Merge branch 'master' into lto

.version

@@ -1 +1 @@
-2.9.0
+2.10.0

doc/manual/anchors.jq

@@ -0,0 +1,31 @@
+"\\[\\]\\{#(?<anchor>[^\\}]+?)\\}" as $empty_anchor_regex |
+"\\[(?<text>[^\\]]+?)\\]\\{#(?<anchor>[^\\}]+?)\\}" as $anchor_regex |
+
+
+def transform_anchors_html:
+    . | gsub($empty_anchor_regex; "<a name=\"" + .anchor + "\"></a>")
+      | gsub($anchor_regex; "<a href=\"#" + .anchor + "\" id=\"" + .anchor + "\">" + .text + "</a>");
+
+
+def transform_anchors_strip:
+    . | gsub($empty_anchor_regex; "")
+      | gsub($anchor_regex; .text);
+
+
+def map_contents_recursively(transformer):
+    . + {
+        Chapter: (.Chapter + {
+            content: .Chapter.content | transformer,
+            sub_items: .Chapter.sub_items | map(map_contents_recursively(transformer)),
+        }),
+    };
+
+
+def process_command:
+    .[0] as $context |
+    .[1] as $body |
+    $body + {
+        sections: $body.sections | map(map_contents_recursively(if $context.renderer == "html" then transform_anchors_html else transform_anchors_strip end)),
+    };
+
+process_command

doc/manual/book.toml

@@ -1,2 +1,7 @@
 [output.html]
 additional-css = ["custom.css"]
+additional-js = ["redirects.js"]
+
+[preprocessor.anchors]
+renderers = ["html"]
+command = "jq --from-file doc/manual/anchors.jq"

doc/manual/local.mk

@@ -97,7 +97,7 @@ doc/manual/generated/man1/nix3-manpages: $(d)/src/command-ref/new-cli
 	done
 	@touch $@
 
-$(docdir)/manual/index.html: $(MANUAL_SRCS) $(d)/book.toml $(d)/custom.css $(d)/src/SUMMARY.md $(d)/src/command-ref/new-cli $(d)/src/command-ref/conf-file.md $(d)/src/expressions/builtins.md $(call rwildcard, $(d)/src, *.md)
+$(docdir)/manual/index.html: $(MANUAL_SRCS) $(d)/book.toml $(d)/anchors.jq $(d)/custom.css $(d)/src/SUMMARY.md $(d)/src/command-ref/new-cli $(d)/src/command-ref/conf-file.md $(d)/src/expressions/builtins.md $(call rwildcard, $(d)/src, *.md)
 	$(trace-gen) RUST_LOG=warn mdbook build doc/manual -d $(DESTDIR)$(docdir)/manual
 
