level) Nix expressions. It
-evaluates the Nix expressions in each of *files* (which defaults to
+The command `nix-instantiate` produces [store derivation]s from (high-level) Nix expressions.
+It evaluates the Nix expressions in each of *files* (which defaults to
 *./default.nix*). Each top-level expression should evaluate to a
 derivation, a list of derivations, or a set of derivations. The paths
 of the resulting store derivations are printed on standard output.
 
+[store derivation]: ../glossary.md#gloss-store-derivation
+
 If *files* is the character `-`, then a Nix expression will be read from
 standard input.
 
@@ -79,8 +80,7 @@ standard input.
 
 # Examples
 
-Instantiating store derivations from a Nix expression, and building them
-using `nix-store`:
+Instantiate [store derivation]s from a Nix expression, and build them using `nix-store`:
 
 ```console
 $ nix-instantiate test.nix (instantiate)

doc/manual/src/command-ref/nix-store.md

@@ -137,8 +137,10 @@ or.
 
 ## Examples
 
-This operation is typically used to build store derivations produced by
-[`nix-instantiate`](nix-instantiate.md):
+This operation is typically used to build [store derivation]s produced by
+[`nix-instantiate`](./nix-instantiate.md):
+
+[store derivation]: ../glossary.md#gloss-store-derivation
 
 ```console
 $ nix-store -r $(nix-instantiate ./test.nix)
@@ -298,7 +300,7 @@ symlink.
 ## Common query options
 
   - `--use-output`; `-u`\
-    For each argument to the query that is a store derivation, apply the
+    For each argument to the query that is a [store derivation], apply the
     query to the output path of the derivation instead.
 
   - `--force-realise`; `-f`\
@@ -318,7 +320,7 @@ symlink.
     This query has one option:
 
       - `--include-outputs`
-        Also include the existing output paths of store derivations,
+        Also include the existing output paths of [store derivation]s,
         and their closures.
 
     This query can be used to implement various kinds of deployment. A
@@ -372,12 +374,12 @@ symlink.
     Prints the references graph of the store paths *paths* in the
     [GraphML](http://graphml.graphdrawing.org/) file format. This can be
     used to visualise dependency graphs. To obtain a build-time
-    dependency graph, apply this to a store derivation. To obtain a
+    dependency graph, apply this to a [store derivation]. To obtain a
     runtime dependency graph, apply it to an output path.
 
   - `--binding` *name*; `-b` *name*\
     Prints the value of the attribute *name* (i.e., environment
-    variable) of the store derivations *paths*. It is an error for a
+    variable) of the [store derivation]s *paths*. It is an error for a
     derivation to not have the specified attribute.
 
   - `--hash`\

doc/manual/src/contributing/hacking.md

@@ -99,8 +99,79 @@ You can run the whole testsuite with `make check`, or the tests for a specific c
 ### Functional tests
 
 The functional tests reside under the `tests` directory and are listed in `tests/local.mk`.
-The whole testsuite can be run with `make install && make installcheck`.
-Individual tests can be run with `make tests/{testName}.sh.test`.
+Each test is a bash script.
+
+The whole test suite can be run with:
+
+```shell-session
+$ make install && make installcheck
+ran test tests/foo.sh... [PASS]
+ran test tests/bar.sh... [PASS]
+...
+```
+
+Individual tests can be run with `make`:
+
+```shell-session
+$ make tests/${testName}.sh.test
+ran test tests/${testName}.sh... [PASS]
+```
+
+or without `make`:
+
+```shell-session
+$ ./mk/run-test.sh tests/${testName}.sh
+ran test tests/${testName}.sh... [PASS]
+```
+
+To see the complete output, one can also run:
+
+```shell-session
+$ ./mk/debug-test.sh tests/${testName}.sh
++ foo
+output from foo
++ bar
+output from bar
+...
+```
+
+The test script will then be traced with `set -x` and the output displayed as it happens, regardless of whether the test succeeds or fails.
+
+#### Debugging failing functional tests
+
+When a functional test fails, it usually does so somewhere in the middle of the script.
+
+To figure out what's wrong, it is convenient to run the test regularly up to the failing `nix` command, and then run that command with a debugger like GDB.
+
+For example, if the script looks like:
+
+```bash
+foo
+nix blah blub
+bar
+```
+edit it like so:
+
+```diff
+ foo
+-nix blah blub
++gdb --args nix blah blub
+ bar
+```
+
+Then, running the test with `./mk/debug-test.sh` will drop you into GDB once the script reaches that point:
+
+```shell-session
+$ ./mk/debug-test.sh tests/${testName}.sh
+...
++ gdb blash blub
+GNU gdb (GDB) 12.1
+...
+(gdb)
+```
+
+One can debug the Nix invocation in all the usual ways.
+For example, enter `run` to start the Nix invocation.
 
 ### Integration tests
 

doc/manual/src/glossary.md

@@ -7,6 +7,18 @@
     translated into low-level *store derivations* (implicitly by
     `nix-env` and `nix-build`, or explicitly by `nix-instantiate`).
 
+    [derivation]: #gloss-derivation
+
+  - [store derivation]{#gloss-store-derivation}\
+    A [derivation] represented as a `.drv` file in the [store].
+    It has a [store path], like any [store object].
+
+    Example: `/nix/store/g946hcz4c8mdvq2g8vxx42z51qb71rvp-git-2.38.1.drv`
+
+    See [`nix show-derivation`](./command-ref/new-cli/nix3-show-derivation.md) (experimental) for displaying the contents of store derivations.
+
+    [store derivation]: #gloss-store-derivation
+
   - [content-addressed derivation]{#gloss-content-addressed-derivation}\
     A derivation which has the
     [`__contentAddressed`](./language/advanced-attributes.md#adv-attr-__contentAddressed)
@@ -34,6 +46,8 @@
       directory on another machine,  accessed via `ssh` or
       served by the `nix-serve` Perl script.
 
+    [store]: #gloss-store
+
   - [chroot store]{#gloss-chroot-store}\
     A local store whose canonical path is anything other than `/nix/store`.
 
@@ -46,9 +60,13 @@
     cache](https://cache.nixos.org).
 
   - [store path]{#gloss-store-path}\
-    The location in the file system of a store object, i.e., an
+    The location of a [store object] in the file system, i.e., an
     immediate child of the Nix store directory.
 
+    Example: `/nix/store/a040m110amc4h71lds2jmr8qrkj2jhxd-git-2.38.1`
+
+    [store path]: #gloss-store-path
+
   - [store object]{#gloss-store-object}\
     A file that is an immediate child of the Nix store directory. These
     can be regular files, but also entire directory trees. Store objects
@@ -56,6 +74,8 @@
     derivation outputs (objects produced by running a build task), or
     derivations (files describing a build task).
 
+    [store object]: #gloss-store-object
+
   - [input-addressed store object]{#gloss-input-addressed-store-object}\
     A store object produced by building a
     non-[content-addressed](#gloss-content-addressed-derivation),
@@ -124,7 +144,9 @@
     references `R` then `R` is also in the closure of `P`.
 
   - [output path]{#gloss-output-path}\
-    A store path produced by a derivation.
+    A [store path] produced by a [derivation].
+
+    [output path]: #gloss-output-path
 
   - [deriver]{#gloss-deriver}\
     The deriver of an *output path* is the store
@@ -156,3 +178,12 @@
 
   - [`ε`]{#gloss-epsilon}\
     The epsilon symbol. In the context of a package, this means the version is empty. More precisely, the derivation does not have a version attribute.
+
+  - [string interpolation]{#gloss-string-interpolation}\
+    Expanding expressions enclosed in `${ }` within a [string], [path], or [attribute name].
+
+    See [String interpolation](./language/string-interpolation.md) for details.
+
+    [string]: ./language/values.md#type-string
+    [path]: ./language/values.md#type-path
+    [attribute name]: ./language/values.md#attribute-set

doc/manual/src/installation/installing-binary.md

@@ -120,10 +120,10 @@ sudo rm -rf /nix /etc/nix /etc/profile/nix.sh ~root/.nix-profile ~root/.nix-defe
 Remove build users and their group:
 
 ```console
-for i in $(seq 30001 30032); do
-  sudo userdel $i
+for i in $(seq 1 32); do
+  sudo userdel nixbld$i
 done
-sudo groupdel 30000
+sudo groupdel nixbld
 ```
 
 There may also be references to Nix in

doc/manual/src/language/operators.md

@@ -1,28 +1,167 @@
 # Operators
 
-The table below lists the operators in the Nix language, in
-order of precedence (from strongest to weakest binding).
+| Name                                   | Syntax                                     | Associativity | Precedence |
+|----------------------------------------|--------------------------------------------|---------------|------------|
+| [Attribute selection]                  | *attrset* `.` *attrpath* \[ `or` *expr* \] | none          | 1          |
+| Function application                   | *func* *expr*                              | left          | 2          |
+| [Arithmetic negation][arithmetic]      | `-` *number*                               | none          | 3          |
+| [Has attribute]                        | *attrset* `?` *attrpath*                   | none          | 4          |
+| List concatenation                     | *list* `++` *list*                         | right         | 5          |
+| [Multiplication][arithmetic]           | *number* `*` *number*                      | left          | 6          |
+| [Division][arithmetic]                 | *number* `/` *number*                      | left          | 6          |
+| [Subtraction][arithmetic]              | *number* `-` *number*                      | left          | 7          |
+| [Addition][arithmetic]                 | *number* `+` *number*                      | left          | 7          |
+| [String concatenation]                 | *string* `+` *string*                      | left          | 7          |
+| [Path concatenation]                   | *path* `+` *path*                          | left          | 7          |
+| [Path and string concatenation]        | *path* `+` *string*                        | left          | 7          |
+| [String and path concatenation]        | *string* `+` *path*                        | left          | 7          |
+| Logical negation (`NOT`)               | `!` *bool*                                 | none          | 8          |
+| [Update]                               | *attrset* `//` *attrset*                   | right         | 9          |
+| [Less than][Comparison]                | *expr* `<` *expr*                          | none          | 10         |
+| [Less than or equal to][Comparison]    | *expr* `<=` *expr*                         | none          | 10         |
+| [Greater than][Comparison]             | *expr* `>` *expr*                          | none          | 10         |
+| [Greater than or equal to][Comparison] | *expr* `>=` *expr*                         | none          | 10         |
+| [Equality]                             | *expr* `==` *expr*                         | none          | 11         |
+| Inequality                             | *expr* `!=` *expr*                         | none          | 11         |
+| Logical conjunction (`AND`)            | *bool* `&&` *bool*                         | left          | 12         |
+| Logical disjunction (`OR`)             | *bool* `||` *bool*                         | left          | 13         |
+| [Logical implication]                  | *bool* `->` *bool*                         | none          | 14         |
+
+[string]: ./values.md#type-string
+[path]: ./values.md#type-path
+[number]: ./values.md#type-number
+[list]: ./values.md#list
+[attribute set]: ./values.md#attribute-set
+
+## Attribute selection
+
+Select the attribute denoted by attribute path *attrpath* from [attribute set] *attrset*.
+If the attribute doesn’t exist, return *value* if provided, otherwise abort evaluation.
+
+<!-- FIXME: the following should to into its own language syntax section, but that needs more work to fit in well -->
+
+An attribute path is a dot-separated list of attribute names.
+An attribute name can be an identifier or a string.
+
+> *attrpath* = *name* [ `.` *name* ]...
+> *name* = *identifier* | *string*
+> *identifier* ~ `[a-zA-Z_][a-zA-Z0-9_'-]*`
+
+[Attribute selection]: #attribute-selection
+
+## Has attribute
+
+> *attrset* `?` *attrpath*
+
+Test whether [attribute set] *attrset* contains the attribute denoted by *attrpath*.
+The result is a [Boolean] value.
+
+[Boolean]: ./values.md#type-boolean
+
+[Has attribute]: #has-attribute
+
+## Arithmetic
+
+Numbers are type-compatible:
+Pure integer operations will always return integers, whereas any operation involving at least one floating point number return a floating point number.
+
+See also [Comparison] and [Equality].
+
+The `+` operator is overloaded to also work on strings and paths.
+
+[arithmetic]: #arithmetic
+
+## String concatenation
+
+> *string* `+` *string*
+
+Concatenate two [string]s and merge their string contexts.
+
+[String concatenation]: #string-concatenation
+
+## Path concatenation
+
+> *path* `+` *path*
+
+Concatenate two [path]s.
+The result is a path.
+
+[Path concatenation]: #path-concatenation
+
+## Path and string concatenation
+
+> *path* + *string*
+
+Concatenate *[path]* with *[string]*.
+The result is a path.
+
+> **Note**
+>
+> The string must not have a string context that refers to a [store path].
+
+[Path and string concatenation]: #path-and-string-concatenation
+
+## String and path concatenation
+
+> *string* + *path*
+
+Concatenate *[string]* with *[path]*.
+The result is a string.
+
+> **Important**
+>
+> The file or directory at *path* must exist and is copied to the [store].
+> The path appears in the result as the corresponding [store path].
+
+[store path]: ../glossary.md#gloss-store-path
+[store]: ../glossary.md#gloss-store
+
+[Path and string concatenation]: #path-and-string-concatenation
+
+## Update
+
+> *attrset1* + *attrset2*
+
+Update [attribute set] *attrset1* with names and values from *attrset2*.
+
+The returned attribute set will have of all the attributes in *e1* and *e2*.
+If an attribute name is present in both, the attribute value from the former is taken.
+
+[Update]: #update
+
+## Comparison
+
+Comparison is
+
+- [arithmetic] for [number]s 
+- lexicographic for [string]s and [path]s
+- item-wise lexicographic for [list]s:
+  elements at the same index in both lists are compared according to their type and skipped if they are equal.
+
+All comparison operators are implemented in terms of `<`, and the following equivalencies hold:
+
+| comparison   | implementation        |
+|--------------|-----------------------|
+| *a* `<=` *b* | `! (` *b* `<` *a* `)` |
+| *a* `>`  *b* |       *b* `<` *a*     |
+| *a* `>=` *b* | `! (` *a* `<` *b* `)` |
+
+[Comparison]: #comparison-operators
+
+## Equality
+
+- [Attribute sets][attribute set] and [list]s are compared recursively, and therefore are fully evaluated.
+- Comparison of [function]s always returns `false`.
+- Numbers are type-compatible, see [arithmetic] operators.
+- Floating point numbers only differ up to a limited precision.
+
+[function]: ./constructs.md#functions
+
+[Equality]: #equality
+
+## Logical implication
+
+Equivalent to `!`*b1* `||` *b2*.
+
+[Logical implication]: #logical-implication
 
-| Name                     | Syntax                              | Associativity | Description                                                                                                                                                                                                                   | Precedence |
-| ------------------------ | ----------------------------------- | ------------- | ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | ---------- |
-| Select                   | *e* `.` *attrpath* \[ `or` *def* \] | none          | Select attribute denoted by the attribute path *attrpath* from set *e*. (An attribute path is a dot-separated list of attribute names.) If the attribute doesn’t exist, return *def* if provided, otherwise abort evaluation. | 1          |
-| Application              | *e1* *e2*                           | left          | Call function *e1* with argument *e2*.                                                                                                                                                                                        | 2          |
-| Arithmetic Negation      | `-` *e*                             | none          | Arithmetic negation.                                                                                                                                                                                                          | 3          |
-| Has Attribute            | *e* `?` *attrpath*                  | none          | Test whether set *e* contains the attribute denoted by *attrpath*; return `true` or `false`.                                                                                                                                  | 4          |
-| List Concatenation       | *e1* `++` *e2*                      | right         | List concatenation.                                                                                                                                                                                                           | 5          |
-| Multiplication           | *e1* `*` *e2*,                      | left          | Arithmetic multiplication.                                                                                                                                                                                                    | 6          |
-| Division                 | *e1* `/` *e2*                       | left          | Arithmetic division.                                                                                                                                                                                                          | 6          |
-| Addition                 | *e1* `+` *e2*                       | left          | Arithmetic addition.                                                                                                                                                                                                          | 7          |
-| Subtraction              | *e1* `-` *e2*                       | left          | Arithmetic subtraction.                                                                                                                                                                                                       | 7          |
-| String Concatenation     | *string1* `+` *string2*             | left          | String concatenation.                                                                                                                                                                                                         | 7          |
-| Not                      | `!` *e*                             | none          | Boolean negation.                                                                                                                                                                                                             | 8          |
-| Update                   | *e1* `//` *e2*                      | right         | Return a set consisting of the attributes in *e1* and *e2* (with the latter taking precedence over the former in case of equally named attributes).                                                                           | 9          |
-| Less Than                | *e1* `<` *e2*,                      | none          | Arithmetic/lexicographic comparison.                                                                                                                                                                                                        | 10         |
-| Less Than or Equal To    | *e1* `<=` *e2*                      | none          | Arithmetic/lexicographic comparison.                                                                                                                                                                                                        | 10         |
-| Greater Than             | *e1* `>` *e2*                       | none          | Arithmetic/lexicographic comparison.                                                                                                                                                                                                        | 10         |
-| Greater Than or Equal To | *e1* `>=` *e2*                      | none          | Arithmetic/lexicographic comparison.                                                                                                                                                                                                        | 10         |
-| Equality                 | *e1* `==` *e2*                      | none          | Equality.                                                                                                                                                                                                                     | 11         |
-| Inequality               | *e1* `!=` *e2*                      | none          | Inequality.                                                                                                                                                                                                                   | 11         |
-| Logical AND              | *e1* `&&` *e2*                      | left          | Logical AND.                                                                                                                                                                                                                  | 12         |
-| Logical OR               | *e1* <code>&#124;&#124;</code> *e2* | left          | Logical OR.                                                                                                                                                                                                                   | 13         |
-| Logical Implication      | *e1* `->` *e2*                      | none          | Logical implication (equivalent to <code>!e1 &#124;&#124; e2</code>).                                                                                                                                                                            | 14         |

doc/manual/src/language/string-interpolation.md

@@ -0,0 +1,82 @@
+# String interpolation
+
+String interpolation is a language feature where a [string], [path], or [attribute name] can contain expressions enclosed in `${ }` (dollar-sign with curly brackets).
+
+Such a string is an *interpolated string*, and an expression inside is an *interpolated expression*.
+
+Interpolated expressions must evaluate to one of the following:
+
+- a [string]
+- a [path]
+- a [derivation]
+
+[string]: ./values.md#type-string
+[path]: ./values.md#type-path
+[attribute name]: ./values.md#attribute-set
+[derivation]: ../glossary.md#gloss-derivation
+
+## Examples
+
+### String
+
+Rather than writing
+
+```nix
+"--with-freetype2-library=" + freetype + "/lib"
+```
+
+(where `freetype` is a [derivation]), you can instead write
+
+```nix
+"--with-freetype2-library=${freetype}/lib"
+```
+
+The latter is automatically translated to the former.
+
+A more complicated example (from the Nix expression for [Qt](http://www.trolltech.com/products/qt)):
+
+```nix
+configureFlags = "
+  -system-zlib -system-libpng -system-libjpeg
+  ${if openglSupport then "-dlopen-opengl
+    -L${mesa}/lib -I${mesa}/include
+    -L${libXmu}/lib -I${libXmu}/include" else ""}
+  ${if threadSupport then "-thread" else "-no-thread"}
+";
+```
+
+Note that Nix expressions and strings can be arbitrarily nested;
+in this case the outer string contains various interpolated expressions that themselves contain strings (e.g., `"-thread"`), some of which in turn contain interpolated expressions (e.g., `${mesa}`).
+
+### Path
+
+Rather than writing
+
+```nix
+./. + "/" + foo + "-" + bar + ".nix"
+```
+
+or
+
+```nix
+./. + "/${foo}-${bar}.nix"
+```
+
+you can instead write
+
+```nix
+./${foo}-${bar}.nix
+```
+
+### Attribute name
+
+Attribute names can be created dynamically with string interpolation:
+
+```nix
+let name = "foo"; in
+{
+  ${name} = "bar";
+}
+```
+
+    { foo = "bar"; }

doc/manual/src/language/values.md

@@ -13,41 +13,9 @@
   returns and tabs can be written as `\n`, `\r` and `\t`,
   respectively.
 
-  You can include the result of an expression into a string by
-  enclosing it in `${...}`, a feature known as *antiquotation*. The
-  enclosed expression must evaluate to something that can be coerced
-  into a string (meaning that it must be a string, a path, or a
-  derivation). For instance, rather than writing
+  You can include the results of other expressions into a string by enclosing them in `${ }`, a feature known as [string interpolation].
 
-  ```nix
-  "--with-freetype2-library=" + freetype + "/lib"
-  ```
-
-  (where `freetype` is a derivation), you can instead write the more
-  natural
-
-  ```nix
-  "--with-freetype2-library=${freetype}/lib"
-  ```
-
-  The latter is automatically translated to the former. A more
-  complicated example (from the Nix expression for
-  [Qt](http://www.trolltech.com/products/qt)):
-
-  ```nix
-  configureFlags = "
-    -system-zlib -system-libpng -system-libjpeg
-    ${if openglSupport then "-dlopen-opengl
-      -L${mesa}/lib -I${mesa}/include
-      -L${libXmu}/lib -I${libXmu}/include" else ""}
-    ${if threadSupport then "-thread" else "-no-thread"}
-  ";
-  ```
-
-  Note that Nix expressions and strings can be arbitrarily nested; in
-  this case the outer string contains various antiquotations that
-  themselves contain strings (e.g., `"-thread"`), some of which in
-  turn contain expressions (e.g., `${mesa}`).
+  [string interpolation]: ./string-interpolation.md
 
   The second way to write string literals is as an *indented string*,
   which is enclosed between pairs of *double single-quotes*, like so:
@@ -75,7 +43,7 @@
   Note that the whitespace and newline following the opening `''` is
   ignored if there is no non-whitespace text on the initial line.
 
-  Antiquotation (`${expr}`) is supported in indented strings.
+  Indented strings support [string interpolation].
 
   Since `${` and `''` have special meaning in indented strings, you
   need a way to quote them. `$` can be escaped by prefixing it with
@@ -117,9 +85,10 @@
   Numbers, which can be *integers* (like `123`) or *floating point*
   (like `123.43` or `.27e13`).
 
-  Numbers are type-compatible: pure integer operations will always
-  return integers, whereas any operation involving at least one
-  floating point number will have a floating point number as a result.
+  See [arithmetic] and [comparison] operators for semantics.
+
+  [arithmetic]: ./operators.md#arithmetic
+  [comparison]: ./operators.md#comparison
 
 - <a id="type-path" href="#type-path">Path</a>
 
@@ -143,26 +112,23 @@
   environment variable `NIX_PATH` will be searched for the given file
   or directory name.
 
-  Antiquotation is supported in any paths except those in angle brackets.
-  `./${foo}-${bar}.nix` is a more convenient way of writing
-  `./. + "/" + foo + "-" + bar + ".nix"` or `./. + "/${foo}-${bar}.nix"`. At
-  least one slash must appear *before* any antiquotations for this to be
-  recognized as a path. `a.${foo}/b.${bar}` is a syntactically valid division
-  operation. `./a.${foo}/b.${bar}` is a path.
+  When an [interpolated string][string interpolation] evaluates to a path, the path is first copied into the Nix store and the resulting string is the [store path] of the newly created [store object].
 
