nix-super/doc/manual/src/release-notes/rl-2.21.md

# Release 2.21.0 (2024-03-11)

- CLI options `--arg-from-file` and `--arg-from-stdin` [#10122](https://github.com/NixOS/nix/pull/10122)

  The new CLI option `--arg-from-file` *name* *path* passes the contents
  of file *path* as a string value via the function argument *name* to a
  Nix expression. Similarly, the new option `--arg-from-stdin` *name*
  reads the contents of the string from standard input.

- Concise error printing in `nix repl` [#9928](https://github.com/NixOS/nix/pull/9928)

  Previously, if an element of a list or attribute set threw an error while
  evaluating, `nix repl` would print the entire error (including source location
  information) inline. This output was clumsy and difficult to parse:

  ```
  nix-repl> { err = builtins.throw "uh oh!"; }
  { err = «error:
         … while calling the 'throw' builtin
           at «string»:1:9:
              1| { err = builtins.throw "uh oh!"; }
               |         ^

         error: uh oh!»; }
  ```

  Now, only the error message is displayed, making the output much more readable.
  ```
  nix-repl> { err = builtins.throw "uh oh!"; }
  { err = «error: uh oh!»; }
  ```

  However, if the whole expression being evaluated throws an error, source
  locations and (if applicable) a stack trace are printed, just like you'd expect:

  ```
  nix-repl> builtins.throw "uh oh!"
  error:
         … while calling the 'throw' builtin
           at «string»:1:1:
              1| builtins.throw "uh oh!"
               | ^

         error: uh oh!
  ```

- `--debugger` can now access bindings from `let` expressions [#8827](https://github.com/NixOS/nix/issues/8827) [#9918](https://github.com/NixOS/nix/pull/9918)

  Breakpoints and errors in the bindings of a `let` expression can now access
  those bindings in the debugger. Previously, only the body of `let` expressions
  could access those bindings.

- Enter the `--debugger` when `builtins.trace` is called if `debugger-on-trace` is set [#9914](https://github.com/NixOS/nix/pull/9914)

  If the `debugger-on-trace` option is set and `--debugger` is given,
  `builtins.trace` calls will behave similarly to `builtins.break` and will enter
  the debug REPL. This is useful for determining where warnings are being emitted
  from.

- Debugger prints source position information [#9913](https://github.com/NixOS/nix/pull/9913)

  The `--debugger` now prints source location information, instead of the
  pointers of source location information. Before:

  ```
  nix-repl> :bt
  0: while evaluating the attribute 'python311.pythonForBuild.pkgs'
  0x600001522598
  ```

  After:

  ```
  0: while evaluating the attribute 'python311.pythonForBuild.pkgs'
  /nix/store/hg65h51xnp74ikahns9hyf3py5mlbbqq-source/overrides/default.nix:132:27

     131|
     132|       bootstrappingBase = pkgs.${self.python.pythonAttr}.pythonForBuild.pkgs;
        |                           ^
     133|     in
  ```

- The `--debugger` will start more reliably in `let` expressions and function calls [#6649](https://github.com/NixOS/nix/issues/6649) [#9917](https://github.com/NixOS/nix/pull/9917)

  Previously, if you attempted to evaluate this file with the debugger:

  ```nix
  let
    a = builtins.trace "before inner break" (
      builtins.break "hello"
    );
    b = builtins.trace "before outer break" (
      builtins.break a
    );
  in
    b
  ```

  Nix would correctly enter the debugger at `builtins.break a`, but if you asked
  it to `:continue`, it would skip over the `builtins.break "hello"` expression
  entirely.

  Now, Nix will correctly enter the debugger at both breakpoints.

- Fix a FOD sandbox escape

  Cooperating Nix derivations could send file descriptors to files in the Nix
  store to each other via Unix domain sockets in the abstract namespace. This
  allowed one derivation to modify the output of the other derivation, after Nix
  has registered the path as "valid" and immutable in the Nix database.
  In particular, this allowed the output of fixed-output derivations to be
  modified from their expected content.

  This isn't the case any more.