 endif

doc/manual/redirects.js

@@ -0,0 +1,337 @@
+// Redirects from old DocBook manual.
+var redirects = {
+  "#part-advanced-topics": "advanced-topics/advanced-topics.html",
+  "#chap-tuning-cores-and-jobs": "advanced-topics/cores-vs-jobs.html",
+  "#chap-diff-hook": "advanced-topics/diff-hook.html",
+  "#check-dirs-are-unregistered": "advanced-topics/diff-hook.html#check-dirs-are-unregistered",
+  "#chap-distributed-builds": "advanced-topics/distributed-builds.html",
+  "#chap-post-build-hook": "advanced-topics/post-build-hook.html",
+  "#chap-post-build-hook-caveats": "advanced-topics/post-build-hook.html#implementation-caveats",
+  "#part-command-ref": "command-ref/command-ref.html",
+  "#conf-allow-import-from-derivation": "command-ref/conf-file.html#conf-allow-import-from-derivation",
+  "#conf-allow-new-privileges": "command-ref/conf-file.html#conf-allow-new-privileges",
+  "#conf-allowed-uris": "command-ref/conf-file.html#conf-allowed-uris",
+  "#conf-allowed-users": "command-ref/conf-file.html#conf-allowed-users",
+  "#conf-auto-optimise-store": "command-ref/conf-file.html#conf-auto-optimise-store",
+  "#conf-binary-cache-public-keys": "command-ref/conf-file.html#conf-binary-cache-public-keys",
+  "#conf-binary-caches": "command-ref/conf-file.html#conf-binary-caches",
+  "#conf-build-compress-log": "command-ref/conf-file.html#conf-build-compress-log",
+  "#conf-build-cores": "command-ref/conf-file.html#conf-build-cores",
+  "#conf-build-extra-chroot-dirs": "command-ref/conf-file.html#conf-build-extra-chroot-dirs",
+  "#conf-build-extra-sandbox-paths": "command-ref/conf-file.html#conf-build-extra-sandbox-paths",
+  "#conf-build-fallback": "command-ref/conf-file.html#conf-build-fallback",
+  "#conf-build-max-jobs": "command-ref/conf-file.html#conf-build-max-jobs",
+  "#conf-build-max-log-size": "command-ref/conf-file.html#conf-build-max-log-size",
+  "#conf-build-max-silent-time": "command-ref/conf-file.html#conf-build-max-silent-time",
+  "#conf-build-repeat": "command-ref/conf-file.html#conf-build-repeat",
+  "#conf-build-timeout": "command-ref/conf-file.html#conf-build-timeout",
+  "#conf-build-use-chroot": "command-ref/conf-file.html#conf-build-use-chroot",
+  "#conf-build-use-sandbox": "command-ref/conf-file.html#conf-build-use-sandbox",
+  "#conf-build-use-substitutes": "command-ref/conf-file.html#conf-build-use-substitutes",
+  "#conf-build-users-group": "command-ref/conf-file.html#conf-build-users-group",
+  "#conf-builders": "command-ref/conf-file.html#conf-builders",
+  "#conf-builders-use-substitutes": "command-ref/conf-file.html#conf-builders-use-substitutes",
+  "#conf-compress-build-log": "command-ref/conf-file.html#conf-compress-build-log",
+  "#conf-connect-timeout": "command-ref/conf-file.html#conf-connect-timeout",
+  "#conf-cores": "command-ref/conf-file.html#conf-cores",
+  "#conf-diff-hook": "command-ref/conf-file.html#conf-diff-hook",
+  "#conf-enforce-determinism": "command-ref/conf-file.html#conf-enforce-determinism",
+  "#conf-env-keep-derivations": "command-ref/conf-file.html#conf-env-keep-derivations",
+  "#conf-extra-binary-caches": "command-ref/conf-file.html#conf-extra-binary-caches",
+  "#conf-extra-platforms": "command-ref/conf-file.html#conf-extra-platforms",
+  "#conf-extra-sandbox-paths": "command-ref/conf-file.html#conf-extra-sandbox-paths",
+  "#conf-extra-substituters": "command-ref/conf-file.html#conf-extra-substituters",
+  "#conf-fallback": "command-ref/conf-file.html#conf-fallback",
+  "#conf-fsync-metadata": "command-ref/conf-file.html#conf-fsync-metadata",
+  "#conf-gc-keep-derivations": "command-ref/conf-file.html#conf-gc-keep-derivations",
+  "#conf-gc-keep-outputs": "command-ref/conf-file.html#conf-gc-keep-outputs",
+  "#conf-hashed-mirrors": "command-ref/conf-file.html#conf-hashed-mirrors",
+  "#conf-http-connections": "command-ref/conf-file.html#conf-http-connections",
+  "#conf-keep-build-log": "command-ref/conf-file.html#conf-keep-build-log",
+  "#conf-keep-derivations": "command-ref/conf-file.html#conf-keep-derivations",
+  "#conf-keep-env-derivations": "command-ref/conf-file.html#conf-keep-env-derivations",
+  "#conf-keep-outputs": "command-ref/conf-file.html#conf-keep-outputs",
+  "#conf-max-build-log-size": "command-ref/conf-file.html#conf-max-build-log-size",
+  "#conf-max-free": "command-ref/conf-file.html#conf-max-free",
+  "#conf-max-jobs": "command-ref/conf-file.html#conf-max-jobs",
+  "#conf-max-silent-time": "command-ref/conf-file.html#conf-max-silent-time",
+  "#conf-min-free": "command-ref/conf-file.html#conf-min-free",
+  "#conf-narinfo-cache-negative-ttl": "command-ref/conf-file.html#conf-narinfo-cache-negative-ttl",
+  "#conf-narinfo-cache-positive-ttl": "command-ref/conf-file.html#conf-narinfo-cache-positive-ttl",
+  "#conf-netrc-file": "command-ref/conf-file.html#conf-netrc-file",
+  "#conf-plugin-files": "command-ref/conf-file.html#conf-plugin-files",
+  "#conf-post-build-hook": "command-ref/conf-file.html#conf-post-build-hook",
+  "#conf-pre-build-hook": "command-ref/conf-file.html#conf-pre-build-hook",
+  "#conf-repeat": "command-ref/conf-file.html#conf-repeat",
+  "#conf-require-sigs": "command-ref/conf-file.html#conf-require-sigs",
+  "#conf-restrict-eval": "command-ref/conf-file.html#conf-restrict-eval",
+  "#conf-run-diff-hook": "command-ref/conf-file.html#conf-run-diff-hook",
+  "#conf-sandbox": "command-ref/conf-file.html#conf-sandbox",
+  "#conf-sandbox-dev-shm-size": "command-ref/conf-file.html#conf-sandbox-dev-shm-size",
+  "#conf-sandbox-paths": "command-ref/conf-file.html#conf-sandbox-paths",
+  "#conf-secret-key-files": "command-ref/conf-file.html#conf-secret-key-files",
+  "#conf-show-trace": "command-ref/conf-file.html#conf-show-trace",
+  "#conf-stalled-download-timeout": "command-ref/conf-file.html#conf-stalled-download-timeout",
+  "#conf-substitute": "command-ref/conf-file.html#conf-substitute",
+  "#conf-substituters": "command-ref/conf-file.html#conf-substituters",
+  "#conf-system": "command-ref/conf-file.html#conf-system",
+  "#conf-system-features": "command-ref/conf-file.html#conf-system-features",
+  "#conf-tarball-ttl": "command-ref/conf-file.html#conf-tarball-ttl",
+  "#conf-timeout": "command-ref/conf-file.html#conf-timeout",
+  "#conf-trace-function-calls": "command-ref/conf-file.html#conf-trace-function-calls",
+  "#conf-trusted-binary-caches": "command-ref/conf-file.html#conf-trusted-binary-caches",
+  "#conf-trusted-public-keys": "command-ref/conf-file.html#conf-trusted-public-keys",
+  "#conf-trusted-substituters": "command-ref/conf-file.html#conf-trusted-substituters",
+  "#conf-trusted-users": "command-ref/conf-file.html#conf-trusted-users",
+  "#extra-sandbox-paths": "command-ref/conf-file.html#extra-sandbox-paths",
+  "#sec-conf-file": "command-ref/conf-file.html",
+  "#env-NIX_PATH": "command-ref/env-common.html#env-NIX_PATH",
+  "#env-common": "command-ref/env-common.html",
+  "#envar-remote": "command-ref/env-common.html#env-NIX_REMOTE",
+  "#sec-common-env": "command-ref/env-common.html",
+  "#ch-files": "command-ref/files.html",
+  "#ch-main-commands": "command-ref/main-commands.html",
+  "#opt-out-link": "command-ref/nix-build.html#opt-out-link",
+  "#sec-nix-build": "command-ref/nix-build.html",
+  "#sec-nix-channel": "command-ref/nix-channel.html",
+  "#sec-nix-collect-garbage": "command-ref/nix-collect-garbage.html",
+  "#sec-nix-copy-closure": "command-ref/nix-copy-closure.html",
+  "#sec-nix-daemon": "command-ref/nix-daemon.html",
+  "#refsec-nix-env-install-examples": "command-ref/nix-env.html#examples",
+  "#rsec-nix-env-install": "command-ref/nix-env.html#operation---install",
+  "#rsec-nix-env-set": "command-ref/nix-env.html#operation---set",
+  "#rsec-nix-env-set-flag": "command-ref/nix-env.html#operation---set-flag",
+  "#rsec-nix-env-upgrade": "command-ref/nix-env.html#operation---upgrade",
+  "#sec-nix-env": "command-ref/nix-env.html",
+  "#ssec-version-comparisons": "command-ref/nix-env.html#versions",
+  "#sec-nix-hash": "command-ref/nix-hash.html",
+  "#sec-nix-instantiate": "command-ref/nix-instantiate.html",
+  "#sec-nix-prefetch-url": "command-ref/nix-prefetch-url.html",
+  "#sec-nix-shell": "command-ref/nix-shell.html",
+  "#ssec-nix-shell-shebang": "command-ref/nix-shell.html#use-as-a--interpreter",
+  "#nixref-queries": "command-ref/nix-store.html#queries",
+  "#opt-add-root": "command-ref/nix-store.html#opt-add-root",
+  "#refsec-nix-store-dump": "command-ref/nix-store.html#operation---dump",
+  "#refsec-nix-store-export": "command-ref/nix-store.html#operation---export",
+  "#refsec-nix-store-import": "command-ref/nix-store.html#operation---import",
+  "#refsec-nix-store-query": "command-ref/nix-store.html#operation---query",
+  "#refsec-nix-store-verify": "command-ref/nix-store.html#operation---verify",
+  "#rsec-nix-store-gc": "command-ref/nix-store.html#operation---gc",
+  "#rsec-nix-store-generate-binary-cache-key": "command-ref/nix-store.html#operation---generate-binary-cache-key",
+  "#rsec-nix-store-realise": "command-ref/nix-store.html#operation---realise",
+  "#rsec-nix-store-serve": "command-ref/nix-store.html#operation---serve",
+  "#sec-nix-store": "command-ref/nix-store.html",
+  "#opt-I": "command-ref/opt-common.html#opt-I",
+  "#opt-attr": "command-ref/opt-common.html#opt-attr",
+  "#opt-common": "command-ref/opt-common.html",
+  "#opt-cores": "command-ref/opt-common.html#opt-cores",
+  "#opt-log-format": "command-ref/opt-common.html#opt-log-format",
+  "#opt-max-jobs": "command-ref/opt-common.html#opt-max-jobs",
+  "#opt-max-silent-time": "command-ref/opt-common.html#opt-max-silent-time",
+  "#opt-timeout": "command-ref/opt-common.html#opt-timeout",
+  "#sec-common-options": "command-ref/opt-common.html",
+  "#ch-utilities": "command-ref/utilities.html",
+  "#chap-hacking": "contributing/hacking.html",
+  "#adv-attr-allowSubstitutes": "expressions/advanced-attributes.html#adv-attr-allowSubstitutes",
+  "#adv-attr-allowedReferences": "expressions/advanced-attributes.html#adv-attr-allowedReferences",
+  "#adv-attr-allowedRequisites": "expressions/advanced-attributes.html#adv-attr-allowedRequisites",
+  "#adv-attr-disallowedReferences": "expressions/advanced-attributes.html#adv-attr-disallowedReferences",
+  "#adv-attr-disallowedRequisites": "expressions/advanced-attributes.html#adv-attr-disallowedRequisites",
+  "#adv-attr-exportReferencesGraph": "expressions/advanced-attributes.html#adv-attr-exportReferencesGraph",
+  "#adv-attr-impureEnvVars": "expressions/advanced-attributes.html#adv-attr-impureEnvVars",
+  "#adv-attr-outputHash": "expressions/advanced-attributes.html#adv-attr-outputHash",
+  "#adv-attr-outputHashAlgo": "expressions/advanced-attributes.html#adv-attr-outputHashAlgo",
+  "#adv-attr-outputHashMode": "expressions/advanced-attributes.html#adv-attr-outputHashMode",
+  "#adv-attr-passAsFile": "expressions/advanced-attributes.html#adv-attr-passAsFile",
+  "#adv-attr-preferLocalBuild": "expressions/advanced-attributes.html#adv-attr-preferLocalBuild",
+  "#fixed-output-drvs": "expressions/advanced-attributes.html#adv-attr-outputHash",
+  "#sec-advanced-attributes": "expressions/advanced-attributes.html",
+  "#sec-arguments": "expressions/arguments-variables.html",
+  "#sec-build-script": "expressions/build-script.html",
+  "#builtin-abort": "expressions/builtins.html#builtins-abort",
+  "#builtin-add": "expressions/builtins.html#builtins-add",
+  "#builtin-all": "expressions/builtins.html#builtins-all",
+  "#builtin-any": "expressions/builtins.html#builtins-any",
+  "#builtin-attrNames": "expressions/builtins.html#builtins-attrNames",
+  "#builtin-attrValues": "expressions/builtins.html#builtins-attrValues",
+  "#builtin-baseNameOf": "expressions/builtins.html#builtins-baseNameOf",
+  "#builtin-bitAnd": "expressions/builtins.html#builtins-bitAnd",
+  "#builtin-bitOr": "expressions/builtins.html#builtins-bitOr",
+  "#builtin-bitXor": "expressions/builtins.html#builtins-bitXor",
+  "#builtin-builtins": "expressions/builtins.html#builtins-builtins",
+  "#builtin-compareVersions": "expressions/builtins.html#builtins-compareVersions",
+  "#builtin-concatLists": "expressions/builtins.html#builtins-concatLists",
+  "#builtin-concatStringsSep": "expressions/builtins.html#builtins-concatStringsSep",
+  "#builtin-currentSystem": "expressions/builtins.html#builtins-currentSystem",
+  "#builtin-deepSeq": "expressions/builtins.html#builtins-deepSeq",
+  "#builtin-derivation": "expressions/builtins.html#builtins-derivation",
+  "#builtin-dirOf": "expressions/builtins.html#builtins-dirOf",
+  "#builtin-div": "expressions/builtins.html#builtins-div",
+  "#builtin-elem": "expressions/builtins.html#builtins-elem",
+  "#builtin-elemAt": "expressions/builtins.html#builtins-elemAt",
+  "#builtin-fetchGit": "expressions/builtins.html#builtins-fetchGit",
+  "#builtin-fetchTarball": "expressions/builtins.html#builtins-fetchTarball",
+  "#builtin-fetchurl": "expressions/builtins.html#builtins-fetchurl",
+  "#builtin-filterSource": "expressions/builtins.html#builtins-filterSource",
+  "#builtin-foldl-prime": "expressions/builtins.html#builtins-foldl-prime",
+  "#builtin-fromJSON": "expressions/builtins.html#builtins-fromJSON",
+  "#builtin-functionArgs": "expressions/builtins.html#builtins-functionArgs",
+  "#builtin-genList": "expressions/builtins.html#builtins-genList",
+  "#builtin-getAttr": "expressions/builtins.html#builtins-getAttr",
+  "#builtin-getEnv": "expressions/builtins.html#builtins-getEnv",
+  "#builtin-hasAttr": "expressions/builtins.html#builtins-hasAttr",
+  "#builtin-hashFile": "expressions/builtins.html#builtins-hashFile",
+  "#builtin-hashString": "expressions/builtins.html#builtins-hashString",
+  "#builtin-head": "expressions/builtins.html#builtins-head",
+  "#builtin-import": "expressions/builtins.html#builtins-import",
+  "#builtin-intersectAttrs": "expressions/builtins.html#builtins-intersectAttrs",
+  "#builtin-isAttrs": "expressions/builtins.html#builtins-isAttrs",
+  "#builtin-isBool": "expressions/builtins.html#builtins-isBool",
+  "#builtin-isFloat": "expressions/builtins.html#builtins-isFloat",
+  "#builtin-isFunction": "expressions/builtins.html#builtins-isFunction",
+  "#builtin-isInt": "expressions/builtins.html#builtins-isInt",
+  "#builtin-isList": "expressions/builtins.html#builtins-isList",
+  "#builtin-isNull": "expressions/builtins.html#builtins-isNull",
+  "#builtin-isString": "expressions/builtins.html#builtins-isString",
+  "#builtin-length": "expressions/builtins.html#builtins-length",
+  "#builtin-lessThan": "expressions/builtins.html#builtins-lessThan",
+  "#builtin-listToAttrs": "expressions/builtins.html#builtins-listToAttrs",
+  "#builtin-map": "expressions/builtins.html#builtins-map",
+  "#builtin-match": "expressions/builtins.html#builtins-match",
+  "#builtin-mul": "expressions/builtins.html#builtins-mul",
+  "#builtin-parseDrvName": "expressions/builtins.html#builtins-parseDrvName",
+  "#builtin-path": "expressions/builtins.html#builtins-path",
+  "#builtin-pathExists": "expressions/builtins.html#builtins-pathExists",
+  "#builtin-placeholder": "expressions/builtins.html#builtins-placeholder",
+  "#builtin-readDir": "expressions/builtins.html#builtins-readDir",
+  "#builtin-readFile": "expressions/builtins.html#builtins-readFile",
+  "#builtin-removeAttrs": "expressions/builtins.html#builtins-removeAttrs",
+  "#builtin-replaceStrings": "expressions/builtins.html#builtins-replaceStrings",
+  "#builtin-seq": "expressions/builtins.html#builtins-seq",
+  "#builtin-sort": "expressions/builtins.html#builtins-sort",
+  "#builtin-split": "expressions/builtins.html#builtins-split",
+  "#builtin-splitVersion": "expressions/builtins.html#builtins-splitVersion",
+  "#builtin-stringLength": "expressions/builtins.html#builtins-stringLength",
+  "#builtin-sub": "expressions/builtins.html#builtins-sub",
+  "#builtin-substring": "expressions/builtins.html#builtins-substring",
+  "#builtin-tail": "expressions/builtins.html#builtins-tail",
+  "#builtin-throw": "expressions/builtins.html#builtins-throw",
+  "#builtin-toFile": "expressions/builtins.html#builtins-toFile",
+  "#builtin-toJSON": "expressions/builtins.html#builtins-toJSON",
+  "#builtin-toPath": "expressions/builtins.html#builtins-toPath",
+  "#builtin-toString": "expressions/builtins.html#builtins-toString",
+  "#builtin-toXML": "expressions/builtins.html#builtins-toXML",
+  "#builtin-trace": "expressions/builtins.html#builtins-trace",
+  "#builtin-tryEval": "expressions/builtins.html#builtins-tryEval",
+  "#builtin-typeOf": "expressions/builtins.html#builtins-typeOf",
+  "#ssec-builtins": "expressions/builtins.html",
+  "#attr-system": "expressions/derivations.html#attr-system",
+  "#ssec-derivation": "expressions/derivations.html",
+  "#ch-expression-language": "expressions/expression-language.html",
+  "#sec-expression-syntax": "expressions/expression-syntax.html",
+  "#sec-generic-builder": "expressions/generic-builder.html",
+  "#sec-constructs": "expressions/language-constructs.html",
+  "#sect-let-expressions": "expressions/language-constructs.html#let-expressions",
+  "#ss-functions": "expressions/language-constructs.html#functions",
+  "#sec-language-operators": "expressions/language-operators.html",
+  "#table-operators": "expressions/language-operators.html",
+  "#ssec-values": "expressions/language-values.html",
+  "#sec-building-simple": "expressions/simple-building-testing.html",
+  "#ch-simple-expression": "expressions/simple-expression.html",
+  "#chap-writing-nix-expressions": "expressions/writing-nix-expressions.html",
+  "#gloss-closure": "glossary.html#gloss-closure",
+  "#gloss-derivation": "glossary.html#gloss-derivation",
+  "#gloss-deriver": "glossary.html#gloss-deriver",
+  "#gloss-nar": "glossary.html#gloss-nar",
+  "#gloss-output-path": "glossary.html#gloss-output-path",
+  "#gloss-profile": "glossary.html#gloss-profile",
+  "#gloss-reachable": "glossary.html#gloss-reachable",
+  "#gloss-reference": "glossary.html#gloss-reference",
+  "#gloss-substitute": "glossary.html#gloss-substitute",
+  "#gloss-user-env": "glossary.html#gloss-user-env",
+  "#gloss-validity": "glossary.html#gloss-validity",
+  "#part-glossary": "glossary.html",
+  "#sec-building-source": "installation/building-source.html",
+  "#ch-env-variables": "installation/env-variables.html",
+  "#sec-installer-proxy-settings": "installation/env-variables.html#proxy-environment-variables",
+  "#sec-nix-ssl-cert-file": "installation/env-variables.html#nix_ssl_cert_file",
+  "#sec-nix-ssl-cert-file-with-nix-daemon-and-macos": "installation/env-variables.html#nix_ssl_cert_file-with-macos-and-the-nix-daemon",
+  "#chap-installation": "installation/installation.html",
+  "#ch-installing-binary": "installation/installing-binary.html",
+  "#sect-macos-installation": "installation/installing-binary.html#macos-installation",
+  "#sect-macos-installation-change-store-prefix": "installation/installing-binary.html#macos-installation",
+  "#sect-macos-installation-encrypted-volume": "installation/installing-binary.html#macos-installation",
+  "#sect-macos-installation-recommended-notes": "installation/installing-binary.html#macos-installation",
+  "#sect-macos-installation-symlink": "installation/installing-binary.html#macos-installation",
+  "#sect-multi-user-installation": "installation/installing-binary.html#multi-user-installation",
+  "#sect-nix-install-binary-tarball": "installation/installing-binary.html#installing-from-a-binary-tarball",
+  "#sect-nix-install-pinned-version-url": "installation/installing-binary.html#installing-a-pinned-nix-version-from-a-url",
+  "#sect-single-user-installation": "installation/installing-binary.html#single-user-installation",
+  "#ch-installing-source": "installation/installing-source.html",
+  "#ssec-multi-user": "installation/multi-user.html",
+  "#ch-nix-security": "installation/nix-security.html",
+  "#sec-obtaining-source": "installation/obtaining-source.html",
+  "#sec-prerequisites-source": "installation/prerequisites-source.html",
+  "#sec-single-user": "installation/single-user.html",
+  "#ch-supported-platforms": "installation/supported-platforms.html",
+  "#ch-upgrading-nix": "installation/upgrading.html",
+  "#ch-about-nix": "introduction.html",
+  "#chap-introduction": "introduction.html",
+  "#ch-basic-package-mgmt": "package-management/basic-package-mgmt.html",
+  "#ssec-binary-cache-substituter": "package-management/binary-cache-substituter.html",
+  "#sec-channels": "package-management/channels.html",
+  "#ssec-copy-closure": "package-management/copy-closure.html",
+  "#sec-garbage-collection": "package-management/garbage-collection.html",
+  "#ssec-gc-roots": "package-management/garbage-collector-roots.html",
+  "#chap-package-management": "package-management/package-management.html",
+  "#sec-profiles": "package-management/profiles.html",
+  "#ssec-s3-substituter": "package-management/s3-substituter.html",
+  "#ssec-s3-substituter-anonymous-reads": "package-management/s3-substituter.html#anonymous-reads-to-your-s3-compatible-binary-cache",
+  "#ssec-s3-substituter-authenticated-reads": "package-management/s3-substituter.html#authenticated-reads-to-your-s3-binary-cache",
+  "#ssec-s3-substituter-authenticated-writes": "package-management/s3-substituter.html#authenticated-writes-to-your-s3-compatible-binary-cache",
+  "#sec-sharing-packages": "package-management/sharing-packages.html",
+  "#ssec-ssh-substituter": "package-management/ssh-substituter.html",
+  "#chap-quick-start": "quick-start.html",
+  "#sec-relnotes": "release-notes/release-notes.html",
+  "#ch-relnotes-0.10.1": "release-notes/rl-0.10.1.html",
+  "#ch-relnotes-0.10": "release-notes/rl-0.10.html",
+  "#ssec-relnotes-0.11": "release-notes/rl-0.11.html",
+  "#ssec-relnotes-0.12": "release-notes/rl-0.12.html",
+  "#ssec-relnotes-0.13": "release-notes/rl-0.13.html",
+  "#ssec-relnotes-0.14": "release-notes/rl-0.14.html",
+  "#ssec-relnotes-0.15": "release-notes/rl-0.15.html",
+  "#ssec-relnotes-0.16": "release-notes/rl-0.16.html",
+  "#ch-relnotes-0.5": "release-notes/rl-0.5.html",
+  "#ch-relnotes-0.6": "release-notes/rl-0.6.html",
+  "#ch-relnotes-0.7": "release-notes/rl-0.7.html",
+  "#ch-relnotes-0.8.1": "release-notes/rl-0.8.1.html",
+  "#ch-relnotes-0.8": "release-notes/rl-0.8.html",
+  "#ch-relnotes-0.9.1": "release-notes/rl-0.9.1.html",
+  "#ch-relnotes-0.9.2": "release-notes/rl-0.9.2.html",
+  "#ch-relnotes-0.9": "release-notes/rl-0.9.html",
+  "#ssec-relnotes-1.0": "release-notes/rl-1.0.html",
+  "#ssec-relnotes-1.1": "release-notes/rl-1.1.html",
+  "#ssec-relnotes-1.10": "release-notes/rl-1.10.html",
+  "#ssec-relnotes-1.11.10": "release-notes/rl-1.11.10.html",
+  "#ssec-relnotes-1.11": "release-notes/rl-1.11.html",
+  "#ssec-relnotes-1.2": "release-notes/rl-1.2.html",
+  "#ssec-relnotes-1.3": "release-notes/rl-1.3.html",
+  "#ssec-relnotes-1.4": "release-notes/rl-1.4.html",
+  "#ssec-relnotes-1.5.1": "release-notes/rl-1.5.1.html",
+  "#ssec-relnotes-1.5.2": "release-notes/rl-1.5.2.html",
+  "#ssec-relnotes-1.5": "release-notes/rl-1.5.html",
+  "#ssec-relnotes-1.6.1": "release-notes/rl-1.6.1.html",
+  "#ssec-relnotes-1.6.0": "release-notes/rl-1.6.html",
+  "#ssec-relnotes-1.7": "release-notes/rl-1.7.html",
+  "#ssec-relnotes-1.8": "release-notes/rl-1.8.html",
+  "#ssec-relnotes-1.9": "release-notes/rl-1.9.html",
+  "#ssec-relnotes-2.0": "release-notes/rl-2.0.html",
+  "#ssec-relnotes-2.1": "release-notes/rl-2.1.html",
+  "#ssec-relnotes-2.2": "release-notes/rl-2.2.html",
+  "#ssec-relnotes-2.3": "release-notes/rl-2.3.html"
+};
+
+var isRoot = (document.location.pathname.endsWith('/') || document.location.pathname.endsWith('/index.html')) && path_to_root === '';
+if (isRoot && redirects[document.location.hash]) {
+  document.location.href = path_to_root + redirects[document.location.hash];
+}