-  When a path appears in an antiquotation, and is thus coerced into a string,
-  the path is first copied into the Nix store and the resulting string is
-  the Nix store path. For instance `"${./foo.txt}" will cause `foo.txt` in
-  the current directory to be copied into the Nix store and result in the
-  string `"/nix/store/<HASH>-foo.txt"`.
+  [store path]: ../glossary.md#gloss-store-path
+  [store object]: ../glossary.md#gloss-store-object
 
-  Note that the Nix language assumes that all input files will remain
-  _unchanged_ during the course of the Nix expression evaluation.
-  If you for example antiquote a file path during a `nix repl` session, and
-  then later in the same session, after having changed the file contents,
-  evaluate the antiquotation with the file path again, then Nix will still
-  return the first store path. It will _not_ reread the file contents to
-  produce a different Nix store path.
+  For instance, evaluating `"${./foo.txt}"` will cause `foo.txt` in the current directory to be copied into the Nix store and result in the string `"/nix/store/<hash>-foo.txt"`.
+
+  Note that the Nix language assumes that all input files will remain _unchanged_ while  evaluating a Nix expression.
+  For example, assume you used a file path in an interpolated string during a `nix repl` session.
+  Later in the same session, after having changed the file contents, evaluating the interpolated string with the file path again might not return a new store path, since Nix might not re-read the file contents.
+
+  Paths themselves, except those in angle brackets (`< >`), support [string interpolation].
+
+  At least one slash (`/`) must appear *before* any interpolated expression for the result to be recognized as a path.
+
+  `a.${foo}/b.${bar}` is a syntactically valid division operation.
+  `./a.${foo}/b.${bar}` is a path.
 
 - <a id="type-boolean" href="#type-boolean">Boolean</a>
 
@@ -235,23 +201,33 @@ will evaluate to `"Xyzzy"` because there is no `c` attribute in the set.
 You can use arbitrary double-quoted strings as attribute names:
 
 ```nix
-{ "foo ${bar}" = 123; "nix-1.0" = 456; }."foo ${bar}"
+{ "$!@#?" = 123; }."$!@#?"
 ```
 
-This will evaluate to `123` (Assuming `bar` is antiquotable). In the
-case where an attribute name is just a single antiquotation, the quotes
-can be dropped:
-
 ```nix
-{ foo = 123; }.${bar} or 456
+let bar = "bar";
+{ "foo ${bar}" = 123; }."foo ${bar}"
 ```
 
-This will evaluate to `123` if `bar` evaluates to `"foo"` when coerced
-to a string and `456` otherwise (again assuming `bar` is antiquotable).
+Both will evaluate to `123`.
+
+Attribute names support [string interpolation]:
+
+```nix
+let bar = "foo"; in
+{ foo = 123; }.${bar}
+```
+
+```nix
+let bar = "foo"; in
+{ ${bar} = 123; }.foo
+```
+
+Both will evaluate to `123`.
 
 In the special case where an attribute name inside of a set declaration
-evaluates to `null` (which is normally an error, as `null` is not
-antiquotable), that attribute is simply not added to the set:
+evaluates to `null` (which is normally an error, as `null` cannot be coerced to
+a string), that attribute is simply not added to the set:
 
 ```nix
 { ${if foo then "bar" else null} = true; }

doc/manual/src/release-notes/rl-next.md

@@ -15,13 +15,9 @@
   # NIX_PATH=nixpkgs=flake:nixpkgs nix-build '<nixpkgs>' -A hello
   ```
 
-* Allow explicitly selecting outputs in a store derivation installable, just like we can do with other sorts of installables.
-  For example,
-  ```shell-session
-  $ nix-build /nix/store/gzaflydcr6sb3567hap9q6srzx8ggdgg-glibc-2.33-78.drv^dev`
-  ```
-  now works just as
-  ```shell-session
-  $ nix-build glibc^dev`
-  ```
-  does already.
+* Instead of "antiquotation", the more common term [string interpolation](../language/string-interpolation.md) is now used consistently.
+  Historical release notes were not changed.
+
+* Error traces have been reworked to provide detailed explanations and more
+  accurate error locations. A short excerpt of the trace is now shown by
+  default when an error occurs.

mk/common-test.sh

@@ -0,0 +1,11 @@
+TESTS_ENVIRONMENT=("TEST_NAME=${test%.*}" 'NIX_REMOTE=')
+
+: ${BASH:=/usr/bin/env bash}
+
+init_test () {
+   cd tests && env "${TESTS_ENVIRONMENT[@]}" $BASH -e init.sh 2>/dev/null > /dev/null
+}
+
+run_test_proper () {
+   cd $(dirname $test) && env "${TESTS_ENVIRONMENT[@]}" $BASH -e $(basename $test)
+}

mk/debug-test.sh

@@ -0,0 +1,11 @@
+#!/usr/bin/env bash
+
+set -eu
+
+test=$1
+
+dir="$(dirname "${BASH_SOURCE[0]}")"
+source "$dir/common-test.sh"
+
+(init_test)
+run_test_proper

mk/run-test.sh

@@ -1,4 +1,4 @@
-#!/bin/sh
+#!/usr/bin/env bash
 
 set -u
 
@@ -7,7 +7,12 @@ green=""
 yellow=""
 normal=""
 
-post_run_msg="ran test $1..."
+test=$1
+
+dir="$(dirname "${BASH_SOURCE[0]}")"
+source "$dir/common-test.sh"
+
+post_run_msg="ran test $test..."
 if [ -t 1 ]; then
     red=""
     green=""
@@ -16,12 +21,12 @@ if [ -t 1 ]; then
 fi
 
 run_test () {
-    (cd tests && env ${TESTS_ENVIRONMENT} init.sh 2>/dev/null > /dev/null)
-    log="$(cd $(dirname $1) && env ${TESTS_ENVIRONMENT} $(basename $1) 2>&1)"
+    (init_test 2>/dev/null > /dev/null)
+    log="$(run_test_proper 2>&1)"
     status=$?
 }
 
-run_test "$1"
+run_test
 
 # Hack: Retry the test if it fails with “unexpected EOF reading a line” as these
 # appear randomly without anyone knowing why.
@@ -32,7 +37,7 @@ if [[ $status -ne 0 && $status -ne 99 && \
 ]]; then
     echo "$post_run_msg [${yellow}FAIL$normal] (possibly flaky, so will be retried)"
     echo "$log" | sed 's/^/    /'
-    run_test "$1"
+    run_test
 fi
 
 if [ $status -eq 0 ]; then

mk/tests.mk

@@ -8,7 +8,11 @@ define run-install-test
 
   .PHONY: $1.test
   $1.test: $1 $(test-deps)
-	@env TEST_NAME=$(basename $1) TESTS_ENVIRONMENT="$(tests-environment)" mk/run_test.sh $1 < /dev/null
+	@env BASH=$(bash) $(bash) mk/run-test.sh $1 < /dev/null
+
+  .PHONY: $1.test-debug
+  $1.test-debug: $1 $(test-deps)
+	@env BASH=$(bash) $(bash) mk/debug-test.sh $1 < /dev/null
 
 endef
 

scripts/install-multi-user.sh

@@ -575,7 +575,7 @@ EOF
     # to extract _just_ the user's note, instead it is prefixed with
     # some plist junk. This was causing the user note to always be set,
     # even if there was no reason for it.
-    if ! poly_user_note_get "$username" | grep -q "Nix build user $coreid"; then
+    if poly_user_note_get "$username" | grep -q "Nix build user $coreid"; then
         row "              Note" "Nix build user $coreid"
     else
         poly_user_note_set "$username" "Nix build user $coreid"

src/libcmd/installables.cc

@@ -183,7 +183,7 @@ SourceExprCommand::SourceExprCommand(bool supportReadOnlyMode)
 
     addFlag({
         .longName = "derivation",
-        .description = "Operate on the store derivation rather than its outputs.",
+        .description = "Operate on the [store derivation](../../glossary.md#gloss-store-derivation) rather than its outputs.",
         .category = installablesCategory,
         .handler = {&operateOn, OperateOn::Derivation},
     });
@@ -667,7 +667,7 @@ ref<eval_cache::EvalCache> openEvalCache(
             auto vFlake = state.allocValue();
             flake::callFlake(state, *lockedFlake, *vFlake);
 
-            state.forceAttrs(*vFlake, noPos);
+            state.forceAttrs(*vFlake, noPos, "while parsing cached flake data");
 
             auto aOutputs = vFlake->attrs->get(state.symbols.create("outputs"));
             assert(aOutputs);
@@ -1128,10 +1128,7 @@ std::vector<std::pair<std::shared_ptr<Installable>, BuiltPathWithResult>> Instal
                                 DrvOutput outputId { *outputHash, output };
                                 auto realisation = store->queryRealisation(outputId);
                                 if (!realisation)
-                                    throw Error(
-                                        "cannot operate on an output of the "
-                                        "unbuilt derivation '%s'",
-                                        outputId.to_string());
+                                    throw MissingRealisation(outputId);
                                 outputs.insert_or_assign(output, realisation->outPath);
                             } else {
                                 // If ca-derivations isn't enabled, assume that

src/libcmd/repl.cc

@@ -397,7 +397,7 @@ StringSet NixRepl::completePrefix(const std::string & prefix)
             Expr * e = parseString(expr);
             Value v;
             e->eval(*state, *env, v);
-            state->forceAttrs(v, noPos);
+            state->forceAttrs(v, noPos, "nevermind, it is ignored anyway");
 
             for (auto & i : *v.attrs) {
                 std::string_view name = state->symbols[i.name];
@@ -590,7 +590,7 @@ bool NixRepl::processLine(std::string line)
         const auto [path, line] = [&] () -> std::pair<Path, uint32_t> {
             if (v.type() == nPath || v.type() == nString) {
                 PathSet context;
-                auto path = state->coerceToPath(noPos, v, context);
+                auto path = state->coerceToPath(noPos, v, context, "while evaluating the filename to edit");
                 return {path, 0};
             } else if (v.isLambda()) {
                 auto pos = state->positions[v.lambda.fun->pos];
@@ -834,7 +834,7 @@ void NixRepl::loadFiles()
 
 void NixRepl::addAttrsToScope(Value & attrs)
 {
-    state->forceAttrs(attrs, [&]() { return attrs.determinePos(noPos); });
+    state->forceAttrs(attrs, [&]() { return attrs.determinePos(noPos); }, "while evaluating an attribute set to be merged in the global scope");
     if (displ + attrs.attrs->size() >= envSize)
         throw Error("environment full; cannot add more variables");
 
@@ -939,7 +939,7 @@ std::ostream & NixRepl::printValue(std::ostream & str, Value & v, unsigned int m
             Bindings::iterator i = v.attrs->find(state->sDrvPath);
             PathSet context;
             if (i != v.attrs->end())
-                str << state->store->printStorePath(state->coerceToStorePath(i->pos, *i->value, context));
+                str << state->store->printStorePath(state->coerceToStorePath(i->pos, *i->value, context, "while evaluating the drvPath of a derivation"));
             else
                 str << "???";
             str << "»";

src/libexpr/attr-path.cc

@@ -118,7 +118,7 @@ std::pair<std::string, uint32_t> findPackageFilename(EvalState & state, Value &
 
     // FIXME: is it possible to extract the Pos object instead of doing this
     //        toString + parsing?
-    auto pos = state.forceString(*v2);
+    auto pos = state.forceString(*v2, noPos, "while evaluating the 'meta.position' attribute of a derivation");
 
     auto colon = pos.rfind(':');
     if (colon == std::string::npos)

src/libexpr/eval-cache.cc

@@ -385,7 +385,7 @@ Value & AttrCursor::getValue()
     if (!_value) {
         if (parent) {
             auto & vParent = parent->first->getValue();
-            root->state.forceAttrs(vParent, noPos);
+            root->state.forceAttrs(vParent, noPos, "while searching for an attribute");
             auto attr = vParent.attrs->get(parent->second);
             if (!attr)
                 throw Error("attribute '%s' is unexpectedly missing", getAttrPathStr());
@@ -571,14 +571,14 @@ std::string AttrCursor::getString()
                 debug("using cached string attribute '%s'", getAttrPathStr());
                 return s->first;
             } else
-                root->state.debugThrowLastTrace(TypeError("'%s' is not a string", getAttrPathStr()));
+                root->state.error("'%s' is not a string", getAttrPathStr()).debugThrow<TypeError>();
         }
     }
 
     auto & v = forceValue();
 
     if (v.type() != nString && v.type() != nPath)
-        root->state.debugThrowLastTrace(TypeError("'%s' is not a string but %s", getAttrPathStr(), showType(v.type())));
+        root->state.error("'%s' is not a string but %s", getAttrPathStr()).debugThrow<TypeError>();
 
     return v.type() == nString ? v.string.s : v.path;
 }
@@ -602,7 +602,7 @@ string_t AttrCursor::getStringWithContext()
                     return *s;
                 }
             } else
-                root->state.debugThrowLastTrace(TypeError("'%s' is not a string", getAttrPathStr()));
+                root->state.error("'%s' is not a string", getAttrPathStr()).debugThrow<TypeError>();
         }
     }
 
@@ -613,7 +613,7 @@ string_t AttrCursor::getStringWithContext()
     else if (v.type() == nPath)
         return {v.path, {}};
     else
-        root->state.debugThrowLastTrace(TypeError("'%s' is not a string but %s", getAttrPathStr(), showType(v.type())));
+        root->state.error("'%s' is not a string but %s", getAttrPathStr()).debugThrow<TypeError>();
 }
 
 bool AttrCursor::getBool()
@@ -626,14 +626,14 @@ bool AttrCursor::getBool()
                 debug("using cached Boolean attribute '%s'", getAttrPathStr());
                 return *b;
             } else
-                root->state.debugThrowLastTrace(TypeError("'%s' is not a Boolean", getAttrPathStr()));
+                root->state.error("'%s' is not a Boolean", getAttrPathStr()).debugThrow<TypeError>();
         }
     }
 
     auto & v = forceValue();
 
     if (v.type() != nBool)
-        root->state.debugThrowLastTrace(TypeError("'%s' is not a Boolean", getAttrPathStr()));
+        root->state.error("'%s' is not a Boolean", getAttrPathStr()).debugThrow<TypeError>();
 
     return v.boolean;
 }
@@ -685,7 +685,7 @@ std::vector<std::string> AttrCursor::getListOfStrings()
     std::vector<std::string> res;
 
     for (auto & elem : v.listItems())
-        res.push_back(std::string(root->state.forceStringNoCtx(*elem)));
+        res.push_back(std::string(root->state.forceStringNoCtx(*elem, noPos, "while evaluating an attribute for caching")));
 
     if (root->db)
         cachedValue = {root->db->setListOfStrings(getKey(), res), res};
@@ -703,14 +703,14 @@ std::vector<Symbol> AttrCursor::getAttrs()
                 debug("using cached attrset attribute '%s'", getAttrPathStr());
                 return *attrs;
             } else
-                root->state.debugThrowLastTrace(TypeError("'%s' is not an attribute set", getAttrPathStr()));
+                root->state.error("'%s' is not an attribute set", getAttrPathStr()).debugThrow<TypeError>();
         }
     }
 
     auto & v = forceValue();
 
     if (v.type() != nAttrs)
-        root->state.debugThrowLastTrace(TypeError("'%s' is not an attribute set", getAttrPathStr()));
+        root->state.error("'%s' is not an attribute set", getAttrPathStr()).debugThrow<TypeError>();
 
     std::vector<Symbol> attrs;
     for (auto & attr : *getValue().attrs)

src/libexpr/eval-inline.hh

@@ -103,33 +103,36 @@ void EvalState::forceValue(Value & v, Callable getPos)
     else if (v.isApp())
         callFunction(*v.app.left, *v.app.right, v, noPos);
     else if (v.isBlackhole())
-        throwEvalError(getPos(), "infinite recursion encountered");
+        error("infinite recursion encountered").atPos(getPos()).template debugThrow<EvalError>();
 }
 
 
 [[gnu::always_inline]]
-inline void EvalState::forceAttrs(Value & v, const PosIdx pos)
+inline void EvalState::forceAttrs(Value & v, const PosIdx pos, std::string_view errorCtx)
 {
-    forceAttrs(v, [&]() { return pos; });
+    forceAttrs(v, [&]() { return pos; }, errorCtx);
 }
 
 
 template <typename Callable>
 [[gnu::always_inline]]
-inline void EvalState::forceAttrs(Value & v, Callable getPos)
+inline void EvalState::forceAttrs(Value & v, Callable getPos, std::string_view errorCtx)
 {
-    forceValue(v, getPos);
-    if (v.type() != nAttrs)
-        throwTypeError(getPos(), "value is %1% while a set was expected", v);
+    forceValue(v, noPos);
+    if (v.type() != nAttrs) {
+        PosIdx pos = getPos();
+        error("value is %1% while a set was expected", showType(v)).withTrace(pos, errorCtx).debugThrow<TypeError>();
+    }
 }
 
 
 [[gnu::always_inline]]
-inline void EvalState::forceList(Value & v, const PosIdx pos)
+inline void EvalState::forceList(Value & v, const PosIdx pos, std::string_view errorCtx)
 {
-    forceValue(v, pos);
-    if (!v.isList())
-        throwTypeError(pos, "value is %1% while a list was expected", v);
+    forceValue(v, noPos);
+    if (!v.isList()) {
+        error("value is %1% while a list was expected", showType(v)).withTrace(pos, errorCtx).debugThrow<TypeError>();
+    }
 }
 
 

src/libexpr/eval.cc

@@ -67,22 +67,19 @@ static char * dupString(const char * s)
 
 // When there's no need to write to the string, we can optimize away empty
 // string allocations.
-// This function handles makeImmutableStringWithLen(null, 0) by returning the
-// empty string.
-static const char * makeImmutableStringWithLen(const char * s, size_t size)
+// This function handles makeImmutableString(std::string_view()) by returning
+// the empty string.
+static const char * makeImmutableString(std::string_view s)
 {
+    const size_t size = s.size();
     if (size == 0)
         return "";
     auto t = allocString(size + 1);
-    memcpy(t, s, size);
-    t[size] = 0;
+    memcpy(t, s.data(), size);
+    t[size] = '\0';
     return t;
 }
 
-static inline const char * makeImmutableString(std::string_view s) {
-    return makeImmutableStringWithLen(s.data(), s.size());
-}
-
 
 RootValue allocRootValue(Value * v)
 {
@@ -321,7 +318,7 @@ static Symbol getName(const AttrName & name, EvalState & state, Env & env)
     } else {
         Value nameValue;
         name.expr->eval(state, env, nameValue);
-        state.forceStringNoCtx(nameValue);
+        state.forceStringNoCtx(nameValue, noPos, "while evaluating an attribute name");
         return state.symbols.create(nameValue.string.s);
     }
 }
@@ -417,6 +414,44 @@ static Strings parseNixPath(const std::string & s)
     return res;
 }
 
+ErrorBuilder & ErrorBuilder::atPos(PosIdx pos)
+{
+    info.errPos = state.positions[pos];
+    return *this;
+}
+
+ErrorBuilder & ErrorBuilder::withTrace(PosIdx pos, const std::string_view text)
+{
+    info.traces.push_front(Trace{ .pos = state.positions[pos], .hint = hintformat(std::string(text)), .frame = false });
+    return *this;
+}
+
+ErrorBuilder & ErrorBuilder::withFrameTrace(PosIdx pos, const std::string_view text)
+{
+    info.traces.push_front(Trace{ .pos = state.positions[pos], .hint = hintformat(std::string(text)), .frame = true });
+    return *this;
+}
+
+ErrorBuilder & ErrorBuilder::withSuggestions(Suggestions & s)
+{
+    info.suggestions = s;
+    return *this;
+}
+
+ErrorBuilder & ErrorBuilder::withFrame(const Env & env, const Expr & expr)
+{
+    // NOTE: This is abusing side-effects.
+    // TODO: check compatibility with nested debugger calls.
+    state.debugTraces.push_front(DebugTrace {
+        .pos = nullptr,
+        .expr = expr,
+        .env = env,
+        .hint = hintformat("Fake frame for debugging purposes"),
+        .isError = true
+    });
+    return *this;
+}
+
 
 EvalState::EvalState(
     const Strings & _searchPath,
@@ -649,25 +684,7 @@ void EvalState::addConstant(const std::string & name, Value * v)
 Value * EvalState::addPrimOp(const std::string & name,
     size_t arity, PrimOpFun primOp)
 {
-    auto name2 = name.substr(0, 2) == "__" ? name.substr(2) : name;
-    auto sym = symbols.create(name2);
-
-    /* Hack to make constants lazy: turn them into a application of
-       the primop to a dummy value. */
-    if (arity == 0) {
-        auto vPrimOp = allocValue();
-        vPrimOp->mkPrimOp(new PrimOp { .fun = primOp, .arity = 1, .name = name2 });
-        Value v;
-        v.mkApp(vPrimOp, vPrimOp);
-        return addConstant(name, v);
-    }
-
-    Value * v = allocValue();
-    v->mkPrimOp(new PrimOp { .fun = primOp, .arity = arity, .name = name2 });
-    staticBaseEnv->vars.emplace_back(symbols.create(name), baseEnvDispl);
-    baseEnv.values[baseEnvDispl++] = v;
-    baseEnv.values[0]->attrs->push_back(Attr(sym, v));
-    return v;
+    return addPrimOp(PrimOp { .fun = primOp, .arity = arity, .name = name });
 }
 
 
@@ -845,176 +862,14 @@ void EvalState::runDebugRepl(const Error * error, const Env & env, const Expr &
     }
 }
 
-/* Every "format" object (even temporary) takes up a few hundred bytes
-   of stack space, which is a real killer in the recursive
-   evaluator.  So here are some helper functions for throwing
-   exceptions. */
-void EvalState::throwEvalError(const PosIdx pos, const char * s, Env & env, Expr & expr)
-{
-    debugThrow(EvalError({
-        .msg = hintfmt(s),
-        .errPos = positions[pos]
-    }), env, expr);
-}
-
-void EvalState::throwEvalError(const PosIdx pos, const char * s)
-{
-    debugThrowLastTrace(EvalError({
-        .msg = hintfmt(s),
-        .errPos = positions[pos]
-    }));
-}
-
-void EvalState::throwEvalError(const char * s, const std::string & s2)
-{
-    debugThrowLastTrace(EvalError(s, s2));
-}
-
-void EvalState::throwEvalError(const PosIdx pos, const Suggestions & suggestions, const char * s,
-    const std::string & s2, Env & env, Expr & expr)
-{
-    debugThrow(EvalError(ErrorInfo{
-        .msg = hintfmt(s, s2),
-        .errPos = positions[pos],
-        .suggestions = suggestions,
-    }), env, expr);
-}
-
-void EvalState::throwEvalError(const PosIdx pos, const char * s, const std::string & s2)
-{
-    debugThrowLastTrace(EvalError({
-        .msg = hintfmt(s, s2),
-        .errPos = positions[pos]
-    }));
-}
-
-void EvalState::throwEvalError(const PosIdx pos, const char * s, const std::string & s2, Env & env, Expr & expr)
-{
-    debugThrow(EvalError({
-        .msg = hintfmt(s, s2),
-        .errPos = positions[pos]
-    }), env, expr);
-}
-
-void EvalState::throwEvalError(const char * s, const std::string & s2,
-    const std::string & s3)
-{
-    debugThrowLastTrace(EvalError({
-        .msg = hintfmt(s, s2, s3),
-        .errPos = positions[noPos]
-    }));
-}
-
-void EvalState::throwEvalError(const PosIdx pos, const char * s, const std::string & s2,
-    const std::string & s3)
-{
-    debugThrowLastTrace(EvalError({
-        .msg = hintfmt(s, s2, s3),
-        .errPos = positions[pos]
-    }));
-}
-
-void EvalState::throwEvalError(const PosIdx pos, const char * s, const std::string & s2,
-    const std::string & s3, Env & env, Expr & expr)
-{
-    debugThrow(EvalError({
-        .msg = hintfmt(s, s2, s3),
-        .errPos = positions[pos]
-    }), env, expr);
-}
-
-void EvalState::throwEvalError(const PosIdx p1, const char * s, const Symbol sym, const PosIdx p2, Env & env, Expr & expr)
-{
-    // p1 is where the error occurred; p2 is a position mentioned in the message.
-    debugThrow(EvalError({
-        .msg = hintfmt(s, symbols[sym], positions[p2]),
-        .errPos = positions[p1]
-    }), env, expr);
-}
-
-void EvalState::throwTypeError(const PosIdx pos, const char * s, const Value & v)
-{
-    debugThrowLastTrace(TypeError({
-        .msg = hintfmt(s, showType(v)),
-        .errPos = positions[pos]
-    }));
-}
-
-void EvalState::throwTypeError(const PosIdx pos, const char * s, const Value & v, Env & env, Expr & expr)
-{
-    debugThrow(TypeError({
-        .msg = hintfmt(s, showType(v)),
-        .errPos = positions[pos]
-    }), env, expr);
-}
-
-void EvalState::throwTypeError(const PosIdx pos, const char * s)
-{
-    debugThrowLastTrace(TypeError({
-        .msg = hintfmt(s),
-        .errPos = positions[pos]
-    }));
-}
-
-void EvalState::throwTypeError(const PosIdx pos, const char * s, const ExprLambda & fun,
-    const Symbol s2, Env & env, Expr &expr)
-{
-    debugThrow(TypeError({
-        .msg = hintfmt(s, fun.showNamePos(*this), symbols[s2]),
-        .errPos = positions[pos]
-    }), env, expr);
-}
-
-void EvalState::throwTypeError(const PosIdx pos, const Suggestions & suggestions, const char * s,
-    const ExprLambda & fun, const Symbol s2, Env & env, Expr &expr)
-{
-    debugThrow(TypeError(ErrorInfo {
-        .msg = hintfmt(s, fun.showNamePos(*this), symbols[s2]),
-        .errPos = positions[pos],
-        .suggestions = suggestions,
-    }), env, expr);
-}
-
-void EvalState::throwTypeError(const char * s, const Value & v, Env & env, Expr &expr)
-{
-    debugThrow(TypeError({
-        .msg = hintfmt(s, showType(v)),
-        .errPos = positions[expr.getPos()],
-    }), env, expr);
-}
-
-void EvalState::throwAssertionError(const PosIdx pos, const char * s, const std::string & s1, Env & env, Expr &expr)
-{
-    debugThrow(AssertionError({
-        .msg = hintfmt(s, s1),
-        .errPos = positions[pos]
-    }), env, expr);
-}
-
-void EvalState::throwUndefinedVarError(const PosIdx pos, const char * s, const std::string & s1, Env & env, Expr &expr)
-{
-    debugThrow(UndefinedVarError({
-        .msg = hintfmt(s, s1),
-        .errPos = positions[pos]
-    }), env, expr);
-}
-
-void EvalState::throwMissingArgumentError(const PosIdx pos, const char * s, const std::string & s1, Env & env, Expr &expr)
-{
-    debugThrow(MissingArgumentError({
-        .msg = hintfmt(s, s1),
-        .errPos = positions[pos]
-    }), env, expr);
-}
-
 void EvalState::addErrorTrace(Error & e, const char * s, const std::string & s2) const
 {
     e.addTrace(nullptr, s, s2);
 }
 