- Nested debuggers are no longer supported [#9920](https://github.com/NixOS/nix/pull/9920)

  Previously, evaluating an expression that throws an error in the debugger would
  enter a second, nested debugger:

  ```
  nix-repl> builtins.throw "what"
  error: what


  Starting REPL to allow you to inspect the current state of the evaluator.

  Welcome to Nix 2.18.1. Type :? for help.

  nix-repl>
  ```

  Now, it just prints the error message like `nix repl`:

  ```
  nix-repl> builtins.throw "what"
  error:
         … while calling the 'throw' builtin
           at «string»:1:1:
              1| builtins.throw "what"
               | ^

         error: what
  ```

- consistent order of lambda formals in printed expressions [#9874](https://github.com/NixOS/nix/pull/9874)

  Always print lambda formals in lexicographic order rather than the internal, creation-time based symbol order.
  This makes printed formals independent of the context they appear in.

- fix duplicate attribute error positions for `inherit` [#9874](https://github.com/NixOS/nix/pull/9874)

  When an inherit caused a duplicate attribute error the position of the error was not reported correctly, placing the error with the inherit itself or at the start of the bindings block instead of the offending attribute name.

- `inherit (x) ...` evaluates `x` only once [#9847](https://github.com/NixOS/nix/pull/9847)

  `inherit (x) a b ...` now evaluates the expression `x` only once for all inherited attributes rather than once for each inherited attribute.
  This does not usually have a measurable impact, but side-effects (such as `builtins.trace`) would be duplicated and expensive expressions (such as derivations) could cause a measurable slowdown.

- Functions are printed with more detail [#7145](https://github.com/NixOS/nix/issues/7145) [#9606](https://github.com/NixOS/nix/pull/9606)

  Functions and `builtins` are printed with more detail in `nix repl`, `nix
  eval`, `builtins.trace`, and most other places values are printed.

  Before:

  ```
  $ nix repl nixpkgs
  nix-repl> builtins.map
  «primop»

  nix-repl> builtins.map lib.id
  «primop-app»

  nix-repl> builtins.trace lib.id "my-value"
  trace: <LAMBDA>
  "my-value"

  $ nix eval --file functions.nix
  { id = <LAMBDA>; primop = <PRIMOP>; primop-app = <PRIMOP-APP>; }
  ```

  After:

  ```
  $ nix repl nixpkgs
  nix-repl> builtins.map
  «primop map»

  nix-repl> builtins.map lib.id
  «partially applied primop map»

  nix-repl> builtins.trace lib.id "my-value"
  trace: «lambda id @ /nix/store/8rrzq23h2zq7sv5l2vhw44kls5w0f654-source/lib/trivial.nix:26:5»
  "my-value"

  $ nix eval --file functions.nix
  { id = «lambda id @ /Users/wiggles/nix/functions.nix:2:8»; primop = «primop map»; primop-app = «partially applied primop map»; }
  ```

  This was actually released in Nix 2.20, but wasn't added to the release notes
  so we're announcing it here. The historical release notes have been updated as well.

  [type-error]: https://github.com/NixOS/nix/pull/9753
  [coercion-error]: https://github.com/NixOS/nix/pull/9754

- Store paths are allowed to start with `.` [#912](https://github.com/NixOS/nix/issues/912) [#9091](https://github.com/NixOS/nix/pull/9091) [#9095](https://github.com/NixOS/nix/pull/9095) [#9120](https://github.com/NixOS/nix/pull/9120) [#9121](https://github.com/NixOS/nix/pull/9121) [#9122](https://github.com/NixOS/nix/pull/9122) [#9130](https://github.com/NixOS/nix/pull/9130) [#9219](https://github.com/NixOS/nix/pull/9219) [#9224](https://github.com/NixOS/nix/pull/9224) [#9867](https://github.com/NixOS/nix/pull/9867)

  Leading periods were allowed by accident in Nix 2.4. The Nix team has considered this to be a bug, but this behavior has since been relied on by users, leading to unnecessary difficulties.
  From now on, leading periods are officially, definitively supported. The names `.` and `..` are disallowed, as well as those starting with `.-` or `..-`.

  Nix versions that denied leading periods are documented [in the issue](https://github.com/NixOS/nix/issues/912#issuecomment-1919583286).

- Nix commands respect Ctrl-C [#7245](https://github.com/NixOS/nix/issues/7245) [#6995](https://github.com/NixOS/nix/pull/6995) [#9687](https://github.com/NixOS/nix/pull/9687)

  Previously, many Nix commands would hang indefinitely if Ctrl-C was pressed
  while performing various operations (including `nix develop`, `nix flake
  update`, and so on). With several fixes to Nix's signal handlers, Nix commands
  will now exit quickly after Ctrl-C is pressed.