doc/manual/src/SUMMARY.md.in

@@ -72,6 +72,7 @@
   - [CLI guideline](contributing/cli-guideline.md)
 - [Release Notes](release-notes/release-notes.md)
   - [Release X.Y (202?-??-??)](release-notes/rl-next.md)
+  - [Release 2.9 (2022-05-30)](release-notes/rl-2.9.md)
   - [Release 2.8 (2022-04-19)](release-notes/rl-2.8.md)
   - [Release 2.7 (2022-03-07)](release-notes/rl-2.7.md)
   - [Release 2.6 (2022-01-24)](release-notes/rl-2.6.md)

doc/manual/src/advanced-topics/diff-hook.md

@@ -101,7 +101,7 @@ In particular, notice the
 `/nix/store/krpqk0l9ib0ibi1d2w52z293zw455cap-unstable.check` output. Nix
 has copied the build results to that directory where you can examine it.
 
-> **Note**
+> []{#check-dirs-are-unregistered} **Note**
 > 
 > Check paths are not protected against garbage collection, and this
 > path will be deleted on the next garbage collection.

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

@@ -2,11 +2,11 @@
 
 Most Nix commands interpret the following environment variables:
 
-  - `IN_NIX_SHELL`\
+  - [`IN_NIX_SHELL`]{#env-IN_NIX_SHELL}\
     Indicator that tells if the current environment was set up by
-    `nix-shell`. Since Nix 2.0 the values are `"pure"` and `"impure"`
+    `nix-shell`. It can have the values `pure` or `impure`.
 
-  - `NIX_PATH`\
+  - [`NIX_PATH`]{#env-NIX_PATH}\
     A colon-separated list of directories used to look up Nix
     expressions enclosed in angle brackets (i.e., `<path>`). For
     instance, the value
@@ -44,7 +44,7 @@ Most Nix commands interpret the following environment variables:
     The Nix search path can also be extended using the `-I` option to
     many Nix commands, which takes precedence over `NIX_PATH`.
 
-  - `NIX_IGNORE_SYMLINK_STORE`\
+  - [`NIX_IGNORE_SYMLINK_STORE`]{#env-NIX_IGNORE_SYMLINK_STORE}\
     Normally, the Nix store directory (typically `/nix/store`) is not
     allowed to contain any symlink components. This is to prevent
     “impure” builds. Builders sometimes “canonicalise” paths by
@@ -66,41 +66,41 @@ Most Nix commands interpret the following environment variables:
 
     Consult the mount 8 manual page for details.
 
-  - `NIX_STORE_DIR`\
+  - [`NIX_STORE_DIR`]{#env-NIX_STORE_DIR}\
     Overrides the location of the Nix store (default `prefix/store`).
 
-  - `NIX_DATA_DIR`\
+  - [`NIX_DATA_DIR`]{#env-NIX_DATA_DIR}\
     Overrides the location of the Nix static data directory (default
     `prefix/share`).
 
-  - `NIX_LOG_DIR`\
+  - [`NIX_LOG_DIR`]{#env-NIX_LOG_DIR}\
     Overrides the location of the Nix log directory (default
     `prefix/var/log/nix`).
 
-  - `NIX_STATE_DIR`\
+  - [`NIX_STATE_DIR`]{#env-NIX_STATE_DIR}\
     Overrides the location of the Nix state directory (default
     `prefix/var/nix`).
 
-  - `NIX_CONF_DIR`\
+  - [`NIX_CONF_DIR`]{#env-NIX_CONF_DIR}\
     Overrides the location of the system Nix configuration directory
     (default `prefix/etc/nix`).
 
-  - `NIX_CONFIG`\
+  - [`NIX_CONFIG`]{#env-NIX_CONFIG}\
     Applies settings from Nix configuration from the environment.
     The content is treated as if it was read from a Nix configuration file.
     Settings are separated by the newline character.
 
-  - `NIX_USER_CONF_FILES`\
+  - [`NIX_USER_CONF_FILES`]{#env-NIX_USER_CONF_FILES}\
     Overrides the location of the user Nix configuration files to load
     from (defaults to the XDG spec locations). The variable is treated
     as a list separated by the `:` token.
 
-  - `TMPDIR`\
+  - [`TMPDIR`]{#env-TMPDIR}\
     Use the specified directory to store temporary files. In particular,
     this includes temporary build directories; these can take up
     substantial amounts of disk space. The default is `/tmp`.
 
-  - `NIX_REMOTE`\
+  - [`NIX_REMOTE`]{#env-NIX_REMOTE}\
     This variable should be set to `daemon` if you want to use the Nix
     daemon to execute Nix operations. This is necessary in [multi-user
     Nix installations](../installation/multi-user.md). If the Nix
@@ -108,16 +108,16 @@ Most Nix commands interpret the following environment variables:
     should be set to `unix://path/to/socket`. Otherwise, it should be
     left unset.
 
-  - `NIX_SHOW_STATS`\
+  - [`NIX_SHOW_STATS`]{#env-NIX_SHOW_STATS}\
     If set to `1`, Nix will print some evaluation statistics, such as
     the number of values allocated.
 
-  - `NIX_COUNT_CALLS`\
+  - [`NIX_COUNT_CALLS`]{#env-NIX_COUNT_CALLS}\
     If set to `1`, Nix will print how often functions were called during
     Nix expression evaluation. This is useful for profiling your Nix
     expressions.
 
-  - `GC_INITIAL_HEAP_SIZE`\
+  - [`GC_INITIAL_HEAP_SIZE`]{#env-GC_INITIAL_HEAP_SIZE}\
     If Nix has been configured to use the Boehm garbage collector, this
     variable sets the initial size of the heap in bytes. It defaults to
     384 MiB. Setting it to a low value reduces memory consumption, but

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

@@ -47,16 +47,16 @@ All options not listed here are passed to `nix-store
 --realise`, except for `--arg` and `--attr` / `-A` which are passed to
 `nix-instantiate`.
 
-  - `--no-out-link`\
+  - [`--no-out-link`]{#opt-no-out-link}\
     Do not create a symlink to the output path. Note that as a result
     the output does not become a root of the garbage collector, and so
     might be deleted by `nix-store
                     --gc`.
 
-  - `--dry-run`\
+  - [`--dry-run`]{#opt-dry-run}\
     Show what store paths would be built or downloaded.
 
-  - `--out-link` / `-o` *outlink*\
+  - [`--out-link`]{#opt-out-link} / `-o` *outlink*\
     Change the name of the symlink to the output path created from
     `result` to *outlink*.
 

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

@@ -22,7 +22,7 @@ This section lists the options that are common to all operations. These
 options are allowed for every subcommand, though they may not always
 have an effect.
 
-  - `--add-root` *path*\
+  - [`--add-root`]{#opt-add-root} *path*\
     Causes the result of a realisation (`--realise` and
     `--force-realise`) to be registered as a root of the garbage
     collector. *path* will be created as a symlink to the resulting

doc/manual/src/command-ref/opt-common.md

@@ -2,13 +2,13 @@
 
 Most Nix commands accept the following command-line options:
 
-  - `--help`\
+  - [`--help`]{#opt-help}\
     Prints out a summary of the command syntax and exits.
 
-  - `--version`\
+  - [`--version`]{#opt-version}\
     Prints out the Nix version number on standard output and exits.
 
-  - `--verbose` / `-v`\
+  - [`--verbose`]{#opt-verbose} / `-v`\
     Increases the level of verbosity of diagnostic messages printed on
     standard error. For each Nix operation, the information printed on
     standard output is well-defined; any diagnostic information is
@@ -37,14 +37,14 @@ Most Nix commands accept the following command-line options:
       - 5\
         “Vomit”: print vast amounts of debug information.
 
-  - `--quiet`\
+  - [`--quiet`]{#opt-quiet}\
     Decreases the level of verbosity of diagnostic messages printed on
     standard error. This is the inverse option to `-v` / `--verbose`.
 
     This option may be specified repeatedly. See the previous verbosity
     levels list.
 
-  - `--log-format` *format*\
+  - [`--log-format`]{#opt-log-format} *format*\
     This option can be used to change the output of the log format, with
     *format* being one of:
 
@@ -66,14 +66,14 @@ Most Nix commands accept the following command-line options:
       - bar-with-logs\
         Display the raw logs, with the progress bar at the bottom.
 
-  - `--no-build-output` / `-Q`\
+  - [`--no-build-output`]{#opt-no-build-output} / `-Q`\
     By default, output written by builders to standard output and
     standard error is echoed to the Nix command's standard error. This
     option suppresses this behaviour. Note that the builder's standard
     output and error are always written to a log file in
     `prefix/nix/var/log/nix`.
 
-  - `--max-jobs` / `-j` *number*\
+  - [`--max-jobs`]{#opt-max-jobs} / `-j` *number*\
     Sets the maximum number of build jobs that Nix will perform in
     parallel to the specified number. Specify `auto` to use the number
     of CPUs in the system. The default is specified by the `max-jobs`
@@ -83,7 +83,7 @@ Most Nix commands accept the following command-line options:
     Setting it to `0` disallows building on the local machine, which is
     useful when you want builds to happen only on remote builders.
 
-  - `--cores`\
+  - [`--cores`]{#opt-cores}\
     Sets the value of the `NIX_BUILD_CORES` environment variable in
     the invocation of builders. Builders can use this variable at
     their discretion to control the maximum amount of parallelism. For
@@ -94,18 +94,18 @@ Most Nix commands accept the following command-line options:
     means that the builder should use all available CPU cores in the
     system.
 
-  - `--max-silent-time`\
+  - [`--max-silent-time`]{#opt-max-silent-time}\
     Sets the maximum number of seconds that a builder can go without
     producing any data on standard output or standard error. The
     default is specified by the `max-silent-time` configuration
     setting. `0` means no time-out.
 
-  - `--timeout`\
+  - [`--timeout`]{#opt-timeout}\
     Sets the maximum number of seconds that a builder can run. The
     default is specified by the `timeout` configuration setting. `0`
     means no timeout.
 
-  - `--keep-going` / `-k`\
+  - [`--keep-going`]{#opt-keep-going} / `-k`\
     Keep going in case of failed builds, to the greatest extent
     possible. That is, if building an input of some derivation fails,
     Nix will still build the other inputs, but not the derivation
@@ -113,13 +113,13 @@ Most Nix commands accept the following command-line options:
     for builds of substitutes), possibly killing builds in progress (in
     case of parallel or distributed builds).
 
-  - `--keep-failed` / `-K`\
+  - [`--keep-failed`]{#opt-keep-failed} / `-K`\
     Specifies that in case of a build failure, the temporary directory
     (usually in `/tmp`) in which the build takes place should not be
     deleted. The path of the build directory is printed as an
     informational message.
 
-  - `--fallback`\
+  - [`--fallback`]{#opt-fallback}\
     Whenever Nix attempts to build a derivation for which substitutes
     are known for each output path, but realising the output paths
     through the substitutes fails, fall back on building the derivation.
@@ -134,12 +134,12 @@ Most Nix commands accept the following command-line options:
     failure in obtaining the substitutes to lead to a full build from
     source (with the related consumption of resources).
 
-  - `--readonly-mode`\
+  - [`--readonly-mode`]{#opt-readonly-mode}\
     When this option is used, no attempt is made to open the Nix
     database. Most Nix operations do need database access, so those
     operations will fail.
 
-  - `--arg` *name* *value*\
+  - [`--arg`]{#opt-arg} *name* *value*\
     This option is accepted by `nix-env`, `nix-instantiate`,
     `nix-shell` and `nix-build`. When evaluating Nix expressions, the
     expression evaluator will automatically try to call functions that
@@ -170,13 +170,13 @@ Most Nix commands accept the following command-line options:
     since the argument is a Nix string literal, you have to escape the
     quotes.)
 
-  - `--argstr` *name* *value*\
+  - [`--argstr`]{#opt-argstr} *name* *value*\
     This option is like `--arg`, only the value is not a Nix
     expression but a string. So instead of `--arg system
     \"i686-linux\"` (the outer quotes are to keep the shell happy) you
     can say `--argstr system i686-linux`.
 
-  - `--attr` / `-A` *attrPath*\
+  - [`--attr`]{#opt-attr} / `-A` *attrPath*\
     Select an attribute from the top-level Nix expression being
     evaluated. (`nix-env`, `nix-instantiate`, `nix-build` and
     `nix-shell` only.) The *attribute path* *attrPath* is a sequence
@@ -191,7 +191,7 @@ Most Nix commands accept the following command-line options:
     attribute of the fourth element of the array in the `foo` attribute
     of the top-level expression.
 