-void EvalState::addErrorTrace(Error & e, const PosIdx pos, const char * s, const std::string & s2) const
+void EvalState::addErrorTrace(Error & e, const PosIdx pos, const char * s, const std::string & s2, bool frame) const
 {
-    e.addTrace(positions[pos], s, s2);
+    e.addTrace(positions[pos], hintfmt(s, s2), frame);
 }
 
 static std::unique_ptr<DebugTraceStacker> makeDebugTraceStacker(
@@ -1091,7 +946,7 @@ inline Value * EvalState::lookupVar(Env * env, const ExprVar & var, bool noEval)
         if (env->type == Env::HasWithExpr) {
             if (noEval) return 0;
             Value * v = allocValue();
-            evalAttrs(*env->up, (Expr *) env->values[0], *v);
+            evalAttrs(*env->up, (Expr *) env->values[0], *v, noPos, "<borked>");
             env->values[0] = v;
             env->type = Env::HasWithAttrs;
         }
@@ -1101,7 +956,7 @@ inline Value * EvalState::lookupVar(Env * env, const ExprVar & var, bool noEval)
             return j->value;
         }
         if (!env->prevWith)
-            throwUndefinedVarError(var.pos, "undefined variable '%1%'", symbols[var.name], *env, const_cast<ExprVar&>(var));
+            error("undefined variable '%1%'", symbols[var.name]).atPos(var.pos).withFrame(*env, var).debugThrow<UndefinedVarError>();
         for (size_t l = env->prevWith; l; --l, env = env->up) ;
     }
 }
@@ -1251,7 +1106,7 @@ void EvalState::cacheFile(
         // computation.
         if (mustBeTrivial &&
             !(dynamic_cast<ExprAttrs *>(e)))
-            throw EvalError("file '%s' must be an attribute set", path);
+            error("file '%s' must be an attribute set", path).debugThrow<EvalError>();
         eval(e, v);
     } catch (Error & e) {
         addErrorTrace(e, "while evaluating the file '%1%':", resolvedPath);
@@ -1269,31 +1124,31 @@ void EvalState::eval(Expr * e, Value & v)
 }
 
 
-inline bool EvalState::evalBool(Env & env, Expr * e)
+inline bool EvalState::evalBool(Env & env, Expr * e, const PosIdx pos, std::string_view errorCtx)
 {
+    try {
         Value v;
         e->eval(*this, env, v);
         if (v.type() != nBool)
-        throwTypeError(noPos, "value is %1% while a Boolean was expected", v, env, *e);
+            error("value is %1% while a Boolean was expected", showType(v)).withFrame(env, *e).debugThrow<TypeError>();
         return v.boolean;
+    } catch (Error & e) {
+        e.addTrace(positions[pos], errorCtx);
+        throw;
+    }
 }
 
 
-inline bool EvalState::evalBool(Env & env, Expr * e, const PosIdx pos)
-{
-    Value v;
-    e->eval(*this, env, v);
-    if (v.type() != nBool)
-        throwTypeError(pos, "value is %1% while a Boolean was expected", v, env, *e);
-    return v.boolean;
-}
-
-
-inline void EvalState::evalAttrs(Env & env, Expr * e, Value & v)
+inline void EvalState::evalAttrs(Env & env, Expr * e, Value & v, const PosIdx pos, std::string_view errorCtx)
 {
+    try {
         e->eval(*this, env, v);
         if (v.type() != nAttrs)
-        throwTypeError(noPos, "value is %1% while a set was expected", v, env, *e);
+            error("value is %1% while a set was expected", showType(v)).withFrame(env, *e).debugThrow<TypeError>();
+    } catch (Error & e) {
+        e.addTrace(positions[pos], errorCtx);
+        throw;
+    }
 }
 
 
@@ -1366,7 +1221,7 @@ void ExprAttrs::eval(EvalState & state, Env & env, Value & v)
            Hence we need __overrides.) */
         if (hasOverrides) {
             Value * vOverrides = (*v.attrs)[overrides->second.displ].value;
-            state.forceAttrs(*vOverrides, [&]() { return vOverrides->determinePos(noPos); });
+            state.forceAttrs(*vOverrides, [&]() { return vOverrides->determinePos(noPos); }, "while evaluating the `__overrides` attribute");
             Bindings * newBnds = state.allocBindings(v.attrs->capacity() + vOverrides->attrs->size());
             for (auto & i : *v.attrs)
                 newBnds->push_back(i);
@@ -1394,11 +1249,11 @@ void ExprAttrs::eval(EvalState & state, Env & env, Value & v)
         state.forceValue(nameVal, i.pos);
         if (nameVal.type() == nNull)
             continue;
-        state.forceStringNoCtx(nameVal);
+        state.forceStringNoCtx(nameVal, i.pos, "while evaluating the name of a dynamic attribute");
         auto nameSym = state.symbols.create(nameVal.string.s);
         Bindings::iterator j = v.attrs->find(nameSym);
         if (j != v.attrs->end())
-            state.throwEvalError(i.pos, "dynamic attribute '%1%' already defined at %2%", nameSym, j->pos, env, *this);
+            state.error("dynamic attribute '%1%' already defined at %2%", state.symbols[nameSym], state.positions[j->pos]).atPos(i.pos).withFrame(env, *this).debugThrow<EvalError>();
 
         i.valueExpr->setName(nameSym);
         /* Keep sorted order so find can catch duplicates */
@@ -1495,15 +1350,14 @@ void ExprSelect::eval(EvalState & state, Env & env, Value & v)
                     return;
                 }
             } else {
-                state.forceAttrs(*vAttrs, pos);
+                state.forceAttrs(*vAttrs, pos, "while selecting an attribute");
                 if ((j = vAttrs->attrs->find(name)) == vAttrs->attrs->end()) {
                     std::set<std::string> allAttrNames;
                     for (auto & attr : *vAttrs->attrs)
                         allAttrNames.insert(state.symbols[attr.name]);
-                    state.throwEvalError(
-                        pos,
-                        Suggestions::bestMatches(allAttrNames, state.symbols[name]),
-                        "attribute '%1%' missing", state.symbols[name], env, *this);
+                    auto suggestions = Suggestions::bestMatches(allAttrNames, state.symbols[name]);
+                    state.error("attribute '%1%' missing", state.symbols[name])
+                        .atPos(pos).withSuggestions(suggestions).withFrame(env, *this).debugThrow<EvalError>();
                 }
             }
             vAttrs = j->value;
@@ -1598,7 +1452,12 @@ void EvalState::callFunction(Value & fun, size_t nrArgs, Value * * args, Value &
             if (!lambda.hasFormals())
                 env2.values[displ++] = args[0];
             else {
-                forceAttrs(*args[0], pos);
+                try {
+                    forceAttrs(*args[0], lambda.pos, "while evaluating the value passed for the lambda argument");
+                } catch (Error & e) {
+                    if (pos) e.addTrace(positions[pos], "from call site");
+                    throw;
+                }
 
                 if (lambda.arg)
                     env2.values[displ++] = args[0];
@@ -1610,8 +1469,15 @@ void EvalState::callFunction(Value & fun, size_t nrArgs, Value * * args, Value &
                 for (auto & i : lambda.formals->formals) {
                     auto j = args[0]->attrs->get(i.name);
                     if (!j) {
-                        if (!i.def) throwTypeError(pos, "%1% called without required argument '%2%'",
-                            lambda, i.name, *fun.lambda.env, lambda);
+                        if (!i.def) {
+                            error("function '%1%' called without required argument '%2%'",
+                                             (lambda.name ? std::string(symbols[lambda.name]) : "anonymous lambda"),
+                                             symbols[i.name])
+                                    .atPos(lambda.pos)
+                                    .withTrace(pos, "from call site")
+                                    .withFrame(*fun.lambda.env, lambda)
+                                    .debugThrow<TypeError>();
+                        }
                         env2.values[displ++] = i.def->maybeThunk(*this, env2);
                     } else {
                         attrsUsed++;
@@ -1629,11 +1495,15 @@ void EvalState::callFunction(Value & fun, size_t nrArgs, Value * * args, Value &
                             std::set<std::string> formalNames;
                             for (auto & formal : lambda.formals->formals)
                                 formalNames.insert(symbols[formal.name]);
-                            throwTypeError(
-                                pos,
-                                Suggestions::bestMatches(formalNames, symbols[i.name]),
-                                "%1% called with unexpected argument '%2%'",
-                                lambda, i.name, *fun.lambda.env, lambda);
+                            auto suggestions = Suggestions::bestMatches(formalNames, symbols[i.name]);
+                            error("function '%1%' called with unexpected argument '%2%'",
+                                             (lambda.name ? std::string(symbols[lambda.name]) : "anonymous lambda"),
+                                             symbols[i.name])
+                                .atPos(lambda.pos)
+                                .withTrace(pos, "from call site")
+                                .withSuggestions(suggestions)
+                                .withFrame(*fun.lambda.env, lambda)
+                                .debugThrow<TypeError>();
                         }
                     abort(); // can't happen
                 }
@@ -1656,11 +1526,15 @@ void EvalState::callFunction(Value & fun, size_t nrArgs, Value * * args, Value &
                 lambda.body->eval(*this, env2, vCur);
             } catch (Error & e) {
                 if (loggerSettings.showTrace.get()) {
-                    addErrorTrace(e, lambda.pos, "while calling %s",
-                        (lambda.name
+                    addErrorTrace(
+                        e,
+                        lambda.pos,
+                        "while calling %s",
+                        lambda.name
                         ? concatStrings("'", symbols[lambda.name], "'")
-                            : "anonymous lambda"));
-                    addErrorTrace(e, pos, "while evaluating call site%s", "");
+                        : "anonymous lambda",
+                        true);
+                    if (pos) addErrorTrace(e, pos, "from call site%s", "", true);
                 }
                 throw;
             }
@@ -1679,9 +1553,17 @@ void EvalState::callFunction(Value & fun, size_t nrArgs, Value * * args, Value &
                 return;
             } else {
                 /* We have all the arguments, so call the primop. */
+                auto name = vCur.primOp->name;
+
                 nrPrimOpCalls++;
-                if (countCalls) primOpCalls[vCur.primOp->name]++;
-                vCur.primOp->fun(*this, pos, args, vCur);
+                if (countCalls) primOpCalls[name]++;
+
+                try {
+                    vCur.primOp->fun(*this, noPos, args, vCur);
+                } catch (Error & e) {
+                    addErrorTrace(e, pos, "while calling the '%1%' builtin", name);
+                    throw;
+                }
 
                 nrArgs -= argsLeft;
                 args += argsLeft;
@@ -1716,9 +1598,20 @@ void EvalState::callFunction(Value & fun, size_t nrArgs, Value * * args, Value &
                 for (size_t i = 0; i < argsLeft; ++i)
                     vArgs[argsDone + i] = args[i];
 
+                auto name = primOp->primOp->name;
                 nrPrimOpCalls++;
-                if (countCalls) primOpCalls[primOp->primOp->name]++;
-                primOp->primOp->fun(*this, pos, vArgs, vCur);
+                if (countCalls) primOpCalls[name]++;
+
+                try {
+                    // TODO:
+                    // 1. Unify this and above code. Heavily redundant.
+                    // 2. Create a fake env (arg1, arg2, etc.) and a fake expr (arg1: arg2: etc: builtins.name arg1 arg2 etc)
+                    //    so the debugger allows to inspect the wrong parameters passed to the builtin.
+                    primOp->primOp->fun(*this, noPos, vArgs, vCur);
+                } catch (Error & e) {
+                    addErrorTrace(e, pos, "while calling the '%1%' builtin", name);
+                    throw;
+                }
 
                 nrArgs -= argsLeft;
                 args += argsLeft;
@@ -1731,14 +1624,18 @@ void EvalState::callFunction(Value & fun, size_t nrArgs, Value * * args, Value &
                heap-allocate a copy and use that instead. */
             Value * args2[] = {allocValue(), args[0]};
             *args2[0] = vCur;
-            /* !!! Should we use the attr pos here? */
-            callFunction(*functor->value, 2, args2, vCur, pos);
+            try {
+                callFunction(*functor->value, 2, args2, vCur, functor->pos);
+            } catch (Error & e) {
+                e.addTrace(positions[pos], "while calling a functor (an attribute set with a '__functor' attribute)");
+                throw;
+            }
             nrArgs--;
             args++;
         }
 
         else
-            throwTypeError(pos, "attempt to call something which is not a function but %1%", vCur);
+            error("attempt to call something which is not a function but %1%", showType(vCur)).atPos(pos).debugThrow<TypeError>();
     }
 
     vRes = vCur;
@@ -1802,13 +1699,12 @@ void EvalState::autoCallFunction(Bindings & args, Value & fun, Value & res)
             if (j != args.end()) {
                 attrs.insert(*j);
             } else if (!i.def) {
-                throwMissingArgumentError(i.pos, R"(cannot evaluate a function that has an argument without a value ('%1%')
-
+                error(R"(cannot evaluate a function that has an argument without a value ('%1%')
 Nix attempted to evaluate a function as a top level expression; in
 this case it must have its arguments supplied either by default
 values, or passed explicitly with '--arg' or '--argstr'. See
-https://nixos.org/manual/nix/stable/language/constructs.html#functions.)", symbols[i.name],
-                *fun.lambda.env, *fun.lambda.fun);
+https://nixos.org/manual/nix/stable/language/constructs.html#functions.)", symbols[i.name])
+                    .atPos(i.pos).withFrame(*fun.lambda.env, *fun.lambda.fun).debugThrow<MissingArgumentError>();
             }
         }
     }
@@ -1831,16 +1727,17 @@ void ExprWith::eval(EvalState & state, Env & env, Value & v)
 
 void ExprIf::eval(EvalState & state, Env & env, Value & v)
 {
-    (state.evalBool(env, cond, pos) ? then : else_)->eval(state, env, v);
+    // We cheat in the parser, and pass the position of the condition as the position of the if itself.
+    (state.evalBool(env, cond, pos, "while evaluating a branch condition") ? then : else_)->eval(state, env, v);
 }
 
 
 void ExprAssert::eval(EvalState & state, Env & env, Value & v)
 {
-    if (!state.evalBool(env, cond, pos)) {
+    if (!state.evalBool(env, cond, pos, "in the condition of the assert statement")) {
         std::ostringstream out;
         cond->show(state.symbols, out);
-        state.throwAssertionError(pos, "assertion '%1%' failed", out.str(), env, *this);
+        state.error("assertion '%1%' failed", out.str()).atPos(pos).withFrame(env, *this).debugThrow<AssertionError>();
     }
     body->eval(state, env, v);
 }
@@ -1848,7 +1745,7 @@ void ExprAssert::eval(EvalState & state, Env & env, Value & v)
 
 void ExprOpNot::eval(EvalState & state, Env & env, Value & v)
 {
-    v.mkBool(!state.evalBool(env, e));
+    v.mkBool(!state.evalBool(env, e, noPos, "in the argument of the not operator")); // XXX: FIXME: !
 }
 
 
@@ -1856,7 +1753,7 @@ void ExprOpEq::eval(EvalState & state, Env & env, Value & v)
 {
     Value v1; e1->eval(state, env, v1);
     Value v2; e2->eval(state, env, v2);
-    v.mkBool(state.eqValues(v1, v2));
+    v.mkBool(state.eqValues(v1, v2, pos, "while testing two values for equality"));
 }
 
 
@@ -1864,33 +1761,33 @@ void ExprOpNEq::eval(EvalState & state, Env & env, Value & v)
 {
     Value v1; e1->eval(state, env, v1);
     Value v2; e2->eval(state, env, v2);
-    v.mkBool(!state.eqValues(v1, v2));
+    v.mkBool(!state.eqValues(v1, v2, pos, "while testing two values for inequality"));
 }
 
 
 void ExprOpAnd::eval(EvalState & state, Env & env, Value & v)
 {
-    v.mkBool(state.evalBool(env, e1, pos) && state.evalBool(env, e2, pos));
+    v.mkBool(state.evalBool(env, e1, pos, "in the left operand of the AND (&&) operator") && state.evalBool(env, e2, pos, "in the right operand of the AND (&&) operator"));
 }
 
 
 void ExprOpOr::eval(EvalState & state, Env & env, Value & v)
 {
-    v.mkBool(state.evalBool(env, e1, pos) || state.evalBool(env, e2, pos));
+    v.mkBool(state.evalBool(env, e1, pos, "in the left operand of the OR (||) operator") || state.evalBool(env, e2, pos, "in the right operand of the OR (||) operator"));
 }
 
 
 void ExprOpImpl::eval(EvalState & state, Env & env, Value & v)
 {
-    v.mkBool(!state.evalBool(env, e1, pos) || state.evalBool(env, e2, pos));
+    v.mkBool(!state.evalBool(env, e1, pos, "in the left operand of the IMPL (->) operator") || state.evalBool(env, e2, pos, "in the right operand of the IMPL (->) operator"));
 }
 
 
 void ExprOpUpdate::eval(EvalState & state, Env & env, Value & v)
 {
     Value v1, v2;
-    state.evalAttrs(env, e1, v1);
-    state.evalAttrs(env, e2, v2);
+    state.evalAttrs(env, e1, v1, pos, "in the left operand of the update (//) operator");
+    state.evalAttrs(env, e2, v2, pos, "in the right operand of the update (//) operator");
 
     state.nrOpUpdates++;
 
@@ -1929,18 +1826,18 @@ void ExprOpConcatLists::eval(EvalState & state, Env & env, Value & v)
     Value v1; e1->eval(state, env, v1);
     Value v2; e2->eval(state, env, v2);
     Value * lists[2] = { &v1, &v2 };
-    state.concatLists(v, 2, lists, pos);
+    state.concatLists(v, 2, lists, pos, "while evaluating one of the elements to concatenate");
 }
 
 
-void EvalState::concatLists(Value & v, size_t nrLists, Value * * lists, const PosIdx pos)
+void EvalState::concatLists(Value & v, size_t nrLists, Value * * lists, const PosIdx pos, std::string_view errorCtx)
 {
     nrListConcats++;
 
     Value * nonEmpty = 0;
     size_t len = 0;
     for (size_t n = 0; n < nrLists; ++n) {
-        forceList(*lists[n], pos);
+        forceList(*lists[n], pos, errorCtx);
         auto l = lists[n]->listSize();
         len += l;
         if (l) nonEmpty = lists[n];
@@ -2017,20 +1914,20 @@ void ExprConcatStrings::eval(EvalState & state, Env & env, Value & v)
                 nf = n;
                 nf += vTmp.fpoint;
             } else
-                state.throwEvalError(i_pos, "cannot add %1% to an integer", showType(vTmp), env, *this);
+                state.error("cannot add %1% to an integer", showType(vTmp)).atPos(i_pos).withFrame(env, *this).debugThrow<EvalError>();
         } else if (firstType == nFloat) {
             if (vTmp.type() == nInt) {
                 nf += vTmp.integer;
             } else if (vTmp.type() == nFloat) {
                 nf += vTmp.fpoint;
             } else
-                state.throwEvalError(i_pos, "cannot add %1% to a float", showType(vTmp), env, *this);
+                state.error("cannot add %1% to a float", showType(vTmp)).atPos(i_pos).withFrame(env, *this).debugThrow<EvalError>();
         } else {
             if (s.empty()) s.reserve(es->size());
             /* skip canonization of first path, which would only be not
             canonized in the first place if it's coming from a ./${foo} type
             path */
-            auto part = state.coerceToString(i_pos, vTmp, context, false, firstType == nString, !first);
+            auto part = state.coerceToString(i_pos, vTmp, context, false, firstType == nString, !first, "while evaluating a path segment");
             sSize += part->size();
             s.emplace_back(std::move(part));
         }
@@ -2044,7 +1941,7 @@ void ExprConcatStrings::eval(EvalState & state, Env & env, Value & v)
         v.mkFloat(nf);
     else if (firstType == nPath) {
         if (!context.empty())
-            state.throwEvalError(pos, "a string that refers to a store path cannot be appended to a path", env, *this);
+            state.error("a string that refers to a store path cannot be appended to a path").atPos(pos).withFrame(env, *this).debugThrow<EvalError>();
         v.mkPath(canonPath(str()));
     } else
         v.mkStringMove(c_str(), context);
@@ -2094,33 +1991,47 @@ void EvalState::forceValueDeep(Value & v)
 }
 
 
-NixInt EvalState::forceInt(Value & v, const PosIdx pos)
+NixInt EvalState::forceInt(Value & v, const PosIdx pos, std::string_view errorCtx)
 {
+    try {
         forceValue(v, pos);
         if (v.type() != nInt)
-        throwTypeError(pos, "value is %1% while an integer was expected", v);
-
+            error("value is %1% while an integer was expected", showType(v)).debugThrow<TypeError>();
         return v.integer;
+    } catch (Error & e) {
+        e.addTrace(positions[pos], errorCtx);
+        throw;
+    }
 }
 
 
-NixFloat EvalState::forceFloat(Value & v, const PosIdx pos)
+NixFloat EvalState::forceFloat(Value & v, const PosIdx pos, std::string_view errorCtx)
 {
+    try {
         forceValue(v, pos);
         if (v.type() == nInt)
             return v.integer;
         else if (v.type() != nFloat)
-        throwTypeError(pos, "value is %1% while a float was expected", v);
+            error("value is %1% while a float was expected", showType(v)).debugThrow<TypeError>();
         return v.fpoint;
+    } catch (Error & e) {
+        e.addTrace(positions[pos], errorCtx);
+        throw;
+    }
 }
 