  This was actually released in Nix 2.20, but wasn't added to the release notes
  so we're announcing it here. The historical release notes have been updated as well.

- `nix repl` pretty-prints values [#9931](https://github.com/NixOS/nix/pull/9931)

  `nix repl` will now pretty-print values:

  ```
  {
    attrs = {
      a = {
        b = {
          c = { };
        };
      };
    };
    list = [ 1 ];
    list' = [
      1
      2
      3
    ];
  }
  ```

- Introduction of `--regex` and `--all` in `nix profile remove` and `nix profile upgrade` [#10166](https://github.com/NixOS/nix/pull/10166)

  Previously the command-line arguments for `nix profile remove` and `nix profile upgrade` matched the package entries using regular expression.
  For instance:

  ```
  nix profile remove '.*vim.*'
  ```

  This would remove all packages that contain `vim` in their name.

  In most cases, only singular package names were used to remove and upgrade packages. Mixing this with regular expressions sometimes lead to unintended behavior. For instance, `python3.1` could match `python311`.

  To avoid unintended behavior, the arguments are now only matching exact names.

  Matching using regular expressions is still possible by using the new `--regex` flag:

  ```
  nix profile remove --regex '.*vim.*'
  ```

  One of the most useful cases for using regular expressions was to upgrade all packages. This was previously accomplished by:

  ```
  nix profile upgrade '.*'
  ```

  With the introduction of the `--all` flag, this now becomes more straightforward:

  ```
  nix profile upgrade --all
  ```

- Visual clutter in `--debugger` is reduced [#9919](https://github.com/NixOS/nix/pull/9919)

  Before:
  ```
  info: breakpoint reached


  Starting REPL to allow you to inspect the current state of the evaluator.

  Welcome to Nix 2.20.0pre20231222_dirty. Type :? for help.

  nix-repl> :continue
  error: uh oh


  Starting REPL to allow you to inspect the current state of the evaluator.

  Welcome to Nix 2.20.0pre20231222_dirty. Type :? for help.

  nix-repl>
  ```

  After:

  ```
  info: breakpoint reached

  Nix 2.20.0pre20231222_dirty debugger
  Type :? for help.
  nix-repl> :continue
  error: uh oh

  nix-repl>
  ```

- `nix repl` now respects Ctrl-C while printing values [#9927](https://github.com/NixOS/nix/pull/9927)

  `nix repl` will now halt immediately when Ctrl-C is pressed while it's printing
  a value. This is useful if you got curious about what would happen if you
  printed all of Nixpkgs.

- Cycle detection in `nix repl` is simpler and more reliable [#8672](https://github.com/NixOS/nix/issues/8672) [#9926](https://github.com/NixOS/nix/pull/9926)

  The cycle detection in `nix repl`, `nix eval`, `builtins.trace`, and everywhere
  else values are printed is now simpler and matches the cycle detection in
  `nix-instantiate --eval` output.

  Before:

  ```
  nix eval --expr 'let self = { inherit self; }; in self'
  { self = { self = «repeated»; }; }
  ```

  After:

  ```
  { self = «repeated»; }
  ```

- In the debugger, `while evaluating the attribute` errors now include position information [#9915](https://github.com/NixOS/nix/pull/9915)

  Before:

  ```
  0: while evaluating the attribute 'python311.pythonForBuild.pkgs'
  0x600001522598
  ```

  After:

  ```
  0: while evaluating the attribute 'python311.pythonForBuild.pkgs'
  /nix/store/hg65h51xnp74ikahns9hyf3py5mlbbqq-source/overrides/default.nix:132:27

     131|
     132|       bootstrappingBase = pkgs.${self.python.pythonAttr}.pythonForBuild.pkgs;
        |                           ^
     133|     in
  ```

- Stack size is increased on macOS [#9860](https://github.com/NixOS/nix/pull/9860)

  Previously, Nix would set the stack size to 64MiB on Linux, but would leave the
  stack size set to the default (approximately 8KiB) on macOS. Now, the stack
  size is correctly set to 64MiB on macOS as well, which should reduce stack
  overflow segfaults in deeply-recursive Nix expressions.