-  - `--expr` / `-E`\
+  - [`--expr`]{#opt-expr} / `-E`\
     Interpret the command line arguments as a list of Nix expressions to
     be parsed and evaluated, rather than as a list of file names of Nix
     expressions. (`nix-instantiate`, `nix-build` and `nix-shell` only.)
@@ -202,17 +202,17 @@ Most Nix commands accept the following command-line options:
     use, give your expression to the `nix-shell -p` convenience flag
     instead.
 
-  - `-I` *path*\
+  - [`-I`]{#opt-I} *path*\
     Add a path to the Nix expression search path. This option may be
     given multiple times. See the `NIX_PATH` environment variable for
     information on the semantics of the Nix search path. Paths added
     through `-I` take precedence over `NIX_PATH`.
 
-  - `--option` *name* *value*\
+  - [`--option`]{#opt-option} *name* *value*\
     Set the Nix configuration option *name* to *value*. This overrides
     settings in the Nix configuration file (see nix.conf5).
 
-  - `--repair`\
+  - [`--repair`]{#opt-repair}\
     Fix corrupted or missing store paths by redownloading or rebuilding
     them. Note that this is slow because it requires computing a
     cryptographic hash of the contents of every path in the closure of

doc/manual/src/expressions/advanced-attributes.md

@@ -2,7 +2,7 @@
 
 Derivations can declare some infrequently used optional attributes.
 
-  - `allowedReferences`\
+  - [`allowedReferences`]{#adv-attr-allowedReferences}\
     The optional attribute `allowedReferences` specifies a list of legal
     references (dependencies) of the output of the builder. For example,
 
@@ -17,7 +17,7 @@ Derivations can declare some infrequently used optional attributes.
     booting Linux don’t have accidental dependencies on other paths in
     the Nix store.
 
-  - `allowedRequisites`\
+  - [`allowedRequisites`]{#adv-attr-allowedRequisites}\
     This attribute is similar to `allowedReferences`, but it specifies
     the legal requisites of the whole closure, so all the dependencies
     recursively. For example,
@@ -30,7 +30,7 @@ Derivations can declare some infrequently used optional attributes.
     runtime dependency than `foobar`, and in addition it enforces that
     `foobar` itself doesn't introduce any other dependency itself.
 
-  - `disallowedReferences`\
+  - [`disallowedReferences`]{#adv-attr-disallowedReferences}\
     The optional attribute `disallowedReferences` specifies a list of
     illegal references (dependencies) of the output of the builder. For
     example,
@@ -42,7 +42,7 @@ Derivations can declare some infrequently used optional attributes.
     enforces that the output of a derivation cannot have a direct
     runtime dependencies on the derivation `foo`.
 
-  - `disallowedRequisites`\
+  - [`disallowedRequisites`]{#adv-attr-disallowedRequisites}\
     This attribute is similar to `disallowedReferences`, but it
     specifies illegal requisites for the whole closure, so all the
     dependencies recursively. For example,
@@ -55,7 +55,7 @@ Derivations can declare some infrequently used optional attributes.
     dependency on `foobar` or any other derivation depending recursively
     on `foobar`.
 
-  - `exportReferencesGraph`\
+  - [`exportReferencesGraph`]{#adv-attr-exportReferencesGraph}\
     This attribute allows builders access to the references graph of
     their inputs. The attribute is a list of inputs in the Nix store
     whose references graph the builder needs to know. The value of
@@ -84,7 +84,7 @@ Derivations can declare some infrequently used optional attributes.
     with a Nix store containing the closure of a bootable NixOS
     configuration).
 
-  - `impureEnvVars`\
+  - [`impureEnvVars`]{#adv-attr-impureEnvVars}\
     This attribute allows you to specify a list of environment variables
     that should be passed from the environment of the calling user to
     the builder. Usually, the environment is cleared completely when the
@@ -112,7 +112,7 @@ Derivations can declare some infrequently used optional attributes.
     > environmental variables come from the environment of the
     > `nix-build`.
 
-  - `outputHash`; `outputHashAlgo`; `outputHashMode`\
+  - [`outputHash`]{#adv-attr-outputHash}; [`outputHashAlgo`]{#adv-attr-outputHashAlgo}; [`outputHashMode`]{#adv-attr-outputHashMode}\
     These attributes declare that the derivation is a so-called
     *fixed-output derivation*, which means that a cryptographic hash of
     the output is already known in advance. When the build of a
@@ -208,7 +208,7 @@ Derivations can declare some infrequently used optional attributes.
     [`nix-hash` command](../command-ref/nix-hash.md) for information
     about converting to and from base-32 notation.)
     
-  - `__contentAddressed`
+  - [`__contentAddressed`]{#adv-attr-__contentAddressed}
     If this **experimental** attribute is set to true, then the derivation
     outputs will be stored in a content-addressed location rather than the
     traditional input-addressed one.
@@ -216,7 +216,7 @@ Derivations can declare some infrequently used optional attributes.
     
     Setting this attribute also requires setting `outputHashMode` and `outputHashAlgo` like for *fixed-output derivations* (see above).
 
-  - `passAsFile`\
+  - [`passAsFile`]{#adv-attr-passAsFile}\
     A list of names of attributes that should be passed via files rather
     than environment variables. For example, if you have
 
@@ -234,7 +234,7 @@ Derivations can declare some infrequently used optional attributes.
     builder, since most operating systems impose a limit on the size
     of the environment (typically, a few hundred kilobyte).
 
-  - `preferLocalBuild`\
+  - [`preferLocalBuild`]{#adv-attr-preferLocalBuild}\
     If this attribute is set to `true` and [distributed building is
     enabled](../advanced-topics/distributed-builds.md), then, if
     possible, the derivation will be built locally instead of forwarded
@@ -242,7 +242,7 @@ Derivations can declare some infrequently used optional attributes.
     where the cost of doing a download or remote build would exceed
     the cost of building locally.
 
-  - `allowSubstitutes`\
+  - [`allowSubstitutes`]{#adv-attr-allowSubstitutes}\
     If this attribute is set to `false`, then Nix will always build this
     derivation; it will not try to substitute its outputs. This is
     useful for very trivial derivations (such as `writeText` in Nixpkgs)

doc/manual/src/expressions/builtin-constants.md

@@ -14,7 +14,7 @@ Here are the constants built into the Nix expression evaluator:
     This allows a Nix expression to fall back gracefully on older Nix
     installations that don’t have the desired built-in function.
 
-  - `builtins.currentSystem`\
+  - [`builtins.currentSystem`]{#builtins-currentSystem}\
     The built-in value `currentSystem` evaluates to the Nix platform
     identifier for the Nix installation on which the expression is being
     evaluated, such as `"i686-linux"` or `"x86_64-darwin"`.

doc/manual/src/expressions/derivations.md

@@ -4,7 +4,7 @@ The most important built-in function is `derivation`, which is used to
 describe a single derivation (a build action). It takes as input a set,
 the attributes of which specify the inputs of the build.
 
-  - There must be an attribute named `system` whose value must be a
+  - There must be an attribute named [`system`]{#attr-system} whose value must be a
     string specifying a Nix system type, such as `"i686-linux"` or
     `"x86_64-darwin"`. (To figure out your system type, run `nix -vv
     --version`.) The build can only be performed on a machine and

doc/manual/src/glossary.md

@@ -1,48 +1,48 @@
 # Glossary
 
-  - derivation\
+  - [derivation]{#gloss-derivation}\
     A description of a build action. The result of a derivation is a
     store object. Derivations are typically specified in Nix expressions
     using the [`derivation` primitive](expressions/derivations.md). These are
     translated into low-level *store derivations* (implicitly by
     `nix-env` and `nix-build`, or explicitly by `nix-instantiate`).
 
-  - store\
+  - [store]{#gloss-store}\
     The location in the file system where store objects live. Typically
     `/nix/store`.
 
-  - store path\
+  - [store path]{#gloss-store-path}\
     The location in the file system of a store object, i.e., an
     immediate child of the Nix store directory.
 
-  - store object\
+  - [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
     can be sources (objects copied from outside of the store),
     derivation outputs (objects produced by running a build action), or
     derivations (files describing a build action).
 
-  - substitute\
+  - [substitute]{#gloss-substitute}\
     A substitute is a command invocation stored in the Nix database that
     describes how to build a store object, bypassing the normal build
     mechanism (i.e., derivations). Typically, the substitute builds the
     store object by downloading a pre-built version of the store object
     from some server.
 
-  - purity\
+  - [purity]{#gloss-purity}\
     The assumption that equal Nix derivations when run always produce
     the same output. This cannot be guaranteed in general (e.g., a
     builder can rely on external inputs such as the network or the
     system time) but the Nix model assumes it.
 
-  - Nix expression\
+  - [Nix expression]{#gloss-nix-expression}\
     A high-level description of software packages and compositions
     thereof. Deploying software using Nix entails writing Nix
     expressions for your packages. Nix expressions are translated to
     derivations that are stored in the Nix store. These derivations can
     then be built.
 
-  - reference\
+  - [reference]{#gloss-reference}\
     A store path `P` is said to have a reference to a store path `Q` if
     the store object at `P` contains the path `Q` somewhere. The
     *references* of a store path are the set of store paths to which it
@@ -52,11 +52,11 @@
     output paths), whereas an output path only references other output
     paths.
 
-  - reachable\
+  - [reachable]{#gloss-reachable}\
     A store path `Q` is reachable from another store path `P` if `Q`
     is in the *closure* of the *references* relation.
 
-  - closure\
+  - [closure]{#gloss-closure}\
     The closure of a store path is the set of store paths that are
     directly or indirectly “reachable” from that store path; that is,
     it’s the closure of the path under the *references* relation. For
@@ -71,34 +71,34 @@
     to path `Q`, then `Q` is in the closure of `P`. Further, if `Q`
     references `R` then `R` is also in the closure of `P`.
 
-  - output path\
+  - [output path]{#gloss-output-path}\
     A store path produced by a derivation.
 
-  - deriver\
+  - [deriver]{#gloss-deriver}\
     The deriver of an *output path* is the store
     derivation that built it.
 
-  - validity\
+  - [validity]{#gloss-validity}\
     A store path is considered *valid* if it exists in the file system,
     is listed in the Nix database as being valid, and if all paths in
     its closure are also valid.
 
-  - user environment\
+  - [user environment]{#gloss-user-env}\
     An automatically generated store object that consists of a set of
     symlinks to “active” applications, i.e., other store paths. These
     are generated automatically by
     [`nix-env`](command-ref/nix-env.md). See *profiles*.
 
-  - profile\
+  - [profile]{#gloss-profile}\
     A symlink to the current *user environment* of a user, e.g.,
     `/nix/var/nix/profiles/default`.
 
-  - NAR\
+  - [NAR]{#gloss-nar}\
     A *N*ix *AR*chive. This is a serialisation of a path in the Nix
     store. It can contain regular files, directories and symbolic
     links.  NARs are generated and unpacked using `nix-store --dump`
     and `nix-store --restore`.
-  - `∅` \
+  - [`∅`]{#gloss-emtpy-set}\
     The empty set symbol. In the context of profile history, this denotes a package is not present in a particular version of the profile.
-  - `ε` \
+  - [`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.

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

@@ -186,7 +186,8 @@ and `/etc/zshrc` which you may remove.
 > read-only root will prevent you from manually deleting the empty `/nix`
 > mountpoint.
 
-# macOS Installation <a name="sect-macos-installation-change-store-prefix"></a><a name="sect-macos-installation-encrypted-volume"></a><a name="sect-macos-installation-symlink"></a><a name="sect-macos-installation-recommended-notes"></a>
+# macOS Installation
+[]{#sect-macos-installation-change-store-prefix}[]{#sect-macos-installation-encrypted-volume}[]{#sect-macos-installation-symlink}[]{#sect-macos-installation-recommended-notes}
 <!-- Note: anchors above to catch permalinks to old explanations -->
 
 We believe we have ironed out how to cleanly support the read-only root

doc/manual/src/release-notes/rl-2.9.md

@@ -0,0 +1,47 @@
+# Release 2.9 (2022-05-30)
+
+* Running Nix with the new `--debugger` flag will cause it to start a
+  repl session if an exception is thrown during evaluation, or if
+  `builtins.break` is called.  From there you can inspect the values
+  of variables and evaluate Nix expressions.  In debug mode, the
+  following new repl commands are available:
+
+  ```
+  :env          Show env stack
+  :bt           Show trace stack
+  :st           Show current trace
+  :st <idx>     Change to another trace in the stack
+  :c            Go until end of program, exception, or builtins.break().
+  :s            Go one step
+  ```
+
+  Read more about the debugger
+  [here](https://www.zknotes.com/note/5970).
+
+* Nix now provides better integration with zsh's `run-help`
+  feature. It is now included in the Nix installation in the form of
+  an autoloadable shell function, `run-help-nix`. It picks up Nix
+  subcommands from the currently typed in command and directs the user
+  to the associated man pages.
+
+* `nix repl` has a new build-and-link (`:bl`) command that builds a
+  derivation while creating GC root symlinks.
+
+* The path produced by `builtins.toFile` is now allowed to be imported
+  or read even with restricted evaluation. Note that this will not
+  work with a read-only store.
+
+* `nix build` has a new `--print-out-paths` flag to print the
+  resulting output paths.  This matches the default behaviour of
+  `nix-build`.
+
+* You can now specify which outputs of a derivation `nix` should
+  operate on using the syntax `installable^outputs`,
+  e.g. `nixpkgs#glibc^dev,static` or `nixpkgs#glibc^*`. By default,
+  `nix` will use the outputs specified by the derivation's
+  `meta.outputsToInstall` attribute if it exists, or all outputs
+  otherwise.
+
+* `builtins.fetchTree` (and flake inputs) can now be used to fetch
+  plain files over the `http(s)` and `file` protocols in addition to
+  directory tarballs.

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

@@ -1,29 +1,4 @@
 # Release X.Y (202?-??-??)
 