 
-bool EvalState::forceBool(Value & v, const PosIdx pos)
+bool EvalState::forceBool(Value & v, const PosIdx pos, std::string_view errorCtx)
 {
+    try {
         forceValue(v, pos);
         if (v.type() != nBool)
-        throwTypeError(pos, "value is %1% while a Boolean was expected", v);
+            error("value is %1% while a Boolean was expected", showType(v)).debugThrow<TypeError>();
         return v.boolean;
+    } catch (Error & e) {
+        e.addTrace(positions[pos], errorCtx);
+        throw;
+    }
 }
 
 
@@ -2130,21 +2041,30 @@ bool EvalState::isFunctor(Value & fun)
 }
 
 
-void EvalState::forceFunction(Value & v, const PosIdx pos)
+void EvalState::forceFunction(Value & v, const PosIdx pos, std::string_view errorCtx)
 {
+    try {
         forceValue(v, pos);
         if (v.type() != nFunction && !isFunctor(v))
-        throwTypeError(pos, "value is %1% while a function was expected", v);
+            error("value is %1% while a function was expected", showType(v)).debugThrow<TypeError>();
+    } catch (Error & e) {
+        e.addTrace(positions[pos], errorCtx);
+        throw;
+    }
 }
 
 
-std::string_view EvalState::forceString(Value & v, const PosIdx pos)
+std::string_view EvalState::forceString(Value & v, const PosIdx pos, std::string_view errorCtx)
 {
+    try {
         forceValue(v, pos);
-    if (v.type() != nString) {
-        throwTypeError(pos, "value is %1% while a string was expected", v);
-    }
+        if (v.type() != nString)
+            error("value is %1% while a string was expected", showType(v)).debugThrow<TypeError>();
         return v.string.s;
+    } catch (Error & e) {
+        e.addTrace(positions[pos], errorCtx);
+        throw;
+    }
 }
 
 
@@ -2188,24 +2108,19 @@ NixStringContext Value::getContext(const Store & store)
 }
 
 
-std::string_view EvalState::forceString(Value & v, PathSet & context, const PosIdx pos)
+std::string_view EvalState::forceString(Value & v, PathSet & context, const PosIdx pos, std::string_view errorCtx)
 {
-    auto s = forceString(v, pos);
+    auto s = forceString(v, pos, errorCtx);
     copyContext(v, context);
     return s;
 }
 
 
-std::string_view EvalState::forceStringNoCtx(Value & v, const PosIdx pos)
+std::string_view EvalState::forceStringNoCtx(Value & v, const PosIdx pos, std::string_view errorCtx)
 {
-    auto s = forceString(v, pos);
+    auto s = forceString(v, pos, errorCtx);
     if (v.string.context) {
-        if (pos)
-            throwEvalError(pos, "the string '%1%' is not allowed to refer to a store path (such as '%2%')",
-                v.string.s, v.string.context[0]);
-        else
-            throwEvalError("the string '%1%' is not allowed to refer to a store path (such as '%2%')",
-                v.string.s, v.string.context[0]);
+        error("the string '%1%' is not allowed to refer to a store path (such as '%2%')", v.string.s, v.string.context[0]).withTrace(pos, errorCtx).debugThrow<EvalError>();
     }
     return s;
 }
@@ -2229,14 +2144,15 @@ std::optional<std::string> EvalState::tryAttrsToString(const PosIdx pos, Value &
     if (i != v.attrs->end()) {
         Value v1;
         callFunction(*i->value, v, v1, pos);
-        return coerceToString(pos, v1, context, coerceMore, copyToStore).toOwned();
+        return coerceToString(pos, v1, context, coerceMore, copyToStore,
+                "while evaluating the result of the `toString` attribute").toOwned();
     }
 
     return {};
 }
 
 BackedStringView EvalState::coerceToString(const PosIdx pos, Value & v, PathSet & context,
-    bool coerceMore, bool copyToStore, bool canonicalizePath)
+    bool coerceMore, bool copyToStore, bool canonicalizePath, std::string_view errorCtx)
 {
     forceValue(v, pos);
 
@@ -2260,12 +2176,12 @@ BackedStringView EvalState::coerceToString(const PosIdx pos, Value & v, PathSet
             return std::move(*maybeString);
         auto i = v.attrs->find(sOutPath);
         if (i == v.attrs->end())
-            throwTypeError(pos, "cannot coerce a set to a string");
-        return coerceToString(pos, *i->value, context, coerceMore, copyToStore);
+            error("cannot coerce a set to a string", showType(v)).withTrace(pos, errorCtx).debugThrow<TypeError>();
+        return coerceToString(pos, *i->value, context, coerceMore, copyToStore, canonicalizePath, errorCtx);
     }
 
     if (v.type() == nExternal)
-        return v.external->coerceToString(positions[pos], context, coerceMore, copyToStore);
+        return v.external->coerceToString(positions[pos], context, coerceMore, copyToStore, errorCtx);
 
     if (coerceMore) {
         /* Note that `false' is represented as an empty string for
@@ -2279,7 +2195,13 @@ BackedStringView EvalState::coerceToString(const PosIdx pos, Value & v, PathSet
         if (v.isList()) {
             std::string result;
             for (auto [n, v2] : enumerate(v.listItems())) {
-                result += *coerceToString(pos, *v2, context, coerceMore, copyToStore);
+                try {
+                    result += *coerceToString(noPos, *v2, context, coerceMore, copyToStore, canonicalizePath,
+                            "while evaluating one element of the list");
+                } catch (Error & e) {
+                    e.addTrace(positions[pos], errorCtx);
+                    throw;
+                }
                 if (n < v.listSize() - 1
                     /* !!! not quite correct */
                     && (!v2->isList() || v2->listSize() != 0))
@@ -2289,14 +2211,14 @@ BackedStringView EvalState::coerceToString(const PosIdx pos, Value & v, PathSet
         }
     }
 
-    throwTypeError(pos, "cannot coerce %1% to a string", v);
+    error("cannot coerce %1% to a string", showType(v)).withTrace(pos, errorCtx).debugThrow<TypeError>();
 }
 
 
 std::string EvalState::copyPathToStore(PathSet & context, const Path & path)
 {
     if (nix::isDerivation(path))
-        throwEvalError("file names are not allowed to end in '%1%'", drvExtension);
+        error("file names are not allowed to end in '%1%'", drvExtension).debugThrow<EvalError>();
 
     Path dstPath;
     auto i = srcToStore.find(path);
@@ -2317,28 +2239,25 @@ std::string EvalState::copyPathToStore(PathSet & context, const Path & path)
 }
 
 
-Path EvalState::coerceToPath(const PosIdx pos, Value & v, PathSet & context)
+Path EvalState::coerceToPath(const PosIdx pos, Value & v, PathSet & context, std::string_view errorCtx)
 {
-    auto path = coerceToString(pos, v, context, false, false).toOwned();
+    auto path = coerceToString(pos, v, context, false, false, true, errorCtx).toOwned();
     if (path == "" || path[0] != '/')
-        throwEvalError(pos, "string '%1%' doesn't represent an absolute path", path);
+        error("string '%1%' doesn't represent an absolute path", path).withTrace(pos, errorCtx).debugThrow<EvalError>();
     return path;
 }
 
 
-StorePath EvalState::coerceToStorePath(const PosIdx pos, Value & v, PathSet & context)
+StorePath EvalState::coerceToStorePath(const PosIdx pos, Value & v, PathSet & context, std::string_view errorCtx)
 {
-    auto path = coerceToString(pos, v, context, false, false).toOwned();
+    auto path = coerceToString(pos, v, context, false, false, true, errorCtx).toOwned();
     if (auto storePath = store->maybeParseStorePath(path))
         return *storePath;
-    throw EvalError({
-        .msg = hintfmt("path '%1%' is not in the Nix store", path),
-        .errPos = positions[pos]
-    });
+    error("path '%1%' is not in the Nix store", path).withTrace(pos, errorCtx).debugThrow<EvalError>();
 }
 
 
-bool EvalState::eqValues(Value & v1, Value & v2)
+bool EvalState::eqValues(Value & v1, Value & v2, const PosIdx pos, std::string_view errorCtx)
 {
     forceValue(v1, noPos);
     forceValue(v2, noPos);
@@ -2358,7 +2277,6 @@ bool EvalState::eqValues(Value & v1, Value & v2)
     if (v1.type() != v2.type()) return false;
 
     switch (v1.type()) {
-
         case nInt:
             return v1.integer == v2.integer;
 
@@ -2377,7 +2295,7 @@ bool EvalState::eqValues(Value & v1, Value & v2)
         case nList:
             if (v1.listSize() != v2.listSize()) return false;
             for (size_t n = 0; n < v1.listSize(); ++n)
-                if (!eqValues(*v1.listElems()[n], *v2.listElems()[n])) return false;
+                if (!eqValues(*v1.listElems()[n], *v2.listElems()[n], pos, errorCtx)) return false;
             return true;
 
         case nAttrs: {
@@ -2387,7 +2305,7 @@ bool EvalState::eqValues(Value & v1, Value & v2)
                 Bindings::iterator i = v1.attrs->find(sOutPath);
                 Bindings::iterator j = v2.attrs->find(sOutPath);
                 if (i != v1.attrs->end() && j != v2.attrs->end())
-                    return eqValues(*i->value, *j->value);
+                    return eqValues(*i->value, *j->value, pos, errorCtx);
             }
 
             if (v1.attrs->size() != v2.attrs->size()) return false;
@@ -2395,7 +2313,7 @@ bool EvalState::eqValues(Value & v1, Value & v2)
             /* Otherwise, compare the attributes one by one. */
             Bindings::iterator i, j;
             for (i = v1.attrs->begin(), j = v2.attrs->begin(); i != v1.attrs->end(); ++i, ++j)
-                if (i->name != j->name || !eqValues(*i->value, *j->value))
+                if (i->name != j->name || !eqValues(*i->value, *j->value, pos, errorCtx))
                     return false;
 
             return true;
@@ -2412,9 +2330,7 @@ bool EvalState::eqValues(Value & v1, Value & v2)
             return v1.fpoint == v2.fpoint;
 
         default:
-            throwEvalError("cannot compare %1% with %2%",
-                showType(v1),
-                showType(v2));
+            error("cannot compare %1% with %2%", showType(v1), showType(v2)).withTrace(pos, errorCtx).debugThrow<EvalError>();
     }
 }
 
@@ -2538,12 +2454,13 @@ void EvalState::printStats()
 }
 
 
-std::string ExternalValueBase::coerceToString(const Pos & pos, PathSet & context, bool copyMore, bool copyToStore) const
+std::string ExternalValueBase::coerceToString(const Pos & pos, PathSet & context, bool copyMore, bool copyToStore, std::string_view errorCtx) const
 {
-    throw TypeError({
-        .msg = hintfmt("cannot coerce %1% to a string", showType()),
-        .errPos = pos
+    auto e = TypeError({
+        .msg = hintfmt("cannot coerce %1% to a string", showType())
     });
+    e.addTrace(pos, errorCtx);
+    throw e;
 }
 
 

src/libexpr/eval.hh

@@ -60,7 +60,6 @@ void copyContext(const Value & v, PathSet & context);
 typedef std::map<Path, StorePath> SrcToStore;
 
 
-std::ostream & printValue(const EvalState & state, std::ostream & str, const Value & v);
 std::string printValue(const EvalState & state, const Value & v);
 std::ostream & operator << (std::ostream & os, const ValueType t);
 
@@ -87,6 +86,43 @@ struct DebugTrace {
 
 void debugError(Error * e, Env & env, Expr & expr);
 
+class ErrorBuilder
+{
+    private:
+        EvalState & state;
+        ErrorInfo info;
+
+        ErrorBuilder(EvalState & s, ErrorInfo && i): state(s), info(i) { }
+
+    public:
+        template<typename... Args>
+        [[nodiscard, gnu::noinline]]
+        static ErrorBuilder * create(EvalState & s, const Args & ... args)
+        {
+            return new ErrorBuilder(s, ErrorInfo { .msg = hintfmt(args...) });
+        }
+
+        [[nodiscard, gnu::noinline]]
+        ErrorBuilder & atPos(PosIdx pos);
+
+        [[nodiscard, gnu::noinline]]
+        ErrorBuilder & withTrace(PosIdx pos, const std::string_view text);
+
+        [[nodiscard, gnu::noinline]]
+        ErrorBuilder & withFrameTrace(PosIdx pos, const std::string_view text);
+
+        [[nodiscard, gnu::noinline]]
+        ErrorBuilder & withSuggestions(Suggestions & s);
+
+        [[nodiscard, gnu::noinline]]
+        ErrorBuilder & withFrame(const Env & e, const Expr & ex);
+
+        template<class ErrorType>
+        [[gnu::noinline, gnu::noreturn]]
+        void debugThrow();
+};
+
+
 class EvalState : public std::enable_shared_from_this<EvalState>
 {
 public:
@@ -146,29 +182,35 @@ public:
 
     template<class E>
     [[gnu::noinline, gnu::noreturn]]
-    void debugThrow(E && error, const Env & env, const Expr & expr)
+    void debugThrowLastTrace(E && error)
     {
-        if (debugRepl)
-            runDebugRepl(&error, env, expr);
-
-        throw std::move(error);
+        debugThrow(error, nullptr, nullptr);
     }
 
     template<class E>
     [[gnu::noinline, gnu::noreturn]]
-    void debugThrowLastTrace(E && e)
+    void debugThrow(E && error, const Env * env, const Expr * expr)
     {
-        // Call this in the situation where Expr and Env are inaccessible.
-        // The debugger will start in the last context that's in the
-        // DebugTrace stack.
-        if (debugRepl && !debugTraces.empty()) {
+        if (debugRepl && ((env && expr) || !debugTraces.empty())) {
+            if (!env || !expr) {
                 const DebugTrace & last = debugTraces.front();
-            runDebugRepl(&e, last.env, last.expr);
+                env = &last.env;
+                expr = &last.expr;
+            }
+            runDebugRepl(&error, *env, *expr);
         }
 
-        throw std::move(e);
+        throw std::move(error);
     }
 
+    ErrorBuilder * errorBuilder;
+
+    template<typename... Args>
+    [[nodiscard, gnu::noinline]]
+    ErrorBuilder & error(const Args & ... args) {
+        errorBuilder = ErrorBuilder::create(*this, args...);
+        return *errorBuilder;
+    }
 
 private:
     SrcToStore srcToStore;
@@ -283,8 +325,8 @@ public:
     /* Evaluation the expression, then verify that it has the expected
        type. */
     inline bool evalBool(Env & env, Expr * e);
-    inline bool evalBool(Env & env, Expr * e, const PosIdx pos);
-    inline void evalAttrs(Env & env, Expr * e, Value & v);
+    inline bool evalBool(Env & env, Expr * e, const PosIdx pos, std::string_view errorCtx);
+    inline void evalAttrs(Env & env, Expr * e, Value & v, const PosIdx pos, std::string_view errorCtx);
 
     /* If `v' is a thunk, enter it and overwrite `v' with the result
        of the evaluation of the thunk.  If `v' is a delayed function
@@ -300,89 +342,25 @@ public:
     void forceValueDeep(Value & v);
 
     /* Force `v', and then verify that it has the expected type. */
-    NixInt forceInt(Value & v, const PosIdx pos);
-    NixFloat forceFloat(Value & v, const PosIdx pos);
-    bool forceBool(Value & v, const PosIdx pos);
+    NixInt forceInt(Value & v, const PosIdx pos, std::string_view errorCtx);
+    NixFloat forceFloat(Value & v, const PosIdx pos, std::string_view errorCtx);
+    bool forceBool(Value & v, const PosIdx pos, std::string_view errorCtx);
 
-    void forceAttrs(Value & v, const PosIdx pos);
+    void forceAttrs(Value & v, const PosIdx pos, std::string_view errorCtx);
 
     template <typename Callable>
-    inline void forceAttrs(Value & v, Callable getPos);
+    inline void forceAttrs(Value & v, Callable getPos, std::string_view errorCtx);
 
-    inline void forceList(Value & v, const PosIdx pos);
-    void forceFunction(Value & v, const PosIdx pos); // either lambda or primop
-    std::string_view forceString(Value & v, const PosIdx pos = noPos);
-    std::string_view forceString(Value & v, PathSet & context, const PosIdx pos = noPos);
-    std::string_view forceStringNoCtx(Value & v, const PosIdx pos = noPos);
-
-    [[gnu::noinline, gnu::noreturn]]
-    void throwEvalError(const PosIdx pos, const char * s);
-    [[gnu::noinline, gnu::noreturn]]
-    void throwEvalError(const PosIdx pos, const char * s,
-        Env & env, Expr & expr);
-    [[gnu::noinline, gnu::noreturn]]
-    void throwEvalError(const char * s, const std::string & s2);
-    [[gnu::noinline, gnu::noreturn]]
-    void throwEvalError(const PosIdx pos, const char * s, const std::string & s2);
-    [[gnu::noinline, gnu::noreturn]]
-    void throwEvalError(const char * s, const std::string & s2,
-        Env & env, Expr & expr);
-    [[gnu::noinline, gnu::noreturn]]
-    void throwEvalError(const PosIdx pos, const char * s, const std::string & s2,
-        Env & env, Expr & expr);
-    [[gnu::noinline, gnu::noreturn]]
-    void throwEvalError(const char * s, const std::string & s2, const std::string & s3,
-        Env & env, Expr & expr);
-    [[gnu::noinline, gnu::noreturn]]
-    void throwEvalError(const PosIdx pos, const char * s, const std::string & s2, const std::string & s3,
-        Env & env, Expr & expr);
-    [[gnu::noinline, gnu::noreturn]]
-    void throwEvalError(const PosIdx pos, const char * s, const std::string & s2, const std::string & s3);
-    [[gnu::noinline, gnu::noreturn]]
-    void throwEvalError(const char * s, const std::string & s2, const std::string & s3);
-    [[gnu::noinline, gnu::noreturn]]
-    void throwEvalError(const PosIdx pos, const Suggestions & suggestions, const char * s, const std::string & s2,
-        Env & env, Expr & expr);
-    [[gnu::noinline, gnu::noreturn]]
-    void throwEvalError(const PosIdx p1, const char * s, const Symbol sym, const PosIdx p2,
-        Env & env, Expr & expr);
-
-    [[gnu::noinline, gnu::noreturn]]
-    void throwTypeError(const PosIdx pos, const char * s, const Value & v);
-    [[gnu::noinline, gnu::noreturn]]
-    void throwTypeError(const PosIdx pos, const char * s, const Value & v,
-        Env & env, Expr & expr);
-    [[gnu::noinline, gnu::noreturn]]
-    void throwTypeError(const PosIdx pos, const char * s);
-    [[gnu::noinline, gnu::noreturn]]
-    void throwTypeError(const PosIdx pos, const char * s,
-        Env & env, Expr & expr);
-    [[gnu::noinline, gnu::noreturn]]
-    void throwTypeError(const PosIdx pos, const char * s, const ExprLambda & fun, const Symbol s2,
-        Env & env, Expr & expr);
-    [[gnu::noinline, gnu::noreturn]]
-    void throwTypeError(const PosIdx pos, const Suggestions & suggestions, const char * s, const ExprLambda & fun, const Symbol s2,
-        Env & env, Expr & expr);
-    [[gnu::noinline, gnu::noreturn]]
-    void throwTypeError(const char * s, const Value & v,
-        Env & env, Expr & expr);
-
-    [[gnu::noinline, gnu::noreturn]]
-    void throwAssertionError(const PosIdx pos, const char * s, const std::string & s1,
-        Env & env, Expr & expr);
-
-    [[gnu::noinline, gnu::noreturn]]
-    void throwUndefinedVarError(const PosIdx pos, const char * s, const std::string & s1,
-        Env & env, Expr & expr);
-
-    [[gnu::noinline, gnu::noreturn]]
-    void throwMissingArgumentError(const PosIdx pos, const char * s, const std::string & s1,
-        Env & env, Expr & expr);
+    inline void forceList(Value & v, const PosIdx pos, std::string_view errorCtx);
+    void forceFunction(Value & v, const PosIdx pos, std::string_view errorCtx); // either lambda or primop
+    std::string_view forceString(Value & v, const PosIdx pos, std::string_view errorCtx);
+    std::string_view forceString(Value & v, PathSet & context, const PosIdx pos, std::string_view errorCtx);
+    std::string_view forceStringNoCtx(Value & v, const PosIdx pos, std::string_view errorCtx);
 
     [[gnu::noinline]]
     void addErrorTrace(Error & e, const char * s, const std::string & s2) const;
     [[gnu::noinline]]
-    void addErrorTrace(Error & e, const PosIdx pos, const char * s, const std::string & s2) const;
+    void addErrorTrace(Error & e, const PosIdx pos, const char * s, const std::string & s2, bool frame = false) const;
 
 public:
     /* Return true iff the value `v' denotes a derivation (i.e. a
@@ -398,17 +376,18 @@ public:
        referenced paths are copied to the Nix store as a side effect. */
     BackedStringView coerceToString(const PosIdx pos, Value & v, PathSet & context,
         bool coerceMore = false, bool copyToStore = true,
-        bool canonicalizePath = true);
+        bool canonicalizePath = true,
+        std::string_view errorCtx = "");
 
     std::string copyPathToStore(PathSet & context, const Path & path);
 
     /* Path coercion.  Converts strings, paths and derivations to a
        path.  The result is guaranteed to be a canonicalised, absolute
        path.  Nothing is copied to the store. */
-    Path coerceToPath(const PosIdx pos, Value & v, PathSet & context);
+    Path coerceToPath(const PosIdx pos, Value & v, PathSet & context, std::string_view errorCtx);
 
     /* Like coerceToPath, but the result must be a store path. */
-    StorePath coerceToStorePath(const PosIdx pos, Value & v, PathSet & context);
+    StorePath coerceToStorePath(const PosIdx pos, Value & v, PathSet & context, std::string_view errorCtx);
 
 public:
 
@@ -468,7 +447,7 @@ public:
 
     /* Do a deep equality test between two values.  That is, list
        elements and attributes are compared recursively. */
-    bool eqValues(Value & v1, Value & v2);
+    bool eqValues(Value & v1, Value & v2, const PosIdx pos, std::string_view errorCtx);
 
     bool isFunctor(Value & fun);
 
@@ -503,7 +482,7 @@ public:
     void mkThunk_(Value & v, Expr * expr);
     void mkPos(Value & v, PosIdx pos);
 
-    void concatLists(Value & v, size_t nrLists, Value * * lists, const PosIdx pos);
+    void concatLists(Value & v, size_t nrLists, Value * * lists, const PosIdx pos, std::string_view errorCtx);
 
     /* Print statistics. */
     void printStats();
@@ -670,6 +649,13 @@ extern EvalSettings evalSettings;
 
 static const std::string corepkgsPrefix{"/__corepkgs__/"};
 
+template<class ErrorType>
+void ErrorBuilder::debugThrow()
+{
+    // NOTE: We always use the -LastTrace version as we push the new trace in withFrame()
+    state.debugThrowLastTrace(ErrorType(info));
+}
+
 }
 
 #include "eval-inline.hh"

src/libexpr/flake/flake.cc

@@ -259,28 +259,28 @@ static Flake getFlake(
             if (setting.value->type() == nString)
                 flake.config.settings.emplace(
                     state.symbols[setting.name],
-                    std::string(state.forceStringNoCtx(*setting.value, setting.pos)));
+                    std::string(state.forceStringNoCtx(*setting.value, setting.pos, "")));
             else if (setting.value->type() == nPath) {
                 PathSet emptyContext = {};
                 flake.config.settings.emplace(
                     state.symbols[setting.name],
-                    state.coerceToString(setting.pos, *setting.value, emptyContext, false, true, true) .toOwned());
+                    state.coerceToString(setting.pos, *setting.value, emptyContext, false, true, true, "") .toOwned());
             }
             else if (setting.value->type() == nInt)
                 flake.config.settings.emplace(
                     state.symbols[setting.name],
-                    state.forceInt(*setting.value, setting.pos));
+                    state.forceInt(*setting.value, setting.pos, ""));
             else if (setting.value->type() == nBool)
                 flake.config.settings.emplace(
                     state.symbols[setting.name],
-                    Explicit<bool> { state.forceBool(*setting.value, setting.pos) });
+                    Explicit<bool> { state.forceBool(*setting.value, setting.pos, "") });
             else if (setting.value->type() == nList) {
                 std::vector<std::string> ss;
                 for (auto elem : setting.value->listItems()) {
                     if (elem->type() != nString)
                         throw TypeError("list element in flake configuration setting '%s' is %s while a string is expected",
                             state.symbols[setting.name], showType(*setting.value));
-                    ss.emplace_back(state.forceStringNoCtx(*elem, setting.pos));
+                    ss.emplace_back(state.forceStringNoCtx(*elem, setting.pos, ""));
                 }
                 flake.config.settings.emplace(state.symbols[setting.name], ss);
             }
@@ -741,7 +741,7 @@ void callFlake(EvalState & state,
 
 static void prim_getFlake(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
-    std::string flakeRefS(state.forceStringNoCtx(*args[0], pos));
+    std::string flakeRefS(state.forceStringNoCtx(*args[0], pos, "while evaluating the argument passed to builtins.getFlake"));
     auto flakeRef = parseFlakeRef(flakeRefS, {}, true);
     if (evalSettings.pureEval && !flakeRef.input.isLocked())
         throw Error("cannot call 'getFlake' on unlocked flake reference '%s', at %s (use --impure to override)", flakeRefS, state.positions[pos]);

src/libexpr/get-drvs.cc

@@ -51,7 +51,7 @@ std::string DrvInfo::queryName() const
     if (name == "" && attrs) {
         auto i = attrs->find(state->sName);
         if (i == attrs->end()) throw TypeError("derivation name missing");
-        name = state->forceStringNoCtx(*i->value);
+        name = state->forceStringNoCtx(*i->value, noPos, "while evaluating the 'name' attribute of a derivation");
     }
     return name;
 }
@@ -61,7 +61,7 @@ std::string DrvInfo::querySystem() const
 {
     if (system == "" && attrs) {
         auto i = attrs->find(state->sSystem);
-        system = i == attrs->end() ? "unknown" : state->forceStringNoCtx(*i->value, i->pos);
+        system = i == attrs->end() ? "unknown" : state->forceStringNoCtx(*i->value, i->pos, "while evaluating the 'system' attribute of a derivation");
     }
     return system;
 }
@@ -75,7 +75,7 @@ std::optional<StorePath> DrvInfo::queryDrvPath() const
         if (i == attrs->end())
             drvPath = {std::nullopt};
         else
-            drvPath = {state->coerceToStorePath(i->pos, *i->value, context)};
+            drvPath = {state->coerceToStorePath(i->pos, *i->value, context, "while evaluating the 'drvPath' attribute of a derivation")};
     }
     return drvPath.value_or(std::nullopt);
 }
@@ -95,7 +95,7 @@ StorePath DrvInfo::queryOutPath() const
         Bindings::iterator i = attrs->find(state->sOutPath);
         PathSet context;
         if (i != attrs->end())
-            outPath = state->coerceToStorePath(i->pos, *i->value, context);
+            outPath = state->coerceToStorePath(i->pos, *i->value, context, "while evaluating the output path of a derivation");
     }
     if (!outPath)
         throw UnimplementedError("CA derivations are not yet supported");
@@ -109,23 +109,23 @@ DrvInfo::Outputs DrvInfo::queryOutputs(bool withPaths, bool onlyOutputsToInstall
         /* Get the ‘outputs’ list. */
         Bindings::iterator i;
         if (attrs && (i = attrs->find(state->sOutputs)) != attrs->end()) {
-            state->forceList(*i->value, i->pos);
+            state->forceList(*i->value, i->pos, "while evaluating the 'outputs' attribute of a derivation");
 
             /* For each output... */
             for (auto elem : i->value->listItems()) {
-                std::string output(state->forceStringNoCtx(*elem, i->pos));
+                std::string output(state->forceStringNoCtx(*elem, i->pos, "while evaluating the name of an output of a derivation"));
 
                 if (withPaths) {
                     /* Evaluate the corresponding set. */
                     Bindings::iterator out = attrs->find(state->symbols.create(output));
                     if (out == attrs->end()) continue; // FIXME: throw error?
-                    state->forceAttrs(*out->value, i->pos);
+                    state->forceAttrs(*out->value, i->pos, "while evaluating an output of a derivation");
 
                     /* And evaluate its ‘outPath’ attribute. */
                     Bindings::iterator outPath = out->value->attrs->find(state->sOutPath);
                     if (outPath == out->value->attrs->end()) continue; // FIXME: throw error?
                     PathSet context;
-                    outputs.emplace(output, state->coerceToStorePath(outPath->pos, *outPath->value, context));
+                    outputs.emplace(output, state->coerceToStorePath(outPath->pos, *outPath->value, context, "while evaluating an output path of a derivation"));
                 } else
                     outputs.emplace(output, std::nullopt);
             }
@@ -137,7 +137,7 @@ DrvInfo::Outputs DrvInfo::queryOutputs(bool withPaths, bool onlyOutputsToInstall
         return outputs;
 
     Bindings::iterator i;
-    if (attrs && (i = attrs->find(state->sOutputSpecified)) != attrs->end() && state->forceBool(*i->value, i->pos)) {
+    if (attrs && (i = attrs->find(state->sOutputSpecified)) != attrs->end() && state->forceBool(*i->value, i->pos, "while evaluating the 'outputSpecified' attribute of a derivation")) {
         Outputs result;
         auto out = outputs.find(queryOutputName());
         if (out == outputs.end())
@@ -169,7 +169,7 @@ std::string DrvInfo::queryOutputName() const
 {
     if (outputName == "" && attrs) {
         Bindings::iterator i = attrs->find(state->sOutputName);
-        outputName = i != attrs->end() ? state->forceStringNoCtx(*i->value) : "";
+        outputName = i != attrs->end() ? state->forceStringNoCtx(*i->value, noPos, "while evaluating the output name of a derivation") : "";
     }
     return outputName;
 }
@@ -181,7 +181,7 @@ Bindings * DrvInfo::getMeta()
     if (!attrs) return 0;
     Bindings::iterator a = attrs->find(state->sMeta);
     if (a == attrs->end()) return 0;
-    state->forceAttrs(*a->value, a->pos);
+    state->forceAttrs(*a->value, a->pos, "while evaluating the 'meta' attribute of a derivation");
     meta = a->value->attrs;
     return meta;
 }
@@ -382,7 +382,7 @@ static void getDerivations(EvalState & state, Value & vIn,
                    `recurseForDerivations = true' attribute. */
                 if (i->value->type() == nAttrs) {
                     Bindings::iterator j = i->value->attrs->find(state.sRecurseForDerivations);
-                    if (j != i->value->attrs->end() && state.forceBool(*j->value, j->pos))
+                    if (j != i->value->attrs->end() && state.forceBool(*j->value, j->pos, "while evaluating the attribute `recurseForDerivations`"))
                         getDerivations(state, *i->value, pathPrefix2, autoArgs, drvs, done, ignoreAssertionFailures);
                 }
             }

src/libexpr/nixexpr.hh

@@ -8,7 +8,6 @@
 #include "error.hh"
 #include "chunked-vector.hh"
 
-
 namespace nix {
 
 

src/libexpr/parser.y

@@ -401,22 +401,22 @@ expr_op
   | '-' expr_op %prec NEGATE { $$ = new ExprCall(CUR_POS, new ExprVar(data->symbols.create("__sub")), {new ExprInt(0), $2}); }
   | expr_op EQ expr_op { $$ = new ExprOpEq($1, $3); }
   | expr_op NEQ expr_op { $$ = new ExprOpNEq($1, $3); }
-  | expr_op '<' expr_op { $$ = new ExprCall(CUR_POS, new ExprVar(data->symbols.create("__lessThan")), {$1, $3}); }
-  | expr_op LEQ expr_op { $$ = new ExprOpNot(new ExprCall(CUR_POS, new ExprVar(data->symbols.create("__lessThan")), {$3, $1})); }
-  | expr_op '>' expr_op { $$ = new ExprCall(CUR_POS, new ExprVar(data->symbols.create("__lessThan")), {$3, $1}); }
-  | expr_op GEQ expr_op { $$ = new ExprOpNot(new ExprCall(CUR_POS, new ExprVar(data->symbols.create("__lessThan")), {$1, $3})); }
-  | expr_op AND expr_op { $$ = new ExprOpAnd(CUR_POS, $1, $3); }
-  | expr_op OR expr_op { $$ = new ExprOpOr(CUR_POS, $1, $3); }
-  | expr_op IMPL expr_op { $$ = new ExprOpImpl(CUR_POS, $1, $3); }
-  | expr_op UPDATE expr_op { $$ = new ExprOpUpdate(CUR_POS, $1, $3); }
+  | expr_op '<' expr_op { $$ = new ExprCall(makeCurPos(@2, data), new ExprVar(data->symbols.create("__lessThan")), {$1, $3}); }
+  | expr_op LEQ expr_op { $$ = new ExprOpNot(new ExprCall(makeCurPos(@2, data), new ExprVar(data->symbols.create("__lessThan")), {$3, $1})); }
+  | expr_op '>' expr_op { $$ = new ExprCall(makeCurPos(@2, data), new ExprVar(data->symbols.create("__lessThan")), {$3, $1}); }
+  | expr_op GEQ expr_op { $$ = new ExprOpNot(new ExprCall(makeCurPos(@2, data), new ExprVar(data->symbols.create("__lessThan")), {$1, $3})); }
+  | expr_op AND expr_op { $$ = new ExprOpAnd(makeCurPos(@2, data), $1, $3); }
+  | expr_op OR expr_op { $$ = new ExprOpOr(makeCurPos(@2, data), $1, $3); }
+  | expr_op IMPL expr_op { $$ = new ExprOpImpl(makeCurPos(@2, data), $1, $3); }
+  | expr_op UPDATE expr_op { $$ = new ExprOpUpdate(makeCurPos(@2, data), $1, $3); }
   | expr_op '?' attrpath { $$ = new ExprOpHasAttr($1, *$3); }
   | expr_op '+' expr_op
-    { $$ = new ExprConcatStrings(CUR_POS, false, new std::vector<std::pair<PosIdx, Expr *>>({{makeCurPos(@1, data), $1}, {makeCurPos(@3, data), $3}})); }
-  | expr_op '-' expr_op { $$ = new ExprCall(CUR_POS, new ExprVar(data->symbols.create("__sub")), {$1, $3}); }
-  | expr_op '*' expr_op { $$ = new ExprCall(CUR_POS, new ExprVar(data->symbols.create("__mul")), {$1, $3}); }
-  | expr_op '/' expr_op { $$ = new ExprCall(CUR_POS, new ExprVar(data->symbols.create("__div")), {$1, $3}); }
-  | expr_op CONCAT expr_op { $$ = new ExprOpConcatLists(CUR_POS, $1, $3); }
-  | expr_op '$' expr_op { $$ = new ExprCall(CUR_POS, $1, {$3}); }
+    { $$ = new ExprConcatStrings(makeCurPos(@2, data), false, new std::vector<std::pair<PosIdx, Expr *> >({{makeCurPos(@1, data), $1}, {makeCurPos(@3, data), $3}})); }
+  | expr_op '-' expr_op { $$ = new ExprCall(makeCurPos(@2, data), new ExprVar(data->symbols.create("__sub")), {$1, $3}); }
+  | expr_op '*' expr_op { $$ = new ExprCall(makeCurPos(@2, data), new ExprVar(data->symbols.create("__mul")), {$1, $3}); }
+  | expr_op '/' expr_op { $$ = new ExprCall(makeCurPos(@2, data), new ExprVar(data->symbols.create("__div")), {$1, $3}); }
+  | expr_op CONCAT expr_op { $$ = new ExprOpConcatLists(makeCurPos(@2, data), $1, $3); }
+  | expr_op '$' expr_op { $$ = new ExprCall(makeCurPos(@2, data), $1, {$3}); }
   | expr_app
   ;
 
@@ -784,13 +784,13 @@ Path EvalState::findFile(SearchPath & searchPath, const std::string_view path, c
     if (hasPrefix(path, "nix/"))
         return concatStrings(corepkgsPrefix, path.substr(4));
 
-    debugThrowLastTrace(ThrownError({
+    debugThrow(ThrownError({
         .msg = hintfmt(evalSettings.pureEval
             ? "cannot look up '<%s>' in pure evaluation mode (use '--impure' to override)"
             : "file '%s' was not found in the Nix search path (add it using $NIX_PATH or -I)",
             path),
         .errPos = positions[pos]
-    }));
+    }), 0, 0);
 }
 
 

src/libexpr/primops/context.cc

@@ -8,7 +8,7 @@ namespace nix {
 static void prim_unsafeDiscardStringContext(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
     PathSet context;
-    auto s = state.coerceToString(pos, *args[0], context);
+    auto s = state.coerceToString(pos, *args[0], context, "while evaluating the argument passed to builtins.unsafeDiscardStringContext");
     v.mkString(*s);
 }
 
@@ -18,7 +18,7 @@ static RegisterPrimOp primop_unsafeDiscardStringContext("__unsafeDiscardStringCo
 static void prim_hasContext(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
     PathSet context;
-    state.forceString(*args[0], context, pos);
+    state.forceString(*args[0], context, pos, "while evaluating the argument passed to builtins.hasContext");
     v.mkBool(!context.empty());
 }
 
@@ -34,7 +34,7 @@ static RegisterPrimOp primop_hasContext("__hasContext", 1, prim_hasContext);
 static void prim_unsafeDiscardOutputDependency(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
     PathSet context;
-    auto s = state.coerceToString(pos, *args[0], context);
+    auto s = state.coerceToString(pos, *args[0], context, "while evaluating the argument passed to builtins.unsafeDiscardOutputDependency");
 
     PathSet context2;
     for (auto & p : context)
@@ -73,7 +73,7 @@ static void prim_getContext(EvalState & state, const PosIdx pos, Value * * args,
         Strings outputs;
     };
     PathSet context;
-    state.forceString(*args[0], context, pos);
+    state.forceString(*args[0], context, pos, "while evaluating the argument passed to builtins.getContext");
     auto contextInfos = std::map<Path, ContextInfo>();
     for (const auto & p : context) {
         Path drv;
@@ -137,9 +137,9 @@ static RegisterPrimOp primop_getContext("__getContext", 1, prim_getContext);
 static void prim_appendContext(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
     PathSet context;
-    auto orig = state.forceString(*args[0], context, pos);
+    auto orig = state.forceString(*args[0], context, noPos, "while evaluating the first argument passed to builtins.appendContext");
 
-    state.forceAttrs(*args[1], pos);
+    state.forceAttrs(*args[1], pos, "while evaluating the second argument passed to builtins.appendContext");
 
     auto sPath = state.symbols.create("path");
     auto sAllOutputs = state.symbols.create("allOutputs");
@@ -147,24 +147,24 @@ static void prim_appendContext(EvalState & state, const PosIdx pos, Value * * ar
         const auto & name = state.symbols[i.name];
         if (!state.store->isStorePath(name))
             throw EvalError({
-                .msg = hintfmt("Context key '%s' is not a store path", name),
+                .msg = hintfmt("context key '%s' is not a store path", name),
                 .errPos = state.positions[i.pos]
             });
         if (!settings.readOnlyMode)
             state.store->ensurePath(state.store->parseStorePath(name));
-        state.forceAttrs(*i.value, i.pos);
+        state.forceAttrs(*i.value, i.pos, "while evaluating the value of a string context");
         auto iter = i.value->attrs->find(sPath);
         if (iter != i.value->attrs->end()) {
-            if (state.forceBool(*iter->value, iter->pos))
+            if (state.forceBool(*iter->value, iter->pos, "while evaluating the `path` attribute of a string context"))
                 context.emplace(name);
         }
 
         iter = i.value->attrs->find(sAllOutputs);
         if (iter != i.value->attrs->end()) {
-            if (state.forceBool(*iter->value, iter->pos)) {
+            if (state.forceBool(*iter->value, iter->pos, "while evaluating the `allOutputs` attribute of a string context")) {
                 if (!isDerivation(name)) {
                     throw EvalError({
-                        .msg = hintfmt("Tried to add all-outputs context of %s, which is not a derivation, to a string", name),
+                        .msg = hintfmt("tried to add all-outputs context of %s, which is not a derivation, to a string", name),
                         .errPos = state.positions[i.pos]
                     });
                 }
@@ -174,15 +174,15 @@ static void prim_appendContext(EvalState & state, const PosIdx pos, Value * * ar
 
         iter = i.value->attrs->find(state.sOutputs);
         if (iter != i.value->attrs->end()) {
-            state.forceList(*iter->value, iter->pos);
+            state.forceList(*iter->value, iter->pos, "while evaluating the `outputs` attribute of a string context");
             if (iter->value->listSize() && !isDerivation(name)) {
                 throw EvalError({
-                    .msg = hintfmt("Tried to add derivation output context of %s, which is not a derivation, to a string", name),
+                    .msg = hintfmt("tried to add derivation output context of %s, which is not a derivation, to a string", name),
                     .errPos = state.positions[i.pos]
                 });
             }
             for (auto elem : iter->value->listItems()) {
-                auto outputName = state.forceStringNoCtx(*elem, iter->pos);
+                auto outputName = state.forceStringNoCtx(*elem, iter->pos, "while evaluating an output name within a string context");
                 context.insert(concatStrings("!", outputName, "!", name));
             }
         }

src/libexpr/primops/fetchClosure.cc

@@ -7,7 +7,7 @@ namespace nix {
 
 static void prim_fetchClosure(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
-    state.forceAttrs(*args[0], pos);
+    state.forceAttrs(*args[0], pos, "while evaluating the argument passed to builtins.fetchClosure");
 
     std::optional<std::string> fromStoreUrl;
     std::optional<StorePath> fromPath;
@@ -19,7 +19,8 @@ static void prim_fetchClosure(EvalState & state, const PosIdx pos, Value * * arg
 
         if (attrName == "fromPath") {
             PathSet context;
-            fromPath = state.coerceToStorePath(attr.pos, *attr.value, context);
+            fromPath = state.coerceToStorePath(attr.pos, *attr.value, context,
+                    "while evaluating the 'fromPath' attribute passed to builtins.fetchClosure");
         }
 
         else if (attrName == "toPath") {
@@ -27,12 +28,14 @@ static void prim_fetchClosure(EvalState & state, const PosIdx pos, Value * * arg
             toCA = true;
             if (attr.value->type() != nString || attr.value->string.s != std::string("")) {
                 PathSet context;
-                toPath = state.coerceToStorePath(attr.pos, *attr.value, context);
+                toPath = state.coerceToStorePath(attr.pos, *attr.value, context,
+                        "while evaluating the 'toPath' attribute passed to builtins.fetchClosure");
             }
         }
 
         else if (attrName == "fromStore")
-            fromStoreUrl = state.forceStringNoCtx(*attr.value, attr.pos);
+            fromStoreUrl = state.forceStringNoCtx(*attr.value, attr.pos,
+                    "while evaluating the 'fromStore' attribute passed to builtins.fetchClosure");
 
         else
             throw Error({

src/libexpr/primops/fetchMercurial.cc

@@ -19,23 +19,21 @@ static void prim_fetchMercurial(EvalState & state, const PosIdx pos, Value * * a
 
     if (args[0]->type() == nAttrs) {
 
-        state.forceAttrs(*args[0], pos);
-
         for (auto & attr : *args[0]->attrs) {
             std::string_view n(state.symbols[attr.name]);
             if (n == "url")
-                url = state.coerceToString(attr.pos, *attr.value, context, false, false).toOwned();
+                url = state.coerceToString(attr.pos, *attr.value, context, false, false, "while evaluating the `url` attribute passed to builtins.fetchMercurial").toOwned();
             else if (n == "rev") {
                 // Ugly: unlike fetchGit, here the "rev" attribute can
                 // be both a revision or a branch/tag name.
-                auto value = state.forceStringNoCtx(*attr.value, attr.pos);
+                auto value = state.forceStringNoCtx(*attr.value, attr.pos, "while evaluating the `rev` attribute passed to builtins.fetchMercurial");
                 if (std::regex_match(value.begin(), value.end(), revRegex))
                     rev = Hash::parseAny(value, htSHA1);
                 else
                     ref = value;
             }
             else if (n == "name")
-                name = state.forceStringNoCtx(*attr.value, attr.pos);
+                name = state.forceStringNoCtx(*attr.value, attr.pos, "while evaluating the `name` attribute passed to builtins.fetchMercurial");
             else
                 throw EvalError({
                     .msg = hintfmt("unsupported argument '%s' to 'fetchMercurial'", state.symbols[attr.name]),
@@ -50,7 +48,7 @@ static void prim_fetchMercurial(EvalState & state, const PosIdx pos, Value * * a
             });
 
     } else
-        url = state.coerceToString(pos, *args[0], context, false, false).toOwned();
+        url = state.coerceToString(pos, *args[0], context, false, false, "while evaluating the first argument passed to builtins.fetchMercurial").toOwned();
 
     // FIXME: git externals probably can be used to bypass the URI
     // whitelist. Ah well.

src/libexpr/primops/fetchTree.cc

@@ -102,7 +102,7 @@ static void fetchTree(
     state.forceValue(*args[0], pos);
 
     if (args[0]->type() == nAttrs) {
-        state.forceAttrs(*args[0], pos);
+        state.forceAttrs(*args[0], pos, "while evaluating the argument passed to builtins.fetchTree");
 
         fetchers::Attrs attrs;
 