-* Nix now provides better integration with zsh's run-help feature. It is now
-  included in the Nix installation in the form of an autoloadable shell
-  function, run-help-nix. It picks up Nix subcommands from the currently typed
-  in command and directs the user to the associated man pages.
-
-* `nix repl` has a new build-'n-link (`:bl`) command that builds a derivation
-  while creating GC root symlinks.
-
-* The path produced by `builtins.toFile` is now allowed to be imported or read
-  even with restricted evaluation. Note that this will not work with a
-  read-only store.
-
-* `nix build` has a new `--print-out-paths` flag to print the resulting output paths.
-  This matches the default behaviour of `nix-build`.
-
 * Nix can now be built with LTO by passing `--enable-lto` to `configure`.
   LTO is currently only supported when building with GCC.
-
-* You can now specify which outputs of a derivation `nix` should
-  operate on using the syntax `installable^outputs`,
-  e.g. `nixpkgs#glibc^dev,static` or `nixpkgs#glibc^*`. By default,
-  `nix` will use the outputs specified by the derivation's
-  `meta.outputsToInstall` attribute if it exists, or all outputs
-  otherwise.
-
-  Selecting derivation outputs using the attribute selection syntax
-  (e.g. `nixpkgs#glibc.dev`) no longer works.

flake.lock

@@ -26,9 +26,10 @@
         "type": "github"
       },
       "original": {
-        "id": "nixpkgs",
+        "owner": "NixOS",
         "ref": "nixos-21.05-small",
-        "type": "indirect"
+        "repo": "nixpkgs",
+        "type": "github"
       }
     },
     "nixpkgs-regression": {
@@ -41,9 +42,10 @@
         "type": "github"
       },
       "original": {
-        "id": "nixpkgs",
+        "owner": "NixOS",
+        "repo": "nixpkgs",
         "rev": "215d4d0fd80ca5163643b03a33fde804a29cc1e2",
-        "type": "indirect"
+        "type": "github"
       }
     },
     "root": {

flake.nix

@@ -1,8 +1,8 @@
 {
   description = "The purely functional package manager";
 
-  inputs.nixpkgs.url = "nixpkgs/nixos-21.05-small";
+  inputs.nixpkgs.url = "github:NixOS/nixpkgs/nixos-21.05-small";
-  inputs.nixpkgs-regression.url = "nixpkgs/215d4d0fd80ca5163643b03a33fde804a29cc1e2";
+  inputs.nixpkgs-regression.url = "github:NixOS/nixpkgs/215d4d0fd80ca5163643b03a33fde804a29cc1e2";
   inputs.lowdown-src = { url = "github:kristapsdz/lowdown"; flake = false; };
 
   outputs = { self, nixpkgs, nixpkgs-regression, lowdown-src }:
@@ -102,7 +102,7 @@
             # Tests
             buildPackages.git
             buildPackages.mercurial # FIXME: remove? only needed for tests
-            buildPackages.jq
+            buildPackages.jq # Also for custom mdBook preprocessor.
           ]
           ++ lib.optionals stdenv.hostPlatform.isLinux [(buildPackages.util-linuxMinimal or buildPackages.utillinuxMinimal)];
 
@@ -135,11 +135,6 @@
             }))
             nlohmann_json
           ];
-
-        perlDeps =
-          [ perl
-            perlPackages.DBDSQLite
-          ];
       };
 
       installScriptFor = systems:
@@ -353,7 +348,7 @@
 
             strictDeps = true;
 
-            passthru.perl-bindings = with final; currentStdenv.mkDerivation {
+            passthru.perl-bindings = with final; perl.pkgs.toPerlModule (currentStdenv.mkDerivation {
               name = "nix-perl-${version}";
 
               src = self;
@@ -383,8 +378,9 @@
               enableParallelBuilding = true;
 
               postUnpack = "sourceRoot=$sourceRoot/perl";
-            };
+            });
 
+            meta.platforms = systems;
           };
 
           lowdown-nix = with final; currentStdenv.mkDerivation rec {
@@ -673,7 +669,7 @@
           outputs = [ "out" "dev" "doc" ];
 
           nativeBuildInputs = nativeBuildDeps;
-          buildInputs = buildDeps ++ propagatedDeps ++ awsDeps ++ perlDeps;
+          buildInputs = buildDeps ++ propagatedDeps ++ awsDeps;
 
           inherit configureFlags;
 

src/libcmd/command.cc

@@ -86,6 +86,11 @@ ref<Store> CopyCommand::getDstStore()
 
 EvalCommand::EvalCommand()
 {
+    addFlag({
+        .longName = "debugger",
+        .description = "start an interactive environment if evaluation fails",
+        .handler = {&startReplOnEvalErrors, true},
+    });
 }
 
 EvalCommand::~EvalCommand()
@@ -103,7 +108,7 @@ ref<Store> EvalCommand::getEvalStore()
 
 ref<EvalState> EvalCommand::getEvalState()
 {
-    if (!evalState)
+    if (!evalState) {
         evalState =
             #if HAVE_BOEHMGC
             std::allocate_shared<EvalState>(traceable_allocator<EvalState>(),
@@ -113,6 +118,11 @@ ref<EvalState> EvalCommand::getEvalState()
                 searchPath, getEvalStore(), getStore())
             #endif
             ;
+
+        if (startReplOnEvalErrors) {
+            evalState->debugRepl = &runRepl;        
+        };
+    }
     return ref<EvalState>(evalState);
 }
 

src/libcmd/command.hh

@@ -57,6 +57,8 @@ struct CopyCommand : virtual StoreCommand
 
 struct EvalCommand : virtual StoreCommand, MixEvalArgs
 {
+    bool startReplOnEvalErrors = false;
+
     EvalCommand();
 
     ~EvalCommand();
@@ -270,4 +272,8 @@ void printClosureDiff(
     const StorePath & afterPath,
     std::string_view indent);
 
+
+void runRepl(
+    ref<EvalState> evalState,
+    const ValMap & extraEnv);
 }

src/libcmd/installables.cc

@@ -623,7 +623,14 @@ std::tuple<std::string, FlakeRef, InstallableValue::DerivationInfo> InstallableF
     std::set<std::string> outputsToInstall;
     std::optional<NixInt> priority;
 
-    if (auto aMeta = attr->maybeGetAttr(state->sMeta)) {
+    if (auto aOutputSpecified = attr->maybeGetAttr(state->sOutputSpecified)) {
+        if (aOutputSpecified->getBool()) {
+            if (auto aOutputName = attr->maybeGetAttr("outputName"))
+                outputsToInstall = { aOutputName->getString() };
+        }
+    }
+
+    else if (auto aMeta = attr->maybeGetAttr(state->sMeta)) {
         if (auto aOutputsToInstall = aMeta->maybeGetAttr("outputsToInstall"))
             for (auto & s : aOutputsToInstall->getListOfStrings())
                 outputsToInstall.insert(s);

src/libcmd/local.mk

@@ -6,9 +6,9 @@ libcmd_DIR := $(d)
 
 libcmd_SOURCES := $(wildcard $(d)/*.cc)
 
-libcmd_CXXFLAGS += -I src/libutil -I src/libstore -I src/libexpr -I src/libmain -I src/libfetchers
+libcmd_CXXFLAGS += -I src/libutil -I src/libstore -I src/libexpr -I src/libmain -I src/libfetchers -I src/nix
 
-libcmd_LDFLAGS += $(LOWDOWN_LIBS) -pthread
+libcmd_LDFLAGS = $(EDITLINE_LIBS) -llowdown -pthread
 
 libcmd_LIBS = libstore libutil libexpr libmain libfetchers
 

src/libcmd/repl.cc

@@ -48,38 +48,42 @@ struct NixRepl
     #endif
 {
     std::string curDir;
-    std::unique_ptr<EvalState> state;
+    ref<EvalState> state;
     Bindings * autoArgs;
 
+    size_t debugTraceIndex;
+
     Strings loadedFiles;
 
     const static int envSize = 32768;
-    StaticEnv staticEnv;
+    std::shared_ptr<StaticEnv> staticEnv;
     Env * env;
     int displ;
     StringSet varNames;
 
     const Path historyFile;
 
-    NixRepl(const Strings & searchPath, nix::ref<Store> store);
+    NixRepl(ref<EvalState> state);
     ~NixRepl();
     void mainLoop(const std::vector<std::string> & files);
     StringSet completePrefix(const std::string & prefix);
-    bool getLine(std::string & input, const std::string &prompt);
+    bool getLine(std::string & input, const std::string & prompt);
     StorePath getDerivationPath(Value & v);
     bool processLine(std::string line);
     void loadFile(const Path & path);
     void loadFlake(const std::string & flakeRef);
     void initEnv();
+    void loadFiles();
     void reloadFiles();
     void addAttrsToScope(Value & attrs);
     void addVarToScope(const Symbol name, Value & v);
     Expr * parseString(std::string s);
     void evalString(std::string s, Value & v);
+    void loadDebugTraceEnv(DebugTrace & dt);
 
     typedef std::set<Value *> ValuesSeen;
-    std::ostream &  printValue(std::ostream & str, Value & v, unsigned int maxDepth);
+    std::ostream & printValue(std::ostream & str, Value & v, unsigned int maxDepth);
-    std::ostream &  printValue(std::ostream & str, Value & v, unsigned int maxDepth, ValuesSeen & seen);
+    std::ostream & printValue(std::ostream & str, Value & v, unsigned int maxDepth, ValuesSeen & seen);
 };
 
 
@@ -92,9 +96,10 @@ std::string removeWhitespace(std::string s)
 }
 
 
-NixRepl::NixRepl(const Strings & searchPath, nix::ref<Store> store)
+NixRepl::NixRepl(ref<EvalState> state)
-    : state(std::make_unique<EvalState>(searchPath, store))
+    : state(state)
-    , staticEnv(false, &state->staticBaseEnv)
+    , debugTraceIndex(0)
+    , staticEnv(new StaticEnv(false, state->staticBaseEnv.get()))
     , historyFile(getDataDir() + "/nix/repl-history")
 {
     curDir = absPath(".");
@@ -198,15 +203,42 @@ namespace {
     }
 }
 
+static std::ostream & showDebugTrace(std::ostream & out, const PosTable & positions, const DebugTrace & dt)
+{
+    if (dt.isError)
+        out << ANSI_RED "error: " << ANSI_NORMAL;
+    out << dt.hint.str() << "\n";
+
+    // prefer direct pos, but if noPos then try the expr.
+    auto pos = *dt.pos
+        ? *dt.pos
+        : positions[dt.expr.getPos() ? dt.expr.getPos() : noPos];
+
+    if (pos) {
+        printAtPos(pos, out);
+
+        auto loc = getCodeLines(pos);
+        if (loc.has_value()) {
+            out << "\n";
+            printCodeLines(out, "", pos, *loc);
+            out << "\n";
+        }
+    }
+
+    return out;
+}
+
 void NixRepl::mainLoop(const std::vector<std::string> & files)
 {
     std::string error = ANSI_RED "error:" ANSI_NORMAL " ";
     notice("Welcome to Nix " + nixVersion + ". Type :? for help.\n");
 
-    for (auto & i : files)
+    if (!files.empty()) {
-        loadedFiles.push_back(i);
+        for (auto & i : files)
+            loadedFiles.push_back(i);
+    }
 
-    reloadFiles();
+    loadFiles();
     if (!loadedFiles.empty()) notice("");
 
     // Allow nix-repl specific settings in .inputrc
@@ -227,9 +259,12 @@ void NixRepl::mainLoop(const std::vector<std::string> & files)
     while (true) {
         // When continuing input from previous lines, don't print a prompt, just align to the same
         // number of chars as the prompt.
-        if (!getLine(input, input.empty() ? "nix-repl> " : "          "))
+        if (!getLine(input, input.empty() ? "nix-repl> " : "          ")) {
+            // ctrl-D should exit the debugger.
+            state->debugStop = false;
+            state->debugQuit = true;
             break;
-
+        }
         try {
             if (!removeWhitespace(input).empty() && !processLine(input)) return;
         } catch (ParseError & e) {
@@ -240,6 +275,14 @@ void NixRepl::mainLoop(const std::vector<std::string> & files)
             } else {
               printMsg(lvlError, e.msg());
             }
+        } catch (EvalError & e) {
+            // in debugger mode, an EvalError should trigger another repl session.
+            // when that session returns the exception will land here.  No need to show it again;
+            // show the error for this repl session instead.
+            if (state->debugRepl && !state->debugTraces.empty())
+                showDebugTrace(std::cout, state->positions, state->debugTraces.front());
+            else
+                printMsg(lvlError, e.msg());
         } catch (Error & e) {
             printMsg(lvlError, e.msg());
         } catch (Interrupted & e) {
@@ -394,6 +437,19 @@ StorePath NixRepl::getDerivationPath(Value & v) {
     return *drvPath;
 }
 
+void NixRepl::loadDebugTraceEnv(DebugTrace & dt)
+{
+    initEnv();
+
+    auto se = state->getStaticEnv(dt.expr);
+    if (se) {
+        auto vm = mapStaticEnvBindings(state->symbols, *se.get(), dt.env);
+
+        // add staticenv vars.
+        for (auto & [name, value] : *(vm.get()))
+            addVarToScope(state->symbols.create(name), *value);
+    }
+}
 
 bool NixRepl::processLine(std::string line)
 {
@@ -429,12 +485,72 @@ bool NixRepl::processLine(std::string line)
              << "  :p <expr>     Evaluate and print expression recursively\n"
              << "  :q            Exit nix-repl\n"
              << "  :r            Reload all files\n"
-             << "  :s <expr>     Build dependencies of derivation, then start nix-shell\n"
+             << "  :sh <expr>    Build dependencies of derivation, then start nix-shell\n"
              << "  :t <expr>     Describe result of evaluation\n"
              << "  :u <expr>     Build derivation, then start nix-shell\n"
              << "  :doc <expr>   Show documentation of a builtin function\n"
              << "  :log <expr>   Show logs for a derivation\n"
-             << "  :st [bool]    Enable, disable or toggle showing traces for errors\n";
+             << "  :te [bool]    Enable, disable or toggle showing traces for errors\n"
+             ;
+        if (state->debugRepl) {
+             std::cout
+             << "\n"
+             << "        Debug mode commands\n"
+             << "  :env          Show env stack\n"
+             << "  :bt           Show trace stack\n"
+             << "  :st           Show current trace\n"
+             << "  :st <idx>     Change to another trace in the stack\n"
+             << "  :c            Go until end of program, exception, or builtins.break\n"
+             << "  :s            Go one step\n"
+             ;
+        }
+
+    }
+
+    else if (state->debugRepl && (command == ":bt" || command == ":backtrace")) {
+        for (const auto & [idx, i] : enumerate(state->debugTraces)) {
+            std::cout << "\n" << ANSI_BLUE << idx << ANSI_NORMAL << ": ";
+            showDebugTrace(std::cout, state->positions, i);
+        }
+    }
+
+    else if (state->debugRepl && (command == ":env")) {
+        for (const auto & [idx, i] : enumerate(state->debugTraces)) {
+            if (idx == debugTraceIndex) {
+                printEnvBindings(*state, i.expr, i.env);
+                break;
+            }
+        }
+    }
+
+    else if (state->debugRepl && (command == ":st")) {
+        try {
+            // change the DebugTrace index.
+            debugTraceIndex = stoi(arg);
+        } catch (...) { }
+
+        for (const auto & [idx, i] : enumerate(state->debugTraces)) {
+             if (idx == debugTraceIndex) {
+                 std::cout << "\n" << ANSI_BLUE << idx << ANSI_NORMAL << ": ";
+                 showDebugTrace(std::cout, state->positions, i);
+                 std::cout << std::endl;
+                 printEnvBindings(*state, i.expr, i.env);
+                 loadDebugTraceEnv(i);
+                 break;
+             }
+        }
+    }
+
+    else if (state->debugRepl && (command == ":s" || command == ":step")) {
+        // set flag to stop at next DebugTrace; exit repl.
+        state->debugStop = true;
+        return false;
+    }
+
+    else if (state->debugRepl && (command == ":c" || command == ":continue")) {
+        // set flag to run to next breakpoint or end of program; exit repl.
+        state->debugStop = false;
+        return false;
     }
 
     else if (command == ":a" || command == ":add") {
@@ -506,7 +622,7 @@ bool NixRepl::processLine(std::string line)
         runNix("nix-shell", {state->store->printStorePath(drvPath)});
     }
 