@@ -112,7 +112,7 @@ static void fetchTree(
                     .msg = hintfmt("unexpected attribute 'type'"),
                     .errPos = state.positions[pos]
                 }));
-            type = state.forceStringNoCtx(*aType->value, aType->pos);
+            type = state.forceStringNoCtx(*aType->value, aType->pos, "while evaluating the `type` attribute passed to builtins.fetchTree");
         } else if (!type)
             state.debugThrowLastTrace(EvalError({
                 .msg = hintfmt("attribute 'type' is missing in call to 'fetchTree'"),
@@ -125,7 +125,7 @@ static void fetchTree(
             if (attr.name == state.sType) continue;
             state.forceValue(*attr.value, attr.pos);
             if (attr.value->type() == nPath || attr.value->type() == nString) {
-                auto s = state.coerceToString(attr.pos, *attr.value, context, false, false).toOwned();
+                auto s = state.coerceToString(attr.pos, *attr.value, context, false, false, "").toOwned();
                 attrs.emplace(state.symbols[attr.name],
                     state.symbols[attr.name] == "url"
                     ? type == "git"
@@ -151,7 +151,7 @@ static void fetchTree(
 
         input = fetchers::Input::fromAttrs(std::move(attrs));
     } else {
-        auto url = state.coerceToString(pos, *args[0], context, false, false).toOwned();
+        auto url = state.coerceToString(pos, *args[0], context, false, false, "while evaluating the first argument passed to the fetcher").toOwned();
 
         if (type == "git") {
             fetchers::Attrs attrs;
@@ -195,16 +195,14 @@ static void fetch(EvalState & state, const PosIdx pos, Value * * args, Value & v
 
     if (args[0]->type() == nAttrs) {
 
-        state.forceAttrs(*args[0], pos);
-
         for (auto & attr : *args[0]->attrs) {
             std::string_view n(state.symbols[attr.name]);
             if (n == "url")
-                url = state.forceStringNoCtx(*attr.value, attr.pos);
+                url = state.forceStringNoCtx(*attr.value, attr.pos, "while evaluating the url we should fetch");
             else if (n == "sha256")
-                expectedHash = newHashAllowEmpty(state.forceStringNoCtx(*attr.value, attr.pos), htSHA256);
+                expectedHash = newHashAllowEmpty(state.forceStringNoCtx(*attr.value, attr.pos, "while evaluating the sha256 of the content we should fetch"), htSHA256);
             else if (n == "name")
-                name = state.forceStringNoCtx(*attr.value, attr.pos);
+                name = state.forceStringNoCtx(*attr.value, attr.pos, "while evaluating the name of the content we should fetch");
             else
                 state.debugThrowLastTrace(EvalError({
                     .msg = hintfmt("unsupported argument '%s' to '%s'", n, who),
@@ -218,7 +216,7 @@ static void fetch(EvalState & state, const PosIdx pos, Value * * args, Value & v
                 .errPos = state.positions[pos]
             }));
     } else
-        url = state.forceStringNoCtx(*args[0], pos);
+        url = state.forceStringNoCtx(*args[0], pos, "while evaluating the url we should fetch");
 
     state.checkURI(*url);
 

src/libexpr/primops/fromTOML.cc

@@ -7,7 +7,7 @@ namespace nix {
 
 static void prim_fromTOML(EvalState & state, const PosIdx pos, Value * * args, Value & val)
 {
-    auto toml = state.forceStringNoCtx(*args[0], pos);
+    auto toml = state.forceStringNoCtx(*args[0], pos, "while evaluating the argument passed to builtins.fromTOML");
 
     std::istringstream tomlStream(std::string{toml});
 

src/libexpr/tests/error_traces.cc

@@ -0,0 +1,94 @@
+#include <gmock/gmock.h>
+#include <gtest/gtest.h>
+
+#include "libexprtests.hh"
+
+namespace nix {
+
+    using namespace testing;
+
+    // Testing eval of PrimOp's
+    class ErrorTraceTest : public LibExprTest { };
+
+#define ASSERT_TRACE1(args, type, message)                                  \
+        ASSERT_THROW(                                                       \
+            try {                                                           \
+                eval("builtins." args);                                     \
+            } catch (BaseError & e) {                                       \
+                ASSERT_EQ(PrintToString(e.info().msg),                      \
+                          PrintToString(message));                          \
+                auto trace = e.info().traces.rbegin();                      \
+                ASSERT_EQ(PrintToString(trace->hint),                       \
+                          PrintToString(hintfmt("while calling the '%s' builtin", "genericClosure"))); \
+                throw;                                                      \
+            }                                                               \
+            , type                                                          \
+        )
+
+#define ASSERT_TRACE2(args, type, message, context)                         \
+        ASSERT_THROW(                                                       \
+            try {                                                           \
+                eval("builtins." args);                                     \
+            } catch (BaseError & e) {                                       \
+                ASSERT_EQ(PrintToString(e.info().msg),                      \
+                          PrintToString(message));                          \
+                auto trace = e.info().traces.rbegin();                      \
+                ASSERT_EQ(PrintToString(trace->hint),                       \
+                          PrintToString(context));                          \
+                ++trace;                                                    \
+                ASSERT_EQ(PrintToString(trace->hint),                       \
+                          PrintToString(hintfmt("while calling the '%s' builtin", "genericClosure"))); \
+                throw;                                                      \
+            }                                                               \
+            , type                                                          \
+        )
+
+    TEST_F(ErrorTraceTest, genericClosure) { \
+        ASSERT_TRACE2("genericClosure 1",
+                      TypeError,
+                      hintfmt("value is %s while a set was expected", "an integer"),
+                      hintfmt("while evaluating the first argument passed to builtins.genericClosure"));
+
+        ASSERT_TRACE1("genericClosure {}",
+                      TypeError,
+                      hintfmt("attribute '%s' missing %s", "startSet", normaltxt("in the attrset passed as argument to builtins.genericClosure")));
+
+        ASSERT_TRACE2("genericClosure { startSet = 1; }",
+                      TypeError,
+                      hintfmt("value is %s while a list was expected", "an integer"),
+                      hintfmt("while evaluating the 'startSet' attribute passed as argument to builtins.genericClosure"));
+
+        // Okay:      "genericClosure { startSet = []; }"
+
+        ASSERT_TRACE2("genericClosure { startSet = [{ key = 1;}]; operator = true; }",
+                      TypeError,
+                      hintfmt("value is %s while a function was expected", "a Boolean"),
+                      hintfmt("while evaluating the 'operator' attribute passed as argument to builtins.genericClosure"));
+
+        ASSERT_TRACE2("genericClosure { startSet = [{ key = 1;}]; operator = item: true; }",
+                      TypeError,
+                      hintfmt("value is %s while a list was expected", "a Boolean"),
+                      hintfmt("while evaluating the return value of the `operator` passed to builtins.genericClosure")); // TODO: inconsistent naming
+
+        ASSERT_TRACE2("genericClosure { startSet = [{ key = 1;}]; operator = item: [ true ]; }",
+                      TypeError,
+                      hintfmt("value is %s while a set was expected", "a Boolean"),
+                      hintfmt("while evaluating one of the elements generated by (or initially passed to) builtins.genericClosure"));
+
+        ASSERT_TRACE1("genericClosure { startSet = [{ key = 1;}]; operator = item: [ {} ]; }",
+                      TypeError,
+                      hintfmt("attribute '%s' missing %s", "key", normaltxt("in one of the attrsets generated by (or initially passed to) builtins.genericClosure")));
+
+        ASSERT_TRACE2("genericClosure { startSet = [{ key = 1;}]; operator = item: [{ key = ''a''; }]; }",
+                      EvalError,
+                      hintfmt("cannot compare %s with %s", "a string", "an integer"),
+                      hintfmt("while comparing the `key` attributes of two genericClosure elements"));
+
+        ASSERT_TRACE2("genericClosure { startSet = [ true ]; operator = item: [{ key = ''a''; }]; }",
+                      TypeError,
+                      hintfmt("value is %s while a set was expected", "a Boolean"),
+                      hintfmt("while evaluating one of the elements generated by (or initially passed to) builtins.genericClosure"));
+
+    }
+
+} /* namespace nix */

src/libexpr/tests/libexprtests.hh

@@ -12,6 +12,7 @@ namespace nix {
     class LibExprTest : public ::testing::Test {
         public:
             static void SetUpTestSuite() {
+                initLibStore();
                 initGC();
             }
 

src/libexpr/tests/primops.cc

@@ -823,4 +823,10 @@ namespace nix {
         for (const auto [n, elem] : enumerate(v.listItems()))
             ASSERT_THAT(*elem, IsStringEq(expected[n]));
     }
+
+    TEST_F(PrimOpTest, genericClosure_not_strict) {
+        // Operator should not be used when startSet is empty
+        auto v = eval("builtins.genericClosure { startSet = []; }");
+        ASSERT_THAT(v, IsListOfSize(0));
+    }
 } /* namespace nix */

src/libexpr/value.hh

@@ -90,7 +90,7 @@ class ExternalValueBase
     /* Coerce the value to a string. Defaults to uncoercable, i.e. throws an
      * error.
      */
-    virtual std::string coerceToString(const Pos & pos, PathSet & context, bool copyMore, bool copyToStore) const;
+    virtual std::string coerceToString(const Pos & pos, PathSet & context, bool copyMore, bool copyToStore, std::string_view errorCtx) const;
 
     /* Compare to another value of the same type. Defaults to uncomparable,
      * i.e. always false.

src/libmain/shared.cc

@@ -235,6 +235,7 @@ void initNix()
 #endif
 
     preloadNSS();
+    initLibStore();
 }
 
 
@@ -362,6 +363,7 @@ void printVersion(const std::string & programName)
             << "\n";
         std::cout << "Store directory: " << settings.nixStore << "\n";
         std::cout << "State directory: " << settings.nixStateDir << "\n";
+        std::cout << "Data directory: " << settings.nixDataDir << "\n";
     }
     throw Exit();
 }
@@ -402,8 +404,6 @@ int handleExceptions(const std::string & programName, std::function<void()> fun)
         return 1;
     } catch (BaseError & e) {
         logError(e.info());
-        if (e.hasTrace() && !loggerSettings.showTrace.get())
-            printError("(use '--show-trace' to show detailed location information)");
         return e.status;
     } catch (std::bad_alloc & e) {
         printError(error + "out of memory");

src/libstore/build/derivation-goal.cc

@@ -39,7 +39,6 @@
 #include <sys/ioctl.h>
 #include <net/if.h>
 #include <netinet/ip.h>
-#include <sys/personality.h>
 #include <sys/mman.h>
 #include <sched.h>
 #include <sys/param.h>
@@ -545,7 +544,8 @@ void DerivationGoal::inputsRealised()
                    However, the impure derivations feature still relies on this
                    fragile way of doing things, because its builds do not have
                    a representation in the store, which is a usability problem
-                   in itself */
+                   in itself. When implementing this logic entirely with lookups
+                   make sure that they're cached. */
                 if (auto outPath = get(inputDrvOutputs, { depDrvPath, j })) {
                     worker.store.computeFSClosure(*outPath, inputPaths);
                 }

src/libstore/build/local-derivation-goal.cc

@@ -15,6 +15,7 @@
 #include "callback.hh"
 #include "json-utils.hh"
 #include "cgroup.hh"
+#include "personality.hh"
 
 #include <regex>
 #include <queue>
@@ -24,7 +25,6 @@
 #include <termios.h>
 #include <unistd.h>
 #include <sys/mman.h>
-#include <sys/utsname.h>
 #include <sys/resource.h>
 #include <sys/socket.h>
 
@@ -37,7 +37,6 @@
 #include <sys/ioctl.h>
 #include <net/if.h>
 #include <netinet/ip.h>
-#include <sys/personality.h>
 #include <sys/mman.h>
 #include <sched.h>
 #include <sys/param.h>
@@ -1964,33 +1963,7 @@ void LocalDerivationGoal::runChild()
         /* Close all other file descriptors. */
         closeMostFDs({STDIN_FILENO, STDOUT_FILENO, STDERR_FILENO});
 
-#if __linux__
-        /* Change the personality to 32-bit if we're doing an
-           i686-linux build on an x86_64-linux machine. */
-        struct utsname utsbuf;
-        uname(&utsbuf);
-        if ((drv->platform == "i686-linux"
-                && (settings.thisSystem == "x86_64-linux"
-                    || (!strcmp(utsbuf.sysname, "Linux") && !strcmp(utsbuf.machine, "x86_64"))))
-            || drv->platform == "armv7l-linux"
-            || drv->platform == "armv6l-linux")
-        {
-            if (personality(PER_LINUX32) == -1)
-                throw SysError("cannot set 32-bit personality");
-        }
-
-        /* Impersonate a Linux 2.6 machine to get some determinism in
-           builds that depend on the kernel version. */
-        if ((drv->platform == "i686-linux" || drv->platform == "x86_64-linux") && settings.impersonateLinux26) {
-            int cur = personality(0xffffffff);
-            if (cur != -1) personality(cur | 0x0020000 /* == UNAME26 */);
-        }
-
-        /* Disable address space randomization for improved
-           determinism. */
-        int cur = personality(0xffffffff);
-        if (cur != -1) personality(cur | ADDR_NO_RANDOMIZE);
-#endif
+        setPersonality(drv->platform);
 
         /* Disable core dumps by default. */
         struct rlimit limit = { 0, RLIM_INFINITY };
@@ -2077,10 +2050,14 @@ void LocalDerivationGoal::runChild()
                     sandboxProfile += "(deny default (with no-log))\n";
                 }
 
-                sandboxProfile += "(import \"sandbox-defaults.sb\")\n";
+                sandboxProfile +=
+                    #include "sandbox-defaults.sb"
+                    ;
 
                 if (!derivationType.isSandboxed())
-                    sandboxProfile += "(import \"sandbox-network.sb\")\n";
+                    sandboxProfile +=
+                        #include "sandbox-network.sb"
+                        ;
 
                 /* Add the output paths we'll use at build-time to the chroot */
                 sandboxProfile += "(allow file-read* file-write* process-exec\n";
@@ -2123,7 +2100,9 @@ void LocalDerivationGoal::runChild()
 
                 sandboxProfile += additionalSandboxProfile;
             } else
-                sandboxProfile += "(import \"sandbox-minimal.sb\")\n";
+                sandboxProfile +=
+                    #include "sandbox-minimal.sb"
+                    ;
 
             debug("Generated sandbox profile:");
             debug(sandboxProfile);
@@ -2148,8 +2127,6 @@ void LocalDerivationGoal::runChild()
                 args.push_back(sandboxFile);
                 args.push_back("-D");
                 args.push_back("_GLOBAL_TMP_DIR=" + globalTmpDir);
-                args.push_back("-D");
-                args.push_back("IMPORT_DIR=" + settings.nixDataDir + "/nix/sandbox/");
                 if (allowLocalNetworking) {
                     args.push_back("-D");
                     args.push_back(std::string("_ALLOW_LOCAL_NETWORKING=1"));

src/libstore/build/personality.cc

@@ -0,0 +1,44 @@
+#include "personality.hh"
+#include "globals.hh"
+
+#if __linux__
+#include <sys/utsname.h>
+#include <sys/personality.h>
+#endif
+
+#include <cstring>
+
+namespace nix {
+
+void setPersonality(std::string_view system)
+{
+#if __linux__
+        /* Change the personality to 32-bit if we're doing an
+           i686-linux build on an x86_64-linux machine. */
+        struct utsname utsbuf;
+        uname(&utsbuf);
+        if ((system == "i686-linux"
+                && (std::string_view(SYSTEM) == "x86_64-linux"
+                    || (!strcmp(utsbuf.sysname, "Linux") && !strcmp(utsbuf.machine, "x86_64"))))
+            || system == "armv7l-linux"
+            || system == "armv6l-linux")
+        {
+            if (personality(PER_LINUX32) == -1)
+                throw SysError("cannot set 32-bit personality");
+        }
+
+        /* Impersonate a Linux 2.6 machine to get some determinism in
+           builds that depend on the kernel version. */
+        if ((system == "i686-linux" || system == "x86_64-linux") && settings.impersonateLinux26) {
+            int cur = personality(0xffffffff);
+            if (cur != -1) personality(cur | 0x0020000 /* == UNAME26 */);
+        }
+
+        /* Disable address space randomization for improved
+           determinism. */
+        int cur = personality(0xffffffff);
+        if (cur != -1) personality(cur | ADDR_NO_RANDOMIZE);
+#endif
+}
+
+}

src/libstore/build/personality.hh

@@ -0,0 +1,11 @@
+#pragma once
+
+#include <string>
+
+namespace nix {
+
+void setPersonality(std::string_view system);
+
+}
+
+

src/libstore/build/sandbox-defaults.sb

@@ -1,3 +1,5 @@
+R""(
+
 (define TMPDIR (param "_GLOBAL_TMP_DIR"))
 
 (deny default)
@@ -104,3 +106,5 @@
        (subpath "/System/Library/Apple/usr/libexec/oah")
        (subpath "/System/Library/LaunchDaemons/com.apple.oahd.plist")
        (subpath "/Library/Apple/System/Library/LaunchDaemons/com.apple.oahd.plist"))
+
+)""

src/libstore/build/sandbox-minimal.sb

@@ -1,5 +1,9 @@
+R""(
+
 (allow default)
 
 ; Disallow creating setuid/setgid binaries, since that
 ; would allow breaking build user isolation.
 (deny file-write-setugid)
+
+)""

src/libstore/build/sandbox-network.sb

@@ -1,3 +1,5 @@
+R""(
+
 ; Allow local and remote network traffic.
 (allow network* (local ip) (remote ip))
 
@@ -18,3 +20,5 @@
 ; Allow access to trustd.
 (allow mach-lookup (global-name "com.apple.trustd"))
 (allow mach-lookup (global-name "com.apple.trustd.agent"))
+
+)""

src/libstore/builtins/buildenv.cc

@@ -95,7 +95,7 @@ static void createLinks(State & state, const Path & srcDir, const Path & dstDir,
                         throw Error(
                                 "files '%1%' and '%2%' have the same priority %3%; "
                                 "use 'nix-env --set-flag priority NUMBER INSTALLED_PKGNAME' "
-                                "or type 'nix profile install --help' if using 'nix profile' to find out how"
+                                "or type 'nix profile install --help' if using 'nix profile' to find out how "
                                 "to change the priority of one of the conflicting packages"
                                 " (0 being the highest priority)",
                                 srcFile, readLink(dstFile), priority);

src/libstore/gc.cc

@@ -77,12 +77,12 @@ Path LocalFSStore::addPermRoot(const StorePath & storePath, const Path & _gcRoot
 }
 
 
-void LocalStore::addTempRoot(const StorePath & path)
+void LocalStore::createTempRootsFile()
 {
-    auto state(_state.lock());
+    auto fdTempRoots(_fdTempRoots.lock());
 
     /* Create the temporary roots file for this process. */
-    if (!state->fdTempRoots) {
+    if (*fdTempRoots) return;
 
     while (1) {
         if (pathExists(fnTempRoots))
@@ -90,47 +90,60 @@ void LocalStore::addTempRoot(const StorePath & path)
                processes with the same pid. */
             unlink(fnTempRoots.c_str());
 
-            state->fdTempRoots = openLockFile(fnTempRoots, true);
+        *fdTempRoots = openLockFile(fnTempRoots, true);
 
         debug("acquiring write lock on '%s'", fnTempRoots);
-            lockFile(state->fdTempRoots.get(), ltWrite, true);
+        lockFile(fdTempRoots->get(), ltWrite, true);
 
         /* Check whether the garbage collector didn't get in our
            way. */
         struct stat st;
-            if (fstat(state->fdTempRoots.get(), &st) == -1)
+        if (fstat(fdTempRoots->get(), &st) == -1)
             throw SysError("statting '%1%'", fnTempRoots);
         if (st.st_size == 0) break;
 
-            /* The garbage collector deleted this file before we could
-               get a lock.  (It won't delete the file after we get a
-               lock.)  Try again. */
+        /* The garbage collector deleted this file before we could get
+           a lock.  (It won't delete the file after we get a lock.)
+           Try again. */
     }
+}
 
+
+void LocalStore::addTempRoot(const StorePath & path)
+{
+    createTempRootsFile();
+
+    /* Open/create the global GC lock file. */
+    {
+        auto fdGCLock(_fdGCLock.lock());
+        if (!*fdGCLock)
+            *fdGCLock = openGCLock();
     }
 
-    if (!state->fdGCLock)
-        state->fdGCLock = openGCLock();
-
  restart:
-    FdLock gcLock(state->fdGCLock.get(), ltRead, false, "");
+    /* Try to acquire a shared global GC lock (non-blocking). This
+       only succeeds if the garbage collector is not currently
+       running. */
+    FdLock gcLock(_fdGCLock.lock()->get(), ltRead, false, "");
 
     if (!gcLock.acquired) {
         /* We couldn't get a shared global GC lock, so the garbage
            collector is running. So we have to connect to the garbage
            collector and inform it about our root. */
-        if (!state->fdRootsSocket) {
+        auto fdRootsSocket(_fdRootsSocket.lock());
+
+        if (!*fdRootsSocket) {
             auto socketPath = stateDir.get() + gcSocketPath;
             debug("connecting to '%s'", socketPath);
-            state->fdRootsSocket = createUnixDomainSocket();
+            *fdRootsSocket = createUnixDomainSocket();
             try {
-                nix::connect(state->fdRootsSocket.get(), socketPath);
+                nix::connect(fdRootsSocket->get(), socketPath);
             } catch (SysError & e) {
                 /* The garbage collector may have exited, so we need to
                    restart. */
                 if (e.errNo == ECONNREFUSED) {
                     debug("GC socket connection refused");
-                    state->fdRootsSocket.close();
+                    fdRootsSocket->close();
                     goto restart;
                 }
                 throw;
@@ -139,9 +152,9 @@ void LocalStore::addTempRoot(const StorePath & path)
 
         try {
             debug("sending GC root '%s'", printStorePath(path));
-            writeFull(state->fdRootsSocket.get(), printStorePath(path) + "\n", false);
+            writeFull(fdRootsSocket->get(), printStorePath(path) + "\n", false);
             char c;
-            readFull(state->fdRootsSocket.get(), &c, 1);
+            readFull(fdRootsSocket->get(), &c, 1);
             assert(c == '1');
             debug("got ack for GC root '%s'", printStorePath(path));
         } catch (SysError & e) {
@@ -149,20 +162,21 @@ void LocalStore::addTempRoot(const StorePath & path)
                restart. */
             if (e.errNo == EPIPE || e.errNo == ECONNRESET) {
                 debug("GC socket disconnected");
-                state->fdRootsSocket.close();
+                fdRootsSocket->close();
                 goto restart;
             }
             throw;
         } catch (EndOfFile & e) {
             debug("GC socket disconnected");
-            state->fdRootsSocket.close();
+            fdRootsSocket->close();
             goto restart;
         }
     }
 
-    /* Append the store path to the temporary roots file. */
+    /* Record the store path in the temporary roots file so it will be
+       seen by a future run of the garbage collector. */
     auto s = printStorePath(path) + '\0';
-    writeFull(state->fdTempRoots.get(), s);
+    writeFull(_fdTempRoots.lock()->get(), s);
 }
 
 

src/libstore/globals.cc

@@ -290,4 +290,18 @@ void initPlugins()
     settings.pluginFiles.pluginsLoaded = true;
 }
 
+static bool initLibStoreDone = false;
+
+void assertLibStoreInitialized() {
+    if (!initLibStoreDone) {
+        printError("The program must call nix::initNix() before calling any libstore library functions.");
+        abort();
+    };
+}
+
+void initLibStore() {
+    initLibStoreDone = true;
+}
+
+
 }

src/libstore/globals.hh

@@ -491,6 +491,9 @@ public:
           for example, `/dev/nvidiactl?` specifies that `/dev/nvidiactl` will
           only be mounted in the sandbox if it exists in the host filesystem.
 