-    else if (command == ":b" || command == ":bl" || command == ":i" || command == ":s" || command == ":log") {
+    else if (command == ":b" || command == ":bl" || command == ":i" || command == ":sh" || command == ":log") {
         Value v;
         evalString(arg, v);
         StorePath drvPath = getDerivationPath(v);
@@ -567,8 +683,11 @@ bool NixRepl::processLine(std::string line)
         printValue(std::cout, v, 1000000000) << std::endl;
     }
 
-    else if (command == ":q" || command == ":quit")
+    else if (command == ":q" || command == ":quit") {
+        state->debugStop = false;
+        state->debugQuit = true;
         return false;
+    }
 
     else if (command == ":doc") {
         Value v;
@@ -593,7 +712,7 @@ bool NixRepl::processLine(std::string line)
             throw Error("value does not have documentation");
     }
 
-    else if (command == ":st" || command == ":show-trace") {
+    else if (command == ":te" || command == ":trace-enable") {
         if (arg == "false" || (arg == "" && loggerSettings.showTrace)) {
             std::cout << "not showing error traces\n";
             loggerSettings.showTrace = false;
@@ -669,10 +788,10 @@ void NixRepl::initEnv()
     env = &state->allocEnv(envSize);
     env->up = &state->baseEnv;
     displ = 0;
-    staticEnv.vars.clear();
+    staticEnv->vars.clear();
 
     varNames.clear();
-    for (auto & i : state->staticBaseEnv.vars)
+    for (auto & i : state->staticBaseEnv->vars)
         varNames.emplace(state->symbols[i.first]);
 }
 
@@ -681,6 +800,12 @@ void NixRepl::reloadFiles()
 {
     initEnv();
 
+    loadFiles();
+}
+
+
+void NixRepl::loadFiles()
+{
     Strings old = loadedFiles;
     loadedFiles.clear();
 
@@ -701,12 +826,12 @@ void NixRepl::addAttrsToScope(Value & attrs)
         throw Error("environment full; cannot add more variables");
 
     for (auto & i : *attrs.attrs) {
-        staticEnv.vars.emplace_back(i.name, displ);
+        staticEnv->vars.emplace_back(i.name, displ);
         env->values[displ++] = i.value;
         varNames.emplace(state->symbols[i.name]);
     }
-    staticEnv.sort();
+    staticEnv->sort();
-    staticEnv.deduplicate();
+    staticEnv->deduplicate();
     notice("Added %1% variables.", attrs.attrs->size());
 }
 
@@ -715,10 +840,10 @@ void NixRepl::addVarToScope(const Symbol name, Value & v)
 {
     if (displ >= envSize)
         throw Error("environment full; cannot add more variables");
-    if (auto oldVar = staticEnv.find(name); oldVar != staticEnv.vars.end())
+    if (auto oldVar = staticEnv->find(name); oldVar != staticEnv->vars.end())
-        staticEnv.vars.erase(oldVar);
+        staticEnv->vars.erase(oldVar);
-    staticEnv.vars.emplace_back(name, displ);
+    staticEnv->vars.emplace_back(name, displ);
-    staticEnv.sort();
+    staticEnv->sort();
     env->values[displ++] = &v;
     varNames.emplace(state->symbols[name]);
 }
@@ -886,6 +1011,21 @@ std::ostream & NixRepl::printValue(std::ostream & str, Value & v, unsigned int m
     return str;
 }
 
+void runRepl(
+    ref<EvalState>evalState,
+    const ValMap & extraEnv)
+{
+    auto repl = std::make_unique<NixRepl>(evalState);
+
+    repl->initEnv();
+
+    // add 'extra' vars.
+    for (auto & [name, value] : extraEnv)
+        repl->addVarToScope(repl->state->symbols.create(name), *value);
+
+    repl->mainLoop({});
+}
+
 struct CmdRepl : StoreCommand, MixEvalArgs
 {
     std::vector<std::string> files;
@@ -914,8 +1054,12 @@ struct CmdRepl : StoreCommand, MixEvalArgs
     void run(ref<Store> store) override
     {
         evalSettings.pureEval = false;
-        auto repl = std::make_unique<NixRepl>(searchPath, openStore());
+
+        auto evalState = make_ref<EvalState>(searchPath, store);
+
+        auto repl = std::make_unique<NixRepl>(evalState);
         repl->autoArgs = getAutoArgs(*repl->state);
+        repl->initEnv();
         repl->mainLoop(files);
     }
 };

src/libexpr/eval-cache.cc

@@ -576,14 +576,14 @@ std::string AttrCursor::getString()
                 debug("using cached string attribute '%s'", getAttrPathStr());
                 return s->first;
             } else
-                throw TypeError("'%s' is not a string", getAttrPathStr());
+                root->state.debugThrowLastTrace(TypeError("'%s' is not a string", getAttrPathStr()));
         }
     }
 
     auto & v = forceValue();
 
     if (v.type() != nString && v.type() != nPath)
-        throw TypeError("'%s' is not a string but %s", getAttrPathStr(), showType(v.type()));
+        root->state.debugThrowLastTrace(TypeError("'%s' is not a string but %s", getAttrPathStr(), showType(v.type())));
 
     return v.type() == nString ? v.string.s : v.path;
 }
@@ -607,7 +607,7 @@ string_t AttrCursor::getStringWithContext()
                     return *s;
                 }
             } else
-                throw TypeError("'%s' is not a string", getAttrPathStr());
+                root->state.debugThrowLastTrace(TypeError("'%s' is not a string", getAttrPathStr()));
         }
     }
 
@@ -618,7 +618,7 @@ string_t AttrCursor::getStringWithContext()
     else if (v.type() == nPath)
         return {v.path, {}};
     else
-        throw TypeError("'%s' is not a string but %s", getAttrPathStr(), showType(v.type()));
+        root->state.debugThrowLastTrace(TypeError("'%s' is not a string but %s", getAttrPathStr(), showType(v.type())));
 }
 
 bool AttrCursor::getBool()
@@ -631,14 +631,14 @@ bool AttrCursor::getBool()
                 debug("using cached Boolean attribute '%s'", getAttrPathStr());
                 return *b;
             } else
-                throw TypeError("'%s' is not a Boolean", getAttrPathStr());
+                root->state.debugThrowLastTrace(TypeError("'%s' is not a Boolean", getAttrPathStr()));
         }
     }
 
     auto & v = forceValue();
 
     if (v.type() != nBool)
-        throw TypeError("'%s' is not a Boolean", getAttrPathStr());
+        root->state.debugThrowLastTrace(TypeError("'%s' is not a Boolean", getAttrPathStr()));
 
     return v.boolean;
 }
@@ -708,14 +708,14 @@ std::vector<Symbol> AttrCursor::getAttrs()
                 debug("using cached attrset attribute '%s'", getAttrPathStr());
                 return *attrs;
             } else
-                throw TypeError("'%s' is not an attribute set", getAttrPathStr());
+                root->state.debugThrowLastTrace(TypeError("'%s' is not an attribute set", getAttrPathStr()));
         }
     }
 
     auto & v = forceValue();
 
     if (v.type() != nAttrs)
-        throw TypeError("'%s' is not an attribute set", getAttrPathStr());
+        root->state.debugThrowLastTrace(TypeError("'%s' is not an attribute set", getAttrPathStr()));
 
     std::vector<Symbol> attrs;
     for (auto & attr : *getValue().attrs)

src/libexpr/eval-inline.hh

@@ -4,7 +4,6 @@
 
 namespace nix {
 
-
 /* Note: Various places expect the allocated memory to be zeroed. */
 [[gnu::always_inline]]
 inline void * allocBytes(size_t n)

src/libexpr/eval.cc

@@ -18,6 +18,7 @@
 #include <sys/resource.h>
 #include <iostream>
 #include <fstream>
+#include <functional>
 
 #include <sys/resource.h>
 
@@ -36,7 +37,6 @@
 
 namespace nix {
 
-
 static char * allocString(size_t size)
 {
     char * t;
@@ -459,10 +459,14 @@ EvalState::EvalState(
     , sKey(symbols.create("key"))
     , sPath(symbols.create("path"))
     , sPrefix(symbols.create("prefix"))
+    , sOutputSpecified(symbols.create("outputSpecified"))
     , repair(NoRepair)
     , emptyBindings(0)
     , store(store)
     , buildStore(buildStore ? buildStore : store)
+    , debugRepl(0)
+    , debugStop(false)
+    , debugQuit(false)
     , regexCache(makeRegexCache())
 #if HAVE_BOEHMGC
     , valueAllocCache(std::allocate_shared<void *>(traceable_allocator<void *>(), nullptr))
@@ -472,7 +476,7 @@ EvalState::EvalState(
     , env1AllocCache(std::make_shared<void *>(nullptr))
 #endif
     , baseEnv(allocEnv(128))
-    , staticBaseEnv(false, 0)
+    , staticBaseEnv{std::make_shared<StaticEnv>(false, nullptr)}
 {
     countCalls = getEnv("NIX_COUNT_CALLS").value_or("0") != "0";
 
@@ -530,7 +534,7 @@ void EvalState::allowPath(const StorePath & storePath)
         allowedPaths->insert(store->toRealPath(storePath));
 }
 
-void EvalState::allowAndSetStorePathString(const StorePath &storePath, Value & v)
+void EvalState::allowAndSetStorePathString(const StorePath & storePath, Value & v)
 {
     allowPath(storePath);
 
@@ -638,7 +642,7 @@ Value * EvalState::addConstant(const std::string & name, Value & v)
 
 void EvalState::addConstant(const std::string & name, Value * v)
 {
-    staticBaseEnv.vars.emplace_back(symbols.create(name), baseEnvDispl);
+    staticBaseEnv->vars.emplace_back(symbols.create(name), baseEnvDispl);
     baseEnv.values[baseEnvDispl++] = v;
     auto name2 = name.substr(0, 2) == "__" ? name.substr(2) : name;
     baseEnv.values[0]->attrs->push_back(Attr(symbols.create(name2), v));
@@ -663,7 +667,7 @@ Value * EvalState::addPrimOp(const std::string & name,
 
     Value * v = allocValue();
     v->mkPrimOp(new PrimOp { .fun = primOp, .arity = arity, .name = name2 });
-    staticBaseEnv.vars.emplace_back(symbols.create(name), baseEnvDispl);
+    staticBaseEnv->vars.emplace_back(symbols.create(name), baseEnvDispl);
     baseEnv.values[baseEnvDispl++] = v;
     baseEnv.values[0]->attrs->push_back(Attr(sym, v));
     return v;
@@ -689,7 +693,7 @@ Value * EvalState::addPrimOp(PrimOp && primOp)
 
     Value * v = allocValue();
     v->mkPrimOp(new PrimOp(primOp));
-    staticBaseEnv.vars.emplace_back(envName, baseEnvDispl);
+    staticBaseEnv->vars.emplace_back(envName, baseEnvDispl);
     baseEnv.values[baseEnvDispl++] = v;
     baseEnv.values[0]->attrs->push_back(Attr(symbols.create(primOp.name), v));
     return v;
@@ -719,128 +723,284 @@ std::optional<EvalState::Doc> EvalState::getDoc(Value & v)
 }
 
 
+// just for the current level of StaticEnv, not the whole chain.
+void printStaticEnvBindings(const SymbolTable & st, const StaticEnv & se)
+{
+    std::cout << ANSI_MAGENTA;
+    for (auto & i : se.vars)
+        std::cout << st[i.first] << " ";
+    std::cout << ANSI_NORMAL;
+    std::cout << std::endl;
+}
+
+// just for the current level of Env, not the whole chain.
+void printWithBindings(const SymbolTable & st, const Env & env)
+{
+    if (env.type == Env::HasWithAttrs) {
+        std::cout << "with: ";
+        std::cout << ANSI_MAGENTA;
+        Bindings::iterator j = env.values[0]->attrs->begin();
+        while (j != env.values[0]->attrs->end()) {
+            std::cout << st[j->name] << " ";
+            ++j;
+        }
+        std::cout << ANSI_NORMAL;
+        std::cout << std::endl;
+    }
+}
+
+void printEnvBindings(const SymbolTable & st, const StaticEnv & se, const Env & env, int lvl)
+{
+    std::cout << "Env level " << lvl << std::endl;
+
+    if (se.up && env.up) {
+        std::cout << "static: ";
+        printStaticEnvBindings(st, se);
+        printWithBindings(st, env);
+        std::cout << std::endl;
+        printEnvBindings(st, *se.up, *env.up, ++lvl);
+    } else {
+        std::cout << ANSI_MAGENTA;
+        // for the top level, don't print the double underscore ones;
+        // they are in builtins.
+        for (auto & i : se.vars)
+            if (!hasPrefix(st[i.first], "__"))
+                std::cout << st[i.first] << " ";
+        std::cout << ANSI_NORMAL;
+        std::cout << std::endl;
+        printWithBindings(st, env);  // probably nothing there for the top level.
+        std::cout << std::endl;
+
+    }
+}
+
+void printEnvBindings(const EvalState &es, const Expr & expr, const Env & env)
+{
+    // just print the names for now
+    auto se = es.getStaticEnv(expr);
+    if (se)
+        printEnvBindings(es.symbols, *se, env, 0);
+}
+
+void mapStaticEnvBindings(const SymbolTable & st, const StaticEnv & se, const Env & env, ValMap & vm)
+{
+    // add bindings for the next level up first, so that the bindings for this level
+    // override the higher levels.
+    // The top level bindings (builtins) are skipped since they are added for us by initEnv()
+    if (env.up && se.up) {
+        mapStaticEnvBindings(st, *se.up, *env.up, vm);
+
+        if (env.type == Env::HasWithAttrs) {
+            // add 'with' bindings.
+            Bindings::iterator j = env.values[0]->attrs->begin();
+            while (j != env.values[0]->attrs->end()) {
+                vm[st[j->name]] = j->value;
+                ++j;
+            }
+        } else {
+            // iterate through staticenv bindings and add them.
+            for (auto & i : se.vars)
+                vm[st[i.first]] = env.values[i.second];
+        }
+    }
+}
+
+std::unique_ptr<ValMap> mapStaticEnvBindings(const SymbolTable & st, const StaticEnv & se, const Env & env)
+{
+    auto vm = std::make_unique<ValMap>();
+    mapStaticEnvBindings(st, se, env, *vm);
+    return vm;
+}
+
+void EvalState::runDebugRepl(const Error * error, const Env & env, const Expr & expr)
+{
+    // double check we've got the debugRepl function pointer.
+    if (!debugRepl)
+        return;
+
+    auto dts =
+        error && expr.getPos()
+        ? std::make_unique<DebugTraceStacker>(
+            *this,
+            DebugTrace {
+                .pos = error->info().errPos ? *error->info().errPos : positions[expr.getPos()],
+                .expr = expr,
+                .env = env,
+                .hint = error->info().msg,
+                .isError = true
+            })
+        : nullptr;
+
+    if (error)
+        printError("%s\n\n" ANSI_BOLD "Starting REPL to allow you to inspect the current state of the evaluator.\n" ANSI_NORMAL, error->what());
+
+    auto se = getStaticEnv(expr);
+    if (se) {
+        auto vm = mapStaticEnvBindings(symbols, *se.get(), env);
+        (debugRepl)(ref<EvalState>(shared_from_this()), *vm);
+    }
+}
+
 /* 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)
-void EvalState::throwEvalError(const PosIdx pos, const char * s) const
 {
-    throw EvalError({
+    debugThrow(EvalError({
         .msg = hintfmt(s),
         .errPos = positions[pos]
-    });
+    }), env, expr);
 }
 
-void EvalState::throwTypeError(const PosIdx pos, const char * s, const Value & v) const
+void EvalState::throwEvalError(const PosIdx pos, const char * s)
 {
-    throw TypeError({
+    debugThrowLastTrace(EvalError({
-        .msg = hintfmt(s, showType(v)),
+        .msg = hintfmt(s),
         .errPos = positions[pos]
-    });
+    }));
 }
 
-void EvalState::throwEvalError(const char * s, const std::string & s2) const
+void EvalState::throwEvalError(const char * s, const std::string & s2)
 {
-    throw EvalError(s, s2);
+    debugThrowLastTrace(EvalError(s, s2));
 }
 
 void EvalState::throwEvalError(const PosIdx pos, const Suggestions & suggestions, const char * s,
-    const std::string & s2) const
+    const std::string & s2, Env & env, Expr & expr)
 {
-    throw EvalError(ErrorInfo {
+    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) const
+void EvalState::throwEvalError(const PosIdx pos, const char * s, const std::string & s2)
 {
-    throw EvalError(ErrorInfo {
+    debugThrowLastTrace(EvalError({
         .msg = hintfmt(s, s2),
         .errPos = positions[pos]
-    });
+    }));
 }
 
-void EvalState::throwEvalError(const char * s, const std::string & s2, const std::string & s3) const
+void EvalState::throwEvalError(const PosIdx pos, const char * s, const std::string & s2, Env & env, Expr & expr)
 {
-    throw EvalError(s, s2, s3);
+    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),
+        .errPos = positions[noPos]
+    }));
 }
 
 void EvalState::throwEvalError(const PosIdx pos, const char * s, const std::string & s2,
-    const std::string & s3) const
+    const std::string & s3)
 {
-    throw EvalError({
+    debugThrowLastTrace(EvalError({
-        .msg = hintfmt(s, s2, s3),
+        .msg = hintfmt(s, s2),
         .errPos = positions[pos]
-    });
+    }));
 }
 
-void EvalState::throwEvalError(const PosIdx p1, const char * s, const Symbol sym, const PosIdx p2) const
+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),
+        .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.
-    throw EvalError({
+    debugThrow(EvalError({
         .msg = hintfmt(s, symbols[sym], positions[p2]),
         .errPos = positions[p1]
-    });
+    }), env, expr);
 }
 
-void EvalState::throwTypeError(const PosIdx pos, const char * s) const
+void EvalState::throwTypeError(const PosIdx pos, const char * s, const Value & v)
 {
-    throw TypeError({
+    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) const
+    const Symbol s2, Env & env, Expr &expr)
 {
-    throw TypeError({
+    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) const
+    const ExprLambda & fun, const Symbol s2, Env & env, Expr &expr)
 {
-    throw TypeError(ErrorInfo {
+    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)
-void EvalState::throwTypeError(const char * s, const Value & v) const
 {
-    throw TypeError(s, showType(v));
+    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) const
+void EvalState::throwAssertionError(const PosIdx pos, const char * s, const std::string & s1, Env & env, Expr &expr)
 {
-    throw AssertionError({
+    debugThrow(AssertionError({
         .msg = hintfmt(s, s1),
         .errPos = positions[pos]
-    });
+    }), env, expr);
 }
 
-void EvalState::throwUndefinedVarError(const PosIdx pos, const char * s, const std::string & s1) const
+void EvalState::throwUndefinedVarError(const PosIdx pos, const char * s, const std::string & s1, Env & env, Expr &expr)
 {
-    throw UndefinedVarError({
+    debugThrow(UndefinedVarError({
         .msg = hintfmt(s, s1),
         .errPos = positions[pos]
-    });
+    }), env, expr);
 }
 
-void EvalState::throwMissingArgumentError(const PosIdx pos, const char * s, const std::string & s1) const
+void EvalState::throwMissingArgumentError(const PosIdx pos, const char * s, const std::string & s1, Env & env, Expr &expr)
 {
-    throw MissingArgumentError({
+    debugThrow(MissingArgumentError({
         .msg = hintfmt(s, s1),
         .errPos = positions[pos]
-    });
+    }), env, expr);
 }
 
 void EvalState::addErrorTrace(Error & e, const char * s, const std::string & s2) const
@@ -853,6 +1013,32 @@ void EvalState::addErrorTrace(Error & e, const PosIdx pos, const char * s, const
     e.addTrace(positions[pos], s, s2);
 }
 
+static std::unique_ptr<DebugTraceStacker> makeDebugTraceStacker(
+    EvalState & state,
+    Expr & expr,
+    Env & env,
+    std::optional<ErrPos> pos,
+    const char * s,
+    const std::string & s2)
+{
+    return std::make_unique<DebugTraceStacker>(state,
+        DebugTrace {
+            .pos = pos,
+            .expr = expr,
+            .env = env,
+            .hint = hintfmt(s, s2),
+            .isError = false
+        });
+}
+
+DebugTraceStacker::DebugTraceStacker(EvalState & evalState, DebugTrace t)
+    : evalState(evalState)
+    , trace(std::move(t))
+{
+    evalState.debugTraces.push_front(trace);
+    if (evalState.debugStop && evalState.debugRepl)
+        evalState.runDebugRepl(nullptr, trace.env, trace.expr);
+}
 
 void Value::mkString(std::string_view s)
 {
@@ -911,12 +1097,11 @@ 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]);
+            throwUndefinedVarError(var.pos, "undefined variable '%1%'", symbols[var.name], *env, const_cast<ExprVar&>(var));
         for (size_t l = env->prevWith; l; --l, env = env->up) ;
     }
 }
 
-
 void EvalState::mkList(Value & v, size_t size)
 {
     v.mkList(size);
@@ -1049,6 +1234,15 @@ void EvalState::cacheFile(
     fileParseCache[resolvedPath] = e;
 
     try {
+        auto dts = debugRepl
+            ? makeDebugTraceStacker(
+                *this,
+                *e,
+                this->baseEnv,
+                e->getPos() ? std::optional(ErrPos(positions[e->getPos()])) : std::nullopt,
+                "while evaluating the file '%1%':", resolvedPath)
+            : nullptr;
+
         // Enforce that 'flake.nix' is a direct attrset, not a
         // computation.
         if (mustBeTrivial &&
@@ -1076,7 +1270,7 @@ inline bool EvalState::evalBool(Env & env, Expr * e)
     Value v;
     e->eval(*this, env, v);
     if (v.type() != nBool)
-        throwTypeError("value is %1% while a Boolean was expected", v);
+        throwTypeError(noPos, "value is %1% while a Boolean was expected", v, env, *e);
     return v.boolean;
 }
 
@@ -1086,7 +1280,7 @@ 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);
+        throwTypeError(pos, "value is %1% while a Boolean was expected", v, env, *e);
     return v.boolean;
 }
 
@@ -1095,7 +1289,7 @@ inline void EvalState::evalAttrs(Env & env, Expr * e, Value & v)
 {
     e->eval(*this, env, v);
     if (v.type() != nAttrs)
-        throwTypeError("value is %1% while a set was expected", v);
+        throwTypeError(noPos, "value is %1% while a set was expected", v, env, *e);
 }
 
 
@@ -1200,7 +1394,7 @@ void ExprAttrs::eval(EvalState & state, Env & env, Value & v)
         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);
+            state.throwEvalError(i.pos, "dynamic attribute '%1%' already defined at %2%", nameSym, j->pos, env, *this);
 
         i.valueExpr->setName(nameSym);
         /* Keep sorted order so find can catch duplicates */
@@ -1274,6 +1468,15 @@ void ExprSelect::eval(EvalState & state, Env & env, Value & v)
     e->eval(state, env, vTmp);
 
     try {
+        auto dts = state.debugRepl
+            ? makeDebugTraceStacker(
+                state,
+                *this,
+                env,
+                state.positions[pos2],
+                "while evaluating the attribute '%1%'",
+                showAttrPath(state, env, attrPath))
+            : nullptr;
 
         for (auto & i : attrPath) {
             state.nrLookups++;
@@ -1296,7 +1499,7 @@ void ExprSelect::eval(EvalState & state, Env & env, Value & v)
                     state.throwEvalError(
                         pos,
                         Suggestions::bestMatches(allAttrNames, state.symbols[name]),
-                        "attribute '%1%' missing", state.symbols[name]);
+                        "attribute '%1%' missing", state.symbols[name], env, *this);
                 }
             }
             vAttrs = j->value;
@@ -1387,7 +1590,6 @@ void EvalState::callFunction(Value & fun, size_t nrArgs, Value * * args, Value &
 
             if (!lambda.hasFormals())
                 env2.values[displ++] = args[0];
-
             else {
                 forceAttrs(*args[0], pos);
 
@@ -1402,7 +1604,7 @@ void EvalState::callFunction(Value & fun, size_t nrArgs, Value * * args, Value &
                     auto j = args[0]->attrs->get(i.name);
                     if (!j) {
                         if (!i.def) throwTypeError(pos, "%1% called without required argument '%2%'",
-                            lambda, i.name);
+                            lambda, i.name, *fun.lambda.env, lambda);
                         env2.values[displ++] = i.def->maybeThunk(*this, env2);
                     } else {
                         attrsUsed++;
@@ -1424,8 +1626,7 @@ void EvalState::callFunction(Value & fun, size_t nrArgs, Value * * args, Value &
                                 pos,
                                 Suggestions::bestMatches(formalNames, symbols[i.name]),
                                 "%1% called with unexpected argument '%2%'",
-                                lambda,
+                                lambda, i.name, *fun.lambda.env, lambda);
-                                i.name);
                         }
                     abort(); // can't happen
                 }
@@ -1436,6 +1637,15 @@ void EvalState::callFunction(Value & fun, size_t nrArgs, Value * * args, Value &
 
             /* Evaluate the body. */
             try {
+                auto dts = debugRepl
+                    ? makeDebugTraceStacker(
+                        *this, *lambda.body, env2, positions[lambda.pos],
+                        "while evaluating %s",
+                        lambda.name
+                        ? concatStrings("'", symbols[lambda.name], "'")
+                        : "anonymous lambda")
+                    : nullptr;
+
                 lambda.body->eval(*this, env2, vCur);
             } catch (Error & e) {
                 if (loggerSettings.showTrace.get()) {
@@ -1590,8 +1800,8 @@ void EvalState::autoCallFunction(Bindings & args, Value & fun, Value & res)
 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/expressions/language-constructs.html#functions.)", symbols[i.name]);
+https://nixos.org/manual/nix/stable/expressions/language-constructs.html#functions.)", symbols[i.name],                 
-
+                *fun.lambda.env, *fun.lambda.fun);
             }
         }
     }
@@ -1623,7 +1833,7 @@ void ExprAssert::eval(EvalState & state, Env & env, Value & v)
     if (!state.evalBool(env, cond, pos)) {
         std::ostringstream out;
         cond->show(state.symbols, out);
-        state.throwAssertionError(pos, "assertion '%1%' failed", out.str());
+        state.throwAssertionError(pos, "assertion '%1%' failed", out.str(), env, *this);
     }
     body->eval(state, env, v);
 }
@@ -1800,14 +2010,14 @@ 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));
+                state.throwEvalError(i_pos, "cannot add %1% to an integer", showType(vTmp), env, *this);
         } 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));
+                state.throwEvalError(i_pos, "cannot add %1% to a float", showType(vTmp), env, *this);
         } else {
             if (s.empty()) s.reserve(es->size());
             /* skip canonization of first path, which would only be not
@@ -1827,7 +2037,7 @@ 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");
+            state.throwEvalError(pos, "a string that refers to a store path cannot be appended to a path", env, *this);
         v.mkPath(canonPath(str()));
     } else
         v.mkStringMove(c_str(), context);
@@ -1854,6 +2064,12 @@ void EvalState::forceValueDeep(Value & v)
         if (v.type() == nAttrs) {
             for (auto & i : *v.attrs)
                 try {
+                    // If the value is a thunk, we're evaling. Otherwise no trace necessary.
+                    auto dts = debugRepl && i.value->isThunk()
+                        ? makeDebugTraceStacker(*this, *i.value->thunk.expr, *i.value->thunk.env, positions[i.pos],
+                            "while evaluating the attribute '%1%'", symbols[i.name])
+                        : nullptr;
+
                     recurse(*i.value);
                 } catch (Error & e) {
                     addErrorTrace(e, i.pos, "while evaluating the attribute '%1%'", symbols[i.name]);
@@ -1876,6 +2092,7 @@ NixInt EvalState::forceInt(Value & v, const PosIdx pos)
     forceValue(v, pos);
     if (v.type() != nInt)
         throwTypeError(pos, "value is %1% while an integer was expected", v);
+
     return v.integer;
 }
 