+          If the source is in the Nix store, then its closure will be added to
+          the sandbox as well.
+
           Depending on how Nix was built, the default value for this option
           may be empty or provide `/bin/sh` as a bind-mount of `bash`.
         )",
@@ -984,4 +987,12 @@ std::vector<Path> getUserConfigFiles();
 
 extern const std::string nixVersion;
 
+/* NB: This is not sufficient. You need to call initNix() */
+void initLibStore();
+
+/* It's important to initialize before doing _anything_, which is why we
+   call upon the programmer to handle this correctly. However, we only add
+   this in a key locations, so as not to litter the code. */
+void assertLibStoreInitialized();
+
 }

src/libstore/local-store.cc

@@ -91,6 +91,7 @@ void migrateCASchema(SQLite& db, Path schemaPath, AutoCloseFD& lockFd)
 
         if (!lockFile(lockFd.get(), ltWrite, false)) {
             printInfo("waiting for exclusive access to the Nix store for ca drvs...");
+            lockFile(lockFd.get(), ltNone, false); // We have acquired a shared lock; release it to prevent deadlocks
             lockFile(lockFd.get(), ltWrite, true);
         }
 
@@ -299,6 +300,7 @@ LocalStore::LocalStore(const Params & params)
 
         if (!lockFile(globalLock.get(), ltWrite, false)) {
             printInfo("waiting for exclusive access to the Nix store...");
+            lockFile(globalLock.get(), ltNone, false); // We have acquired a shared lock; release it to prevent deadlocks
             lockFile(globalLock.get(), ltWrite, true);
         }
 
@@ -439,9 +441,9 @@ LocalStore::~LocalStore()
     }
 
     try {
-        auto state(_state.lock());
-        if (state->fdTempRoots) {
-            state->fdTempRoots = -1;
+        auto fdTempRoots(_fdTempRoots.lock());
+        if (*fdTempRoots) {
+            *fdTempRoots = -1;
             unlink(fnTempRoots.c_str());
         }
     } catch (...) {

src/libstore/local-store.hh

@@ -59,15 +59,6 @@ private:
         struct Stmts;
         std::unique_ptr<Stmts> stmts;
 
-        /* The global GC lock  */
-        AutoCloseFD fdGCLock;
-
-        /* The file to which we write our temporary roots. */
-        AutoCloseFD fdTempRoots;
-
-        /* Connection to the garbage collector. */
-        AutoCloseFD fdRootsSocket;
-
         /* The last time we checked whether to do an auto-GC, or an
            auto-GC finished. */
         std::chrono::time_point<std::chrono::steady_clock> lastGCCheck;
@@ -156,6 +147,21 @@ public:
 
     void addTempRoot(const StorePath & path) override;
 
+private:
+
+    void createTempRootsFile();
+
+    /* The file to which we write our temporary roots. */
+    Sync<AutoCloseFD> _fdTempRoots;
+
+    /* The global GC lock. */
+    Sync<AutoCloseFD> _fdGCLock;
+
+    /* Connection to the garbage collector. */
+    Sync<AutoCloseFD> _fdRootsSocket;
+
+public:
+
     void addIndirectRoot(const Path & path) override;
 
 private:

src/libstore/local.mk

@@ -13,10 +13,6 @@ ifdef HOST_LINUX
  libstore_LDFLAGS += -ldl
 endif
 
-ifdef HOST_DARWIN
-libstore_FILES = sandbox-defaults.sb sandbox-minimal.sb sandbox-network.sb
-endif
-
 $(foreach file,$(libstore_FILES),$(eval $(call install-data-in,$(d)/$(file),$(datadir)/nix/sandbox)))
 
 ifeq ($(ENABLE_S3), 1)

src/libstore/lock.cc

@@ -123,8 +123,12 @@ struct AutoUserLock : UserLock
 
     std::vector<gid_t> getSupplementaryGIDs() override { return {}; }
 
-    static std::unique_ptr<UserLock> acquire(uid_t nrIds, bool useChroot)
+    static std::unique_ptr<UserLock> acquire(uid_t nrIds, bool useUserNamespace)
     {
+        #if !defined(__linux__)
+        useUserNamespace = false;
+        #endif
+
         settings.requireExperimentalFeature(Xp::AutoAllocateUids);
         assert(settings.startId > 0);
         assert(settings.uidCount % maxIdsPerBuild == 0);
@@ -157,7 +161,7 @@ struct AutoUserLock : UserLock
                 auto lock = std::make_unique<AutoUserLock>();
                 lock->fdUserLock = std::move(fd);
                 lock->firstUid = firstUid;
-                if (useChroot)
+                if (useUserNamespace)
                     lock->firstGid = firstUid;
                 else {
                     struct group * gr = getgrnam(settings.buildUsersGroup.get().c_str());
@@ -174,10 +178,10 @@ struct AutoUserLock : UserLock
     }
 };
 
-std::unique_ptr<UserLock> acquireUserLock(uid_t nrIds, bool useChroot)
+std::unique_ptr<UserLock> acquireUserLock(uid_t nrIds, bool useUserNamespace)
 {
     if (settings.autoAllocateUids)
-        return AutoUserLock::acquire(nrIds, useChroot);
+        return AutoUserLock::acquire(nrIds, useUserNamespace);
     else
         return SimpleUserLock::acquire();
 }

src/libstore/lock.hh

@@ -31,7 +31,7 @@ struct UserLock
 
 /* Acquire a user lock for a UID range of size `nrIds`. Note that this
    may return nullptr if no user is available. */
-std::unique_ptr<UserLock> acquireUserLock(uid_t nrIds, bool useChroot);
+std::unique_ptr<UserLock> acquireUserLock(uid_t nrIds, bool useUserNamespace);
 
 bool useBuildUsers();
 

src/libstore/realisation.hh

@@ -93,4 +93,14 @@ struct RealisedPath {
     GENERATE_CMP(RealisedPath, me->raw);
 };
 
+class MissingRealisation : public Error
+{
+public:
+    MissingRealisation(DrvOutput & outputId)
+        : Error( "cannot operate on an output of the "
+                "unbuilt derivation '%s'",
+                outputId.to_string())
+    {}
+};
+
 }

src/libstore/remote-store.cc

@@ -879,10 +879,7 @@ std::vector<BuildResult> RemoteStore::buildPathsWithResults(
                                 auto realisation =
                                     queryRealisation(outputId);
                                 if (!realisation)
-                                    throw Error(
-                                        "cannot operate on an output of unbuilt "
-                                        "content-addressed derivation '%s'",
-                                        outputId.to_string());
+                                    throw MissingRealisation(outputId);
                                 res.builtOutputs.emplace(realisation->id, *realisation);
                             } else {
                                 // If ca-derivations isn't enabled, assume that

src/libstore/sqlite.cc

@@ -47,9 +47,13 @@ SQLite::SQLite(const Path & path, bool create)
     // `unix-dotfile` is needed on NFS file systems and on Windows' Subsystem
     // for Linux (WSL) where useSQLiteWAL should be false by default.
     const char *vfs = settings.useSQLiteWAL ? 0 : "unix-dotfile";
-    if (sqlite3_open_v2(path.c_str(), &db,
-            SQLITE_OPEN_READWRITE | (create ? SQLITE_OPEN_CREATE : 0), vfs) != SQLITE_OK)
-        throw Error("cannot open SQLite database '%s'", path);
+    int flags = SQLITE_OPEN_READWRITE;
+    if (create) flags |= SQLITE_OPEN_CREATE;
+    int ret = sqlite3_open_v2(path.c_str(), &db, flags, vfs);
+    if (ret != SQLITE_OK) {
+        const char * err = sqlite3_errstr(ret);
+        throw Error("cannot open SQLite database '%s': %s", path, err);
+    }
 
     if (sqlite3_busy_timeout(db, 60 * 60 * 1000) != SQLITE_OK)
         SQLiteError::throw_(db, "setting timeout");

src/libstore/store-api.cc

@@ -458,6 +458,7 @@ Store::Store(const Params & params)
     : StoreConfig(params)
     , state({(size_t) pathInfoCacheSize})
 {
+    assertLibStoreInitialized();
 }
 
 

src/libutil/error.cc

@@ -9,9 +9,9 @@ namespace nix {
 
 const std::string nativeSystem = SYSTEM;
 
-void BaseError::addTrace(std::shared_ptr<AbstractPos> && e, hintformat hint)
+void BaseError::addTrace(std::shared_ptr<AbstractPos> && e, hintformat hint, bool frame)
 {
-    err.traces.push_front(Trace { .pos = std::move(e), .hint = hint });
+    err.traces.push_front(Trace { .pos = std::move(e), .hint = hint, .frame = frame });
 }
 
 // c++ std::exception descendants must have a 'const char* what()' function.
@@ -200,13 +200,125 @@ std::ostream & showErrorInfo(std::ostream & out, const ErrorInfo & einfo, bool s
 
     auto noSource = ANSI_ITALIC " (source not available)" ANSI_NORMAL "\n";
 
-    // traces
-    if (showTrace && !einfo.traces.empty()) {
+    /*
+     * Traces
+     * ------
+     *
+     *  The semantics of traces is a bit weird. We have only one option to
+     *  print them and to make them verbose (--show-trace). In the code they
+     *  are always collected, but they are not printed by default. The code
+     *  also collects more traces when the option is on. This means that there
+     *  is no way to print the simplified traces at all.
+     *
+     *  I (layus) designed the code to attach positions to a restricted set of
+     *  messages. This means that we have  a lot of traces with no position at
+     *  all, including most of the base error messages. For example "type
+     *  error: found a string while a set was expected" has no position, but
+     *  will come with several traces detailing it's precise relation to the
+     *  closest know position. This makes erroring without printing traces
+     *  quite useless.
+     *
+     *  This is why I introduced the idea to always print a few traces on
+     *  error. The number 3 is quite arbitrary, and was selected so as not to
+     *  clutter the console on error. For the same reason, a trace with an
+     *  error position takes more space, and counts as two traces towards the
+     *  limit.
+     *
+     *  The rest is truncated, unless --show-trace is passed. This preserves
+     *  the same bad semantics of --show-trace to both show the trace and
+     *  augment it with new data. Not too sure what is the best course of
+     *  action.
+     *
+     *  The issue is that it is fundamentally hard to provide a trace for a
+     *  lazy language. The trace will only cover the current spine of the
+     *  evaluation, missing things that have been evaluated before. For
+     *  example, most type errors are hard to inspect because there is not
+     *  trace for the faulty value. These errors should really print the faulty
+     *  value itself.
+     *
+     *  In function calls, the --show-trace flag triggers extra traces for each
+     *  function invocation. These work as scopes, allowing to follow the
+     *  current spine of the evaluation graph. Without that flag, the error
+     *  trace should restrict itself to a restricted prefix of that trace,
+     *  until the first scope. If we ever get to such a precise error
+     *  reporting, there would be no need to add an arbitrary limit here. We
+     *  could always print the full trace, and it would just be small without
+     *  the flag.
+     *
+     *  One idea I had is for XxxError.addTrace() to perform nothing if one
+     *  scope has already been traced. Alternatively, we could stop here when
+     *  we encounter such a scope instead of after an arbitrary number of
+     *  traces. This however requires to augment traces with the notion of
+     *  "scope".
+     *
+     *  This is particularly visible in code like evalAttrs(...) where we have
+     *  to make a decision between the two following options.
+     *
+     *  ``` long traces
+     *  inline void EvalState::evalAttrs(Env & env, Expr * e, Value & v, const Pos & pos, std::string_view errorCtx)
+     *  {
+     *      try {
+     *          e->eval(*this, env, v);
+     *          if (v.type() != nAttrs)
+     *              throwTypeError("value is %1% while a set was expected", v);
+     *      } catch (Error & e) {
+     *          e.addTrace(pos, errorCtx);
+     *          throw;
+     *      }
+     *  }
+     *  ```
+     *
+     *  ``` short traces
+     *  inline void EvalState::evalAttrs(Env & env, Expr * e, Value & v, const Pos & pos, std::string_view errorCtx)
+     *  {
+     *      e->eval(*this, env, v);
+     *      try {
+     *          if (v.type() != nAttrs)
+     *              throwTypeError("value is %1% while a set was expected", v);
+     *      } catch (Error & e) {
+     *          e.addTrace(pos, errorCtx);
+     *          throw;
+     *      }
+     *  }
+     *  ```
+     *
+     *  The second example can be rewritten more concisely, but kept in this
+     *  form to highlight the symmetry. The first option adds more information,
+     *  because whatever caused an error down the line, in the generic eval
+     *  function, will get annotated with the code location that uses and
+     *  required it. The second option is less verbose, but does not provide
+     *  any context at all as to where and why a failing value was required.
+     *
+     *  Scopes would fix that, by adding context only when --show-trace is
+     *  passed, and keeping the trace terse otherwise.
+     *
+     */
+
+    // Enough indent to align with with the `... `
+    // prepended to each element of the trace
+    auto ellipsisIndent = "  ";
+
+    bool frameOnly = false;
+    if (!einfo.traces.empty()) {
+        size_t count = 0;
         for (const auto & trace : einfo.traces) {
+            if (!showTrace && count > 3) {
+                oss << "\n" << ANSI_WARNING "(stack trace truncated; use '--show-trace' to show the full trace)" ANSI_NORMAL << "\n";
+                break;
+            }
+
+            if (trace.hint.str().empty()) continue;
+            if (frameOnly && !trace.frame) continue;
+
+            count++;
+            frameOnly = trace.frame;
+
             oss << "\n" << "… " << trace.hint.str() << "\n";
 
             if (trace.pos) {
-                oss << "\n" << ANSI_BLUE << "at " ANSI_WARNING << *trace.pos << ANSI_NORMAL << ":";
+                count++;
+
+                oss << "\n" << ellipsisIndent << ANSI_BLUE << "at " ANSI_WARNING << *trace.pos << ANSI_NORMAL << ":";
 
                 if (auto loc = trace.pos->getCodeLines()) {
                     oss << "\n";

src/libutil/error.hh

@@ -86,6 +86,7 @@ void printCodeLines(std::ostream & out,
 struct Trace {
     std::shared_ptr<AbstractPos> pos;
     hintformat hint;
+    bool frame;
 };
 
 struct ErrorInfo {
@@ -114,6 +115,8 @@ protected:
 public:
     unsigned int status = 1; // exit status
 
+    BaseError(const BaseError &) = default;
+
     template<typename... Args>
     BaseError(unsigned int status, const Args & ... args)
         : err { .level = lvlError, .msg = hintfmt(args...) }
@@ -152,15 +155,22 @@ public:
     const std::string & msg() const { return calcWhat(); }
     const ErrorInfo & info() const { calcWhat(); return err; }
 
-    template<typename... Args>
-    void addTrace(std::shared_ptr<AbstractPos> && e, const std::string & fs, const Args & ... args)
+    void pushTrace(Trace trace)
     {
-        addTrace(std::move(e), hintfmt(fs, args...));
+        err.traces.push_front(trace);
     }
 
-    void addTrace(std::shared_ptr<AbstractPos> && e, hintformat hint);
+    template<typename... Args>
+    void addTrace(std::shared_ptr<AbstractPos> && e, std::string_view fs, const Args & ... args)
+    {
+        addTrace(std::move(e), hintfmt(std::string(fs), args...));
+    }
+
+    void addTrace(std::shared_ptr<AbstractPos> && e, hintformat hint, bool frame = false);
 
     bool hasTrace() const { return !err.traces.empty(); }
+
+    const ErrorInfo & info() { return err; };
 };
 
 #define MakeError(newClass, superClass) \

src/nix-env/user-env.cc

@@ -134,9 +134,9 @@ bool createUserEnv(EvalState & state, DrvInfos & elems,
     state.forceValue(topLevel, [&]() { return topLevel.determinePos(noPos); });
     PathSet context;
     Attr & aDrvPath(*topLevel.attrs->find(state.sDrvPath));
-    auto topLevelDrv = state.coerceToStorePath(aDrvPath.pos, *aDrvPath.value, context);
+    auto topLevelDrv = state.coerceToStorePath(aDrvPath.pos, *aDrvPath.value, context, "");
     Attr & aOutPath(*topLevel.attrs->find(state.sOutPath));
-    auto topLevelOut = state.coerceToStorePath(aOutPath.pos, *aOutPath.value, context);
+    auto topLevelOut = state.coerceToStorePath(aOutPath.pos, *aOutPath.value, context, "");
 
     /* Realise the resulting store expression. */
     debug("building user environment");

src/nix/bundle.cc

@@ -97,13 +97,13 @@ struct CmdBundle : InstallableCommand
             throw Error("the bundler '%s' does not produce a derivation", bundler.what());
 
         PathSet context2;
-        auto drvPath = evalState->coerceToStorePath(attr1->pos, *attr1->value, context2);
+        auto drvPath = evalState->coerceToStorePath(attr1->pos, *attr1->value, context2, "");
 
         auto attr2 = vRes->attrs->get(evalState->sOutPath);
         if (!attr2)
             throw Error("the bundler '%s' does not produce a derivation", bundler.what());
 
-        auto outPath = evalState->coerceToStorePath(attr2->pos, *attr2->value, context2);
+        auto outPath = evalState->coerceToStorePath(attr2->pos, *attr2->value, context2, "");
 
         store->buildPaths({ DerivedPath::Built { drvPath } });
 
@@ -113,7 +113,7 @@ struct CmdBundle : InstallableCommand
             auto * attr = vRes->attrs->get(evalState->sName);
             if (!attr)
                 throw Error("attribute 'name' missing");
-            outLink = evalState->forceStringNoCtx(*attr->value, attr->pos);
+            outLink = evalState->forceStringNoCtx(*attr->value, attr->pos, "");
         }
 
         // TODO: will crash if not a localFSStore?

src/nix/develop.cc

@@ -164,6 +164,14 @@ struct BuildEnvironment
     {
         return vars == other.vars && bashFunctions == other.bashFunctions;
     }
+
+    std::string getSystem() const
+    {
+        if (auto v = get(vars, "system"))
+            return getString(*v);
+        else
+            return settings.thisSystem;
+    }
 };
 
 const static std::string getEnvSh =
@@ -570,7 +578,7 @@ struct CmdDevelop : Common, MixEnvironment
             }
         }
 
-        runProgramInStore(store, shell, args);
+        runProgramInStore(store, shell, args, buildEnvironment.getSystem());
     }
 };
 

src/nix/eval.cc

@@ -112,7 +112,7 @@ struct CmdEval : MixJSON, InstallableCommand
 
         else if (raw) {
             stopProgressBar();
-            std::cout << *state->coerceToString(noPos, *v, context);
+            std::cout << *state->coerceToString(noPos, *v, context, "while generating the eval command output");
         }
 
         else if (json) {

src/nix/flake.cc

@@ -126,12 +126,12 @@ static void enumerateOutputs(EvalState & state, Value & vFlake,
     std::function<void(const std::string & name, Value & vProvide, const PosIdx pos)> callback)
 {
     auto pos = vFlake.determinePos(noPos);
-    state.forceAttrs(vFlake, pos);
+    state.forceAttrs(vFlake, pos, "while evaluating a flake to get its outputs");
 
     auto aOutputs = vFlake.attrs->get(state.symbols.create("outputs"));
     assert(aOutputs);
 
-    state.forceAttrs(*aOutputs->value, pos);
+    state.forceAttrs(*aOutputs->value, pos, "while evaluating the outputs of a flake");
 
     auto sHydraJobs = state.symbols.create("hydraJobs");
 
@@ -381,23 +381,6 @@ struct CmdFlakeCheck : FlakeCommand
         auto checkModule = [&](const std::string & attrPath, Value & v, const PosIdx pos) {
             try {
                 state->forceValue(v, pos);
-                if (v.isLambda()) {
-                    if (!v.lambda.fun->hasFormals() || !v.lambda.fun->formals->ellipsis)
-                        throw Error("module must match an open attribute set ('{ config, ... }')");
-                } else if (v.type() == nAttrs) {
-                    for (auto & attr : *v.attrs)
-                        try {
-                            state->forceValue(*attr.value, attr.pos);
-                        } catch (Error & e) {
-                            e.addTrace(
-                                state->positions[attr.pos],
-                                hintfmt("while evaluating the option '%s'", state->symbols[attr.name]));
-                            throw;
-                        }
-                } else
-                    throw Error("module must be a function or an attribute set");
-                // FIXME: if we have a 'nixpkgs' input, use it to
-                // check the module.
             } catch (Error & e) {
                 e.addTrace(resolve(pos), hintfmt("while checking the NixOS module '%s'", attrPath));
                 reportError(e);
@@ -408,13 +391,13 @@ struct CmdFlakeCheck : FlakeCommand
 
         checkHydraJobs = [&](const std::string & attrPath, Value & v, const PosIdx pos) {
             try {
-                state->forceAttrs(v, pos);
+                state->forceAttrs(v, pos, "");
 
                 if (state->isDerivation(v))
                     throw Error("jobset should not be a derivation at top-level");
 
                 for (auto & attr : *v.attrs) {
-                    state->forceAttrs(*attr.value, attr.pos);
+                    state->forceAttrs(*attr.value, attr.pos, "");
                     auto attrPath2 = concatStrings(attrPath, ".", state->symbols[attr.name]);
                     if (state->isDerivation(*attr.value)) {
                         Activity act(*logger, lvlChatty, actUnknown,
@@ -436,7 +419,7 @@ struct CmdFlakeCheck : FlakeCommand
                     fmt("checking NixOS configuration '%s'", attrPath));
                 Bindings & bindings(*state->allocBindings(0));
                 auto vToplevel = findAlongAttrPath(*state, "config.system.build.toplevel", bindings, v).first;
-                state->forceAttrs(*vToplevel, pos);
+                state->forceValue(*vToplevel, pos);
                 if (!state->isDerivation(*vToplevel))
                     throw Error("attribute 'config.system.build.toplevel' is not a derivation");
             } catch (Error & e) {
@@ -450,12 +433,12 @@ struct CmdFlakeCheck : FlakeCommand
                 Activity act(*logger, lvlChatty, actUnknown,
                     fmt("checking template '%s'", attrPath));
 
-                state->forceAttrs(v, pos);
+                state->forceAttrs(v, pos, "");
 
                 if (auto attr = v.attrs->get(state->symbols.create("path"))) {
                     if (attr->name == state->symbols.create("path")) {
                         PathSet context;
-                        auto path = state->coerceToPath(attr->pos, *attr->value, context);
+                        auto path = state->coerceToPath(attr->pos, *attr->value, context, "");
                         if (!store->isInStore(path))
                             throw Error("template '%s' has a bad 'path' attribute");
                         // TODO: recursively check the flake in 'path'.
@@ -464,7 +447,7 @@ struct CmdFlakeCheck : FlakeCommand
                     throw Error("template '%s' lacks attribute 'path'", attrPath);
 
                 if (auto attr = v.attrs->get(state->symbols.create("description")))
-                    state->forceStringNoCtx(*attr->value, attr->pos);
+                    state->forceStringNoCtx(*attr->value, attr->pos, "");
                 else
                     throw Error("template '%s' lacks attribute 'description'", attrPath);
 
@@ -521,11 +504,11 @@ struct CmdFlakeCheck : FlakeCommand
                             warn("flake output attribute '%s' is deprecated; use '%s' instead", name, replacement);
 
                         if (name == "checks") {
-                            state->forceAttrs(vOutput, pos);
+                            state->forceAttrs(vOutput, pos, "");
                             for (auto & attr : *vOutput.attrs) {
                                 const auto & attr_name = state->symbols[attr.name];
                                 checkSystemName(attr_name, attr.pos);
-                                state->forceAttrs(*attr.value, attr.pos);
+                                state->forceAttrs(*attr.value, attr.pos, "");
                                 for (auto & attr2 : *attr.value->attrs) {
                                     auto drvPath = checkDerivation(
                                         fmt("%s.%s.%s", name, attr_name, state->symbols[attr2.name]),
@@ -537,7 +520,7 @@ struct CmdFlakeCheck : FlakeCommand
                         }
 
                         else if (name == "formatter") {
-                            state->forceAttrs(vOutput, pos);
+                            state->forceAttrs(vOutput, pos, "");
                             for (auto & attr : *vOutput.attrs) {
                                 const auto & attr_name = state->symbols[attr.name];
                                 checkSystemName(attr_name, attr.pos);
@@ -548,11 +531,11 @@ struct CmdFlakeCheck : FlakeCommand
                         }
 
                         else if (name == "packages" || name == "devShells") {
-                            state->forceAttrs(vOutput, pos);
+                            state->forceAttrs(vOutput, pos, "");
                             for (auto & attr : *vOutput.attrs) {
                                 const auto & attr_name = state->symbols[attr.name];
                                 checkSystemName(attr_name, attr.pos);
-                                state->forceAttrs(*attr.value, attr.pos);
+                                state->forceAttrs(*attr.value, attr.pos, "");
                                 for (auto & attr2 : *attr.value->attrs)
                                     checkDerivation(
                                         fmt("%s.%s.%s", name, attr_name, state->symbols[attr2.name]),
@@ -561,11 +544,11 @@ struct CmdFlakeCheck : FlakeCommand
                         }
 
                         else if (name == "apps") {
-                            state->forceAttrs(vOutput, pos);
+                            state->forceAttrs(vOutput, pos, "");
                             for (auto & attr : *vOutput.attrs) {
                                 const auto & attr_name = state->symbols[attr.name];
                                 checkSystemName(attr_name, attr.pos);
-                                state->forceAttrs(*attr.value, attr.pos);
+                                state->forceAttrs(*attr.value, attr.pos, "");
                                 for (auto & attr2 : *attr.value->attrs)
                                     checkApp(
                                         fmt("%s.%s.%s", name, attr_name, state->symbols[attr2.name]),
@@ -574,7 +557,7 @@ struct CmdFlakeCheck : FlakeCommand
                         }
 
                         else if (name == "defaultPackage" || name == "devShell") {
-                            state->forceAttrs(vOutput, pos);
+                            state->forceAttrs(vOutput, pos, "");
                             for (auto & attr : *vOutput.attrs) {
                                 const auto & attr_name = state->symbols[attr.name];
                                 checkSystemName(attr_name, attr.pos);
@@ -585,7 +568,7 @@ struct CmdFlakeCheck : FlakeCommand
                         }
 
                         else if (name == "defaultApp") {
-                            state->forceAttrs(vOutput, pos);
+                            state->forceAttrs(vOutput, pos, "");
                             for (auto & attr : *vOutput.attrs) {
                                 const auto & attr_name = state->symbols[attr.name];
                                 checkSystemName(attr_name, attr.pos);
@@ -596,7 +579,7 @@ struct CmdFlakeCheck : FlakeCommand
                         }
 
                         else if (name == "legacyPackages") {
-                            state->forceAttrs(vOutput, pos);
+                            state->forceAttrs(vOutput, pos, "");
                             for (auto & attr : *vOutput.attrs) {
                                 checkSystemName(state->symbols[attr.name], attr.pos);
                                 // FIXME: do getDerivations?
@@ -607,7 +590,7 @@ struct CmdFlakeCheck : FlakeCommand
                             checkOverlay(name, vOutput, pos);
 
                         else if (name == "overlays") {
-                            state->forceAttrs(vOutput, pos);
+                            state->forceAttrs(vOutput, pos, "");
                             for (auto & attr : *vOutput.attrs)
                                 checkOverlay(fmt("%s.%s", name, state->symbols[attr.name]),
                                     *attr.value, attr.pos);
@@ -617,14 +600,14 @@ struct CmdFlakeCheck : FlakeCommand
                             checkModule(name, vOutput, pos);
 
                         else if (name == "nixosModules") {
-                            state->forceAttrs(vOutput, pos);
+                            state->forceAttrs(vOutput, pos, "");
                             for (auto & attr : *vOutput.attrs)
                                 checkModule(fmt("%s.%s", name, state->symbols[attr.name]),
                                     *attr.value, attr.pos);
                         }
 
                         else if (name == "nixosConfigurations") {
-                            state->forceAttrs(vOutput, pos);
+                            state->forceAttrs(vOutput, pos, "");
                             for (auto & attr : *vOutput.attrs)
                                 checkNixOSConfiguration(fmt("%s.%s", name, state->symbols[attr.name]),
                                     *attr.value, attr.pos);
@@ -637,14 +620,14 @@ struct CmdFlakeCheck : FlakeCommand
                             checkTemplate(name, vOutput, pos);
 
                         else if (name == "templates") {
-                            state->forceAttrs(vOutput, pos);
+                            state->forceAttrs(vOutput, pos, "");
                             for (auto & attr : *vOutput.attrs)
                                 checkTemplate(fmt("%s.%s", name, state->symbols[attr.name]),
                                     *attr.value, attr.pos);
                         }
 
                         else if (name == "defaultBundler") {
-                            state->forceAttrs(vOutput, pos);
+                            state->forceAttrs(vOutput, pos, "");
                             for (auto & attr : *vOutput.attrs) {
                                 const auto & attr_name = state->symbols[attr.name];
                                 checkSystemName(attr_name, attr.pos);
@@ -655,11 +638,11 @@ struct CmdFlakeCheck : FlakeCommand
                         }
 
                         else if (name == "bundlers") {
-                            state->forceAttrs(vOutput, pos);
+                            state->forceAttrs(vOutput, pos, "");
                             for (auto & attr : *vOutput.attrs) {
                                 const auto & attr_name = state->symbols[attr.name];
                                 checkSystemName(attr_name, attr.pos);
-                                state->forceAttrs(*attr.value, attr.pos);
+                                state->forceAttrs(*attr.value, attr.pos, "");
                                 for (auto & attr2 : *attr.value->attrs) {
                                     checkBundler(
                                         fmt("%s.%s.%s", name, attr_name, state->symbols[attr2.name]),

src/nix/main.cc

@@ -199,7 +199,7 @@ static void showHelp(std::vector<std::string> subcommand, MultiCommand & topleve
     if (!attr)
         throw UsageError("Nix has no subcommand '%s'", concatStringsSep("", subcommand));
 
-    auto markdown = state.forceString(*attr->value);
+    auto markdown = state.forceString(*attr->value, noPos, "while evaluating the lowdown help text");
 
     RunPager pager;
     std::cout << renderMarkdownToTerminal(markdown) << "\n";

src/nix/nix.md

@@ -115,12 +115,11 @@ the Nix store. Here are the recognised types of installables:
 
 * **Store derivations**: `/nix/store/p7gp6lxdg32h4ka1q398wd9r2zkbbz2v-hello-2.10.drv`
 
-  Store derivations are store paths with extension `.drv` and are a
-  low-level representation of a build-time dependency graph used
-  internally by Nix. By default, if you pass a store derivation to a
-  `nix` subcommand, it will operate on the *output paths* of the
-  derivation. For example, `nix path-info` prints information about
-  the output paths:
+  By default, if you pass a [store derivation] path to a `nix` subcommand, the command will operate on the [output path]s of the derivation.
+
+  [output path]: ../../glossary.md#gloss-output-path
+
+  For example, `nix path-info` prints information about the output paths:
 
   ```console
   # nix path-info --json /nix/store/p7gp6lxdg32h4ka1q398wd9r2zkbbz2v-hello-2.10.drv
@@ -202,9 +201,11 @@ operate are determined as follows:
   a command like `nix shell nixpkgs#libxml2` will provide only those
   two outputs by default.
 
-  Note that a store derivation (given by `.drv` file store path) doesn't have
+  Note that a [store derivation] (given by its `.drv` file store path) doesn't have
   any attributes like `meta`, and thus this case doesn't apply to it.
 
+  [store derivation]: ../../glossary.md#gloss-store-derivation
+
 * Otherwise, Nix will use all outputs of the derivation.
 
 # Nix stores

src/nix/path-info.md

@@ -68,7 +68,9 @@ R""(
   ]
   ```
 
-* Print the path of the store derivation produced by `nixpkgs#hello`:
+* Print the path of the [store derivation] produced by `nixpkgs#hello`:
+
+  [store derivation]: ../../glossary.md#gloss-store-derivation
 
   ```console
   # nix path-info --derivation nixpkgs#hello

src/nix/prefetch.cc

@@ -28,17 +28,17 @@ std::string resolveMirrorUrl(EvalState & state, const std::string & url)
     Value vMirrors;
     // FIXME: use nixpkgs flake
     state.eval(state.parseExprFromString("import <nixpkgs/pkgs/build-support/fetchurl/mirrors.nix>", "."), vMirrors);
-    state.forceAttrs(vMirrors, noPos);
+    state.forceAttrs(vMirrors, noPos, "while evaluating the set of all mirrors");
 
     auto mirrorList = vMirrors.attrs->find(state.symbols.create(mirrorName));
     if (mirrorList == vMirrors.attrs->end())
         throw Error("unknown mirror name '%s'", mirrorName);
-    state.forceList(*mirrorList->value, noPos);
+    state.forceList(*mirrorList->value, noPos, "while evaluating one mirror configuration");
 
     if (mirrorList->value->listSize() < 1)
         throw Error("mirror URL '%s' did not expand to anything", url);
 
-    std::string mirror(state.forceString(*mirrorList->value->listElems()[0]));
+    std::string mirror(state.forceString(*mirrorList->value->listElems()[0], noPos, "while evaluating the first available mirror"));
     return mirror + (hasSuffix(mirror, "/") ? "" : "/") + s.substr(p + 1);
 }
 
@@ -196,29 +196,29 @@ static int main_nix_prefetch_url(int argc, char * * argv)
             Value vRoot;
             state->evalFile(path, vRoot);
             Value & v(*findAlongAttrPath(*state, attrPath, autoArgs, vRoot).first);
-            state->forceAttrs(v, noPos);
+            state->forceAttrs(v, noPos, "while evaluating the source attribute to prefetch");
 
             /* Extract the URL. */
             auto * attr = v.attrs->get(state->symbols.create("urls"));
             if (!attr)
                 throw Error("attribute 'urls' missing");
-            state->forceList(*attr->value, noPos);
+            state->forceList(*attr->value, noPos, "while evaluating the urls to prefetch");
             if (attr->value->listSize() < 1)
                 throw Error("'urls' list is empty");
-            url = state->forceString(*attr->value->listElems()[0]);
+            url = state->forceString(*attr->value->listElems()[0], noPos, "while evaluating the first url from the urls list");
 
             /* Extract the hash mode. */
             auto attr2 = v.attrs->get(state->symbols.create("outputHashMode"));
             if (!attr2)
                 printInfo("warning: this does not look like a fetchurl call");
             else
-                unpack = state->forceString(*attr2->value) == "recursive";
+                unpack = state->forceString(*attr2->value, noPos, "while evaluating the outputHashMode of the source to prefetch") == "recursive";
 
             /* Extract the name. */
             if (!name) {
                 auto attr3 = v.attrs->get(state->symbols.create("name"));
                 if (!attr3)
-                    name = state->forceString(*attr3->value);
+                    name = state->forceString(*attr3->value, noPos, "while evaluating the name of the source to prefetch");
             }
         }
 

src/nix/run.cc

@@ -9,6 +9,7 @@
 #include "fs-accessor.hh"
 #include "progress-bar.hh"
 #include "eval.hh"
+#include "build/personality.hh"
 
 #if __linux__
 #include <sys/mount.h>
@@ -24,7 +25,8 @@ namespace nix {
 
 void runProgramInStore(ref<Store> store,
     const std::string & program,
-    const Strings & args)
+    const Strings & args,
+    std::optional<std::string_view> system)
 {
     stopProgressBar();
 
@@ -44,7 +46,7 @@ void runProgramInStore(ref<Store> store,
         throw Error("store '%s' is not a local store so it does not support command execution", store->getUri());
 
     if (store->storeDir != store2->getRealStoreDir()) {
-        Strings helperArgs = { chrootHelperName, store->storeDir, store2->getRealStoreDir(), program };
+        Strings helperArgs = { chrootHelperName, store->storeDir, store2->getRealStoreDir(), std::string(system.value_or("")), program };
         for (auto & arg : args) helperArgs.push_back(arg);
 
         execv(getSelfExe().value_or("nix").c_str(), stringsToCharPtrs(helperArgs).data());
@@ -52,6 +54,9 @@ void runProgramInStore(ref<Store> store,
         throw SysError("could not execute chroot helper");
     }
 
+    if (system)
+        setPersonality(*system);
+
     execvp(program.c_str(), stringsToCharPtrs(args).data());
 
     throw SysError("unable to execute '%s'", program);
@@ -241,6 +246,7 @@ void chrootHelper(int argc, char * * argv)
     int p = 1;
     std::string storeDir = argv[p++];
     std::string realStoreDir = argv[p++];
+    std::string system = argv[p++];
     std::string cmd = argv[p++];
     Strings args;
     while (p < argc)
@@ -304,6 +310,9 @@ void chrootHelper(int argc, char * * argv)
     writeFile("/proc/self/uid_map", fmt("%d %d %d", uid, uid, 1));
     writeFile("/proc/self/gid_map", fmt("%d %d %d", gid, gid, 1));
 
+    if (system != "")
+        setPersonality(system);
+
     execvp(cmd.c_str(), stringsToCharPtrs(args).data());
 
     throw SysError("unable to exec '%s'", cmd);

src/nix/run.hh

@@ -6,6 +6,7 @@ namespace nix {
 
 void runProgramInStore(ref<Store> store,
     const std::string & program,
-    const Strings & args);
+    const Strings & args,
+    std::optional<std::string_view> system = std::nullopt);
 
 }

src/nix/show-derivation.md

@@ -2,9 +2,11 @@ R""(
 
 # Examples
 
-* Show the store derivation that results from evaluating the Hello
+* Show the [store derivation] that results from evaluating the Hello
   package:
 
+  [store derivation]: ../../glossary.md#gloss-store-derivation
+
   ```console
   # nix show-derivation nixpkgs#hello
   {
@@ -37,7 +39,7 @@ R""(
 # Description
 
 This command prints on standard output a JSON representation of the
-store derivations to which *installables* evaluate. Store derivations
+[store derivation]s to which *installables* evaluate. Store derivations
 are used internally by Nix. They are store paths with extension `.drv`
 that represent the build-time dependency graph to which a Nix
 expression evaluates.

src/nix/store-copy-log.md

@@ -18,7 +18,9 @@ R""(
   (The flag `--substituters ''` avoids querying
   `https://cache.nixos.org` for the log.)
 
-* To copy the log for a specific store derivation via SSH:
+* To copy the log for a specific [store derivation] via SSH:
+
+  [store derivation]: ../../glossary.md#gloss-store-derivation
 
   ```console
   # nix store copy-log --to ssh-ng://machine /nix/store/ilgm50plpmcgjhcp33z6n4qbnpqfhxym-glibc-2.33-59.drv

src/nix/upgrade-nix.cc

@@ -144,7 +144,7 @@ struct CmdUpgradeNix : MixDryRun, StoreCommand
         Bindings & bindings(*state->allocBindings(0));
         auto v2 = findAlongAttrPath(*state, settings.thisSystem, bindings, *v).first;
 
-        return store->parseStorePath(state->forceString(*v2));
+        return store->parseStorePath(state->forceString(*v2, noPos, "while evaluating the path tho latest nix version"));
     }
 };
 

src/nix/why-depends.cc

@@ -95,23 +95,13 @@ struct CmdWhyDepends : SourceExprCommand
          * to build.
          */
         auto dependency = parseInstallable(store, _dependency);
-        auto derivedDependency = dependency->toDerivedPath();
-        auto optDependencyPath = std::visit(overloaded {
-            [](const DerivedPath::Opaque & nodrv) -> std::optional<StorePath> {
-                return { nodrv.path };
-            },
-            [&](const DerivedPath::Built & hasdrv) -> std::optional<StorePath> {
-                if (hasdrv.outputs.size() != 1) {
-                    throw Error("argument '%s' should evaluate to one store path", dependency->what());
+        auto optDependencyPath = [&]() -> std::optional<StorePath> {
+            try {
+                return {Installable::toStorePath(getEvalStore(), store, Realise::Derivation, operateOn, dependency)};
+            } catch (MissingRealisation &) {
+                return std::nullopt;
             }
-                auto outputMap = store->queryPartialDerivationOutputMap(hasdrv.drvPath);
-                auto maybePath = outputMap.find(*hasdrv.outputs.begin());
-                if (maybePath == outputMap.end()) {
-                    throw Error("unexpected end of iterator");
-                }
-                return maybePath->second;
-            },
-        }, derivedDependency.raw());
+        }();
 
         StorePathSet closure;
         store->computeFSClosure({packagePath}, closure, false, false);

tests/containers.nix

@@ -20,7 +20,7 @@ makeTest ({
               (import ./systemd-nspawn.nix { inherit nixpkgs; }).toplevel
             ];
           virtualisation.memorySize = 4096;
-          nix.binaryCaches = lib.mkForce [ ];
+          nix.settings.substituters = lib.mkForce [ ];
           nix.extraOptions =
             ''
               extra-experimental-features = nix-command auto-allocate-uids cgroups

tests/flakes/check.sh

@@ -41,9 +41,9 @@ nix flake check $flakeDir
 cat > $flakeDir/flake.nix <<EOF
 {
   outputs = { self }: {
-    nixosModules.foo = {
+    nixosModules.foo = assert false; {
       a.b.c = 123;
-      foo = assert false; true;
+      foo = true;
     };
   };
 }
@@ -63,18 +63,6 @@ EOF
 
 nix flake check $flakeDir
 
-cat > $flakeDir/flake.nix <<EOF
-{
-  outputs = { self }: {
-    nixosModule = { config, pkgs }: {
-      a.b.c = 123;
-    };
-  };
-}
-EOF
-
-(! nix flake check $flakeDir)
-
 cat > $flakeDir/flake.nix <<EOF
 {
   outputs = { self }: {

tests/github-flakes.nix

@@ -149,7 +149,7 @@ makeTest (
           virtualisation.diskSize = 2048;
           virtualisation.additionalPaths = [ pkgs.hello pkgs.fuse ];
           virtualisation.memorySize = 4096;
-          nix.binaryCaches = lib.mkForce [ ];
+          nix.settings.substituters = lib.mkForce [ ];
           nix.extraOptions = "experimental-features = nix-command flakes";
           networking.hosts.${(builtins.head nodes.github.config.networking.interfaces.eth1.ipv4.addresses).address} =
             [ "channels.nixos.org" "api.github.com" "github.com" ];

tests/lang/eval-okay-ind-string.nix

@@ -110,7 +110,7 @@ let
     And finally to interpret \n etc. as in a string: ''\n, ''\r, ''\t.
   '';
 
-  # Regression test: antiquotation in '${x}' should work, but didn't.
+  # Regression test: string interpolation in '${x}' should work, but didn't.
   s15 = let x = "bla"; in ''
     foo
     '${x}'

tests/lang/eval-okay-intersectAttrs.exp

@@ -0,0 +1 @@
+[ { } { a = 1; } { a = 1; } { a = "a"; } { m = 1; } { m = "m"; } { n = 1; } { n = "n"; } { n = 1; p = 2; } { n = "n"; p = "p"; } { n = 1; p = 2; } { n = "n"; p = "p"; } { a = "a"; b = "b"; c = "c"; d = "d"; e = "e"; f = "f"; g = "g"; h = "h"; i = "i"; j = "j"; k = "k"; l = "l"; m = "m"; n = "n"; o = "o"; p = "p"; q = "q"; r = "r"; s = "s"; t = "t"; u = "u"; v = "v"; w = "w"; x = "x"; y = "y"; z = "z"; } true ]

tests/lang/eval-okay-intersectAttrs.nix

@@ -0,0 +1,50 @@
+let
+  alphabet =
+  { a = "a";
+    b = "b";
+    c = "c";
+    d = "d";
+    e = "e";
+    f = "f";
+    g = "g";
+    h = "h";
+    i = "i";
+    j = "j";
+    k = "k";
+    l = "l";
+    m = "m";
+    n = "n";
+    o = "o";
+    p = "p";
+    q = "q";
+    r = "r";
+    s = "s";
+    t = "t";
+    u = "u";
+    v = "v";
+    w = "w";
+    x = "x";
+    y = "y";
+    z = "z";
+  };
+  foo = {
+    inherit (alphabet) f o b a r z q u x;
+    aa = throw "aa";
+  };
+  alphabetFail = builtins.mapAttrs throw alphabet;
+in
+[ (builtins.intersectAttrs { a = abort "l1"; } { b = abort "r1"; })
+  (builtins.intersectAttrs { a = abort "l2"; } { a = 1; })
+  (builtins.intersectAttrs alphabetFail { a = 1; })
+  (builtins.intersectAttrs  { a = abort "laa"; } alphabet)
+  (builtins.intersectAttrs alphabetFail { m = 1; })
+  (builtins.intersectAttrs  { m = abort "lam"; } alphabet)
+  (builtins.intersectAttrs alphabetFail { n = 1; })
+  (builtins.intersectAttrs  { n = abort "lan"; } alphabet)
+  (builtins.intersectAttrs alphabetFail { n = 1; p = 2; })
+  (builtins.intersectAttrs  { n = abort "lan2"; p = abort "lap"; } alphabet)
+  (builtins.intersectAttrs alphabetFail { n = 1; p = 2; })
+  (builtins.intersectAttrs  { n = abort "lan2"; p = abort "lap"; } alphabet)
+  (builtins.intersectAttrs alphabetFail alphabet)
+  (builtins.intersectAttrs alphabet foo == builtins.intersectAttrs foo alphabet)
+]

tests/local.mk

@@ -92,6 +92,7 @@ nix_tests = \
   fmt.sh \
   eval-store.sh \
   why-depends.sh \
+  ca/why-depends.sh \
   import-derivation.sh \
   ca/import-derivation.sh \
   nix_path.sh \
@@ -134,8 +135,6 @@ endif
 
 install-tests += $(foreach x, $(nix_tests), tests/$(x))
 
-tests-environment = NIX_REMOTE= $(bash) -e
-
 clean-files += $(d)/common.sh $(d)/config.nix $(d)/ca/config.nix
 
 test-deps += tests/common.sh tests/config.nix tests/ca/config.nix

tests/nix-copy-closure.nix

@@ -15,7 +15,7 @@ makeTest (let pkgA = pkgs.cowsay; pkgB = pkgs.wget; pkgC = pkgs.hello; pkgD = pk
         { config, lib, pkgs, ... }:
         { virtualisation.writableStore = true;
           virtualisation.additionalPaths = [ pkgA pkgD.drvPath ];
-          nix.binaryCaches = lib.mkForce [ ];
+          nix.settings.substituters = lib.mkForce [ ];
         };
 
       server =

tests/nss-preload.nix

@@ -98,9 +98,9 @@ rec {
         { address = "192.168.0.10"; prefixLength = 24; }
       ];
 
-      nix.sandboxPaths = lib.mkForce [];
-      nix.binaryCaches = lib.mkForce [];
-      nix.useSandbox = lib.mkForce true;
+      nix.settings.extra-sandbox-paths = lib.mkForce [];
+      nix.settings.substituters = lib.mkForce [];
+      nix.settings.sandbox = lib.mkForce true;
     };
   };
 

tests/remote-builds.nix

@@ -16,7 +16,7 @@ let
     { config, pkgs, ... }:
     { services.openssh.enable = true;
       virtualisation.writableStore = true;
-      nix.useSandbox = true;
+      nix.settings.sandbox = true;
     };
 
   # Trivial Nix expression to build remotely.
@@ -44,7 +44,7 @@ in
 
       client =
         { config, lib, pkgs, ... }:
-        { nix.maxJobs = 0; # force remote building
+        { nix.settings.max-jobs = 0; # force remote building
           nix.distributedBuilds = true;
           nix.buildMachines =
             [ { hostName = "builder1";
@@ -62,7 +62,7 @@ in
             ];
           virtualisation.writableStore = true;
           virtualisation.additionalPaths = [ config.system.build.extraUtils ];
-          nix.binaryCaches = lib.mkForce [ ];
+          nix.settings.substituters = lib.mkForce [ ];
           programs.ssh.extraConfig = "ConnectTimeout 30";
         };
     };

tests/setuid.nix

@@ -13,7 +13,7 @@ makeTest {
   nodes.machine =
     { config, lib, pkgs, ... }:
     { virtualisation.writableStore = true;
-      nix.binaryCaches = lib.mkForce [ ];
+      nix.settings.substituters = lib.mkForce [ ];
       nix.nixPath = [ "nixpkgs=${lib.cleanSource pkgs.path}" ];
       virtualisation.additionalPaths = [ pkgs.stdenv pkgs.pkgsi686Linux.stdenv ];
     };

tests/sourcehut-flakes.nix

@@ -108,7 +108,7 @@ makeTest (
             virtualisation.diskSize = 2048;
             virtualisation.additionalPaths = [ pkgs.hello pkgs.fuse ];
             virtualisation.memorySize = 4096;
-            nix.binaryCaches = lib.mkForce [ ];
+            nix.settings.substituters = lib.mkForce [ ];
             nix.extraOptions = ''
               experimental-features = nix-command flakes
               flake-registry = https://git.sr.ht/~NixOS/flake-registry/blob/master/flake-registry.json

tests/why-depends.sh

@@ -6,6 +6,9 @@ cp ./dependencies.nix ./dependencies.builder0.sh ./config.nix $TEST_HOME
 
 cd $TEST_HOME
 
+nix why-depends --derivation --file ./dependencies.nix input2_drv input1_drv
+nix why-depends --file ./dependencies.nix input2_drv input1_drv
+
 nix-build ./dependencies.nix -A input0_drv -o dep
 nix-build ./dependencies.nix -o toplevel