@@ -1918,10 +2135,7 @@ std::string_view EvalState::forceString(Value & v, const PosIdx pos)
 {
     forceValue(v, pos);
     if (v.type() != nString) {
-        if (pos)
+        throwTypeError(pos, "value is %1% while a string was expected", v);
-            throwTypeError(pos, "value is %1% while a string was expected", v);
-        else
-            throwTypeError("value is %1% while a string was expected", v);
     }
     return v.string.s;
 }
@@ -2038,7 +2252,8 @@ BackedStringView EvalState::coerceToString(const PosIdx pos, Value & v, PathSet
         if (maybeString)
             return std::move(*maybeString);
         auto i = v.attrs->find(sOutPath);
-        if (i == v.attrs->end()) throwTypeError(pos, "cannot coerce a set to a string");
+        if (i == v.attrs->end())
+            throwTypeError(pos, "cannot coerce a set to a string");
         return coerceToString(pos, *i->value, context, coerceMore, copyToStore);
     }
 
@@ -2046,7 +2261,6 @@ BackedStringView EvalState::coerceToString(const PosIdx pos, Value & v, PathSet
         return v.external->coerceToString(positions[pos], context, coerceMore, copyToStore);
 
     if (coerceMore) {
-
         /* Note that `false' is represented as an empty string for
            shell scripting convenience, just like `null'. */
         if (v.type() == nBool && v.boolean) return "1";
@@ -2191,7 +2405,9 @@ bool EvalState::eqValues(Value & v1, Value & v2)
             return v1.fpoint == v2.fpoint;
 
         default:
-            throwEvalError("cannot compare %1% with %2%", showType(v1), showType(v2));
+            throwEvalError("cannot compare %1% with %2%",
+                showType(v1),
+                showType(v2));
     }
 }
 

src/libexpr/eval.hh

@@ -13,7 +13,6 @@
 #include <unordered_map>
 #include <mutex>
 
-
 namespace nix {
 
 
@@ -25,7 +24,6 @@ enum RepairFlag : bool;
 
 typedef void (* PrimOpFun) (EvalState & state, const PosIdx pos, Value * * args, Value & v);
 
-
 struct PrimOp
 {
     PrimOpFun fun;
@@ -35,6 +33,11 @@ struct PrimOp
     const char * doc = nullptr;
 };
 
+#if HAVE_BOEHMGC
+    typedef std::map<std::string, Value *, std::less<std::string>, traceable_allocator<std::pair<const std::string, Value *> > > ValMap;
+#else
+    typedef std::map<std::string, Value *> ValMap;
+#endif
 
 struct Env
 {
@@ -44,6 +47,10 @@ struct Env
     Value * values[0];
 };
 
+void printEnvBindings(const EvalState &es, const Expr & expr, const Env & env);
+void printEnvBindings(const SymbolTable & st, const StaticEnv & se, const Env & env, int lvl = 0);
+
+std::unique_ptr<ValMap> mapStaticEnvBindings(const SymbolTable & st, const StaticEnv & se, const Env & env);
 
 void copyContext(const Value & v, PathSet & context);
 
@@ -70,8 +77,17 @@ struct RegexCache;
 
 std::shared_ptr<RegexCache> makeRegexCache();
 
+struct DebugTrace {
+    std::optional<ErrPos> pos;
+    const Expr & expr;
+    const Env & env;
+    hintformat hint;
+    bool isError;
+};
 
-class EvalState
+void debugError(Error * e, Env & env, Expr & expr);
+
+class EvalState : public std::enable_shared_from_this<EvalState>
 {
 public:
     SymbolTable symbols;
@@ -87,7 +103,8 @@ public:
         sOutputHash, sOutputHashAlgo, sOutputHashMode,
         sRecurseForDerivations,
         sDescription, sSelf, sEpsilon, sStartSet, sOperator, sKey, sPath,
-        sPrefix;
+        sPrefix,
+        sOutputSpecified;
     Symbol sDerivationNix;
 
     /* If set, force copying files to the Nix store even if they
@@ -109,12 +126,55 @@ public:
     RootValue vCallFlake = nullptr;
     RootValue vImportedDrvToDerivation = nullptr;
 
+    /* Debugger */
+    void (* debugRepl)(ref<EvalState> es, const ValMap & extraEnv);
+    bool debugStop;
+    bool debugQuit;
+    std::list<DebugTrace> debugTraces;
+    std::map<const Expr*, const std::shared_ptr<const StaticEnv>> exprEnvs;
+    const std::shared_ptr<const StaticEnv> getStaticEnv(const Expr & expr) const
+    {
+        auto i = exprEnvs.find(&expr);
+        if (i != exprEnvs.end())
+            return i->second;
+        else
+            return std::shared_ptr<const StaticEnv>();;
+    }
+
+    void runDebugRepl(const Error * error, const Env & env, const Expr & expr);
+
+    template<class E>
+    [[gnu::noinline, gnu::noreturn]]
+    void debugThrow(E && error, const Env & env, const Expr & expr)
+    {
+        if (debugRepl)
+            runDebugRepl(&error, env, expr);
+
+        throw error;
+    }
+
+    template<class E>
+    [[gnu::noinline, gnu::noreturn]]
+    void debugThrowLastTrace(E && e)
+    {
+        // 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()) {
+            const DebugTrace & last = debugTraces.front();
+            runDebugRepl(&e, last.env, last.expr);
+        }
+
+        throw e;
+    }
+
+
 private:
     SrcToStore srcToStore;
 
     /* A cache from path names to parse trees. */
 #if HAVE_BOEHMGC
-    typedef std::map<Path, Expr *, std::less<Path>, traceable_allocator<std::pair<const Path, Expr *> > > FileParseCache;
+    typedef std::map<Path, Expr *, std::less<Path>, traceable_allocator<std::pair<const Path, Expr *>>> FileParseCache;
 #else
     typedef std::map<Path, Expr *> FileParseCache;
 #endif
@@ -122,7 +182,7 @@ private:
 
     /* A cache from path names to values. */
 #if HAVE_BOEHMGC
-    typedef std::map<Path, Value, std::less<Path>, traceable_allocator<std::pair<const Path, Value> > > FileEvalCache;
+    typedef std::map<Path, Value, std::less<Path>, traceable_allocator<std::pair<const Path, Value>>> FileEvalCache;
 #else
     typedef std::map<Path, Value> FileEvalCache;
 #endif
@@ -185,10 +245,10 @@ public:
 
     /* Parse a Nix expression from the specified file. */
     Expr * parseExprFromFile(const Path & path);
-    Expr * parseExprFromFile(const Path & path, StaticEnv & staticEnv);
+    Expr * parseExprFromFile(const Path & path, std::shared_ptr<StaticEnv> & staticEnv);
 
     /* Parse a Nix expression from the specified string. */
-    Expr * parseExprFromString(std::string s, const Path & basePath, StaticEnv & staticEnv);
+    Expr * parseExprFromString(std::string s, const Path & basePath, std::shared_ptr<StaticEnv> & staticEnv);
     Expr * parseExprFromString(std::string s, const Path & basePath);
 
     Expr * parseStdin();
@@ -198,7 +258,7 @@ public:
        trivial (i.e. doesn't require arbitrary computation). */
     void evalFile(const Path & path, Value & v, bool mustBeTrivial = false);
 
-    /* Like `cacheFile`, but with an already parsed expression. */
+    /* Like `evalFile`, but with an already parsed expression. */
     void cacheFile(
         const Path & path,
         const Path & resolvedPath,
@@ -255,37 +315,68 @@ public:
     std::string_view forceStringNoCtx(Value & v, const PosIdx pos = noPos);
 
     [[gnu::noinline, gnu::noreturn]]
-    void throwEvalError(const PosIdx pos, const char * s) const;
+    void throwEvalError(const PosIdx pos, const char * s);
     [[gnu::noinline, gnu::noreturn]]
-    void throwTypeError(const PosIdx pos, const char * s, const Value & v) const;
+    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) const;
+    void throwEvalError(const char * s, const std::string & s2);
     [[gnu::noinline, gnu::noreturn]]
-    void throwEvalError(const PosIdx pos, const Suggestions & suggestions, const char * s,
+    void throwEvalError(const PosIdx pos, const char * s, const std::string & s2);
-        const std::string & s2) const;
     [[gnu::noinline, gnu::noreturn]]
-    void throwEvalError(const PosIdx pos, const char * s, const std::string & s2) const;
+    void throwEvalError(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) const;
+    void throwEvalError(const PosIdx pos, 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, const std::string & s3) const;
+    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 p1, const char * s, const Symbol sym, const PosIdx p2) const;
+    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 throwTypeError(const PosIdx pos, const char * s) const;
+    void throwEvalError(const PosIdx pos, const char * s, const std::string & s2, const std::string & s3);
     [[gnu::noinline, gnu::noreturn]]
-    void throwTypeError(const PosIdx pos, const char * s, const ExprLambda & fun, const Symbol s2) const;
+    void throwEvalError(const char * s, const std::string & s2, const std::string & s3);
     [[gnu::noinline, gnu::noreturn]]
-    void throwTypeError(const PosIdx pos, const Suggestions & suggestions, const char * s,
+    void throwEvalError(const PosIdx pos, const Suggestions & suggestions, const char * s, const std::string & s2,
-        const ExprLambda & fun, const Symbol s2) const;
+        Env & env, Expr & expr);
     [[gnu::noinline, gnu::noreturn]]
-    void throwTypeError(const char * s, const Value & v) const;
+    void throwEvalError(const PosIdx p1, const char * s, const Symbol sym, const PosIdx p2,
+        Env & env, Expr & expr);
+
     [[gnu::noinline, gnu::noreturn]]
-    void throwAssertionError(const PosIdx pos, const char * s, const std::string & s1) const;
+    void throwTypeError(const PosIdx pos, const char * s, const Value & v);
     [[gnu::noinline, gnu::noreturn]]
-    void throwUndefinedVarError(const PosIdx pos, const char * s, const std::string & s1) const;
+    void throwTypeError(const PosIdx pos, const char * s, const Value & v,
+        Env & env, Expr & expr);
     [[gnu::noinline, gnu::noreturn]]
-    void throwMissingArgumentError(const PosIdx pos, const char * s, const std::string & s1) const;
+    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);
 
     [[gnu::noinline]]
     void addErrorTrace(Error & e, const char * s, const std::string & s2) const;
@@ -325,7 +416,7 @@ public:
     Env & baseEnv;
 
     /* The same, but used during parsing to resolve variables. */
-    StaticEnv staticBaseEnv; // !!! should be private
+    std::shared_ptr<StaticEnv> staticBaseEnv; // !!! should be private
 
 private:
 
@@ -366,7 +457,7 @@ private:
     friend struct ExprLet;
 
     Expr * parse(char * text, size_t length, FileOrigin origin, const PathView path,
-        const PathView basePath, StaticEnv & staticEnv);
+        const PathView basePath, std::shared_ptr<StaticEnv> & staticEnv);
 
 public:
 
@@ -461,6 +552,16 @@ private:
     friend struct Value;
 };
 
+struct DebugTraceStacker {
+    DebugTraceStacker(EvalState & evalState, DebugTrace t);
+    ~DebugTraceStacker()
+    {
+        // assert(evalState.debugTraces.front() == trace);
+        evalState.debugTraces.pop_front();
+    }
+    EvalState & evalState;
+    DebugTrace trace;
+};
 
 /* Return a string representing the type of the value `v'. */
 std::string_view showType(ValueType type);

src/libexpr/get-drvs.cc

@@ -132,23 +132,36 @@ DrvInfo::Outputs DrvInfo::queryOutputs(bool withPaths, bool onlyOutputsToInstall
         } else
             outputs.emplace("out", withPaths ? std::optional{queryOutPath()} : std::nullopt);
     }
+
     if (!onlyOutputsToInstall || !attrs)
         return outputs;
 
-    /* Check for `meta.outputsToInstall` and return `outputs` reduced to that. */
+    Bindings::iterator i;
-    const Value * outTI = queryMeta("outputsToInstall");
+    if (attrs && (i = attrs->find(state->sOutputSpecified)) != attrs->end() && state->forceBool(*i->value, i->pos)) {
-    if (!outTI) return outputs;
+        Outputs result;
-    const auto errMsg = Error("this derivation has bad 'meta.outputsToInstall'");
+        auto out = outputs.find(queryOutputName());
-        /* ^ this shows during `nix-env -i` right under the bad derivation */
+        if (out == outputs.end())
-    if (!outTI->isList()) throw errMsg;
+            throw Error("derivation does not have output '%s'", queryOutputName());
-    Outputs result;
-    for (auto elem : outTI->listItems()) {
-        if (elem->type() != nString) throw errMsg;
-        auto out = outputs.find(elem->string.s);
-        if (out == outputs.end()) throw errMsg;
         result.insert(*out);
+        return result;
+    }
+
+    else {
+        /* Check for `meta.outputsToInstall` and return `outputs` reduced to that. */
+        const Value * outTI = queryMeta("outputsToInstall");
+        if (!outTI) return outputs;
+        const auto errMsg = Error("this derivation has bad 'meta.outputsToInstall'");
+            /* ^ this shows during `nix-env -i` right under the bad derivation */
+        if (!outTI->isList()) throw errMsg;
+        Outputs result;
+        for (auto elem : outTI->listItems()) {
+            if (elem->type() != nString) throw errMsg;
+            auto out = outputs.find(elem->string.s);
+            if (out == outputs.end()) throw errMsg;
+            result.insert(*out);
+        }
+        return result;
     }
-    return result;
 }
 
 

src/libexpr/get-drvs.hh

@@ -73,7 +73,7 @@ public:
 
 
 #if HAVE_BOEHMGC
-typedef std::list<DrvInfo, traceable_allocator<DrvInfo> > DrvInfos;
+typedef std::list<DrvInfo, traceable_allocator<DrvInfo>> DrvInfos;
 #else
 typedef std::list<DrvInfo> DrvInfos;
 #endif

src/libexpr/lexer.l

@@ -198,7 +198,7 @@ or          { return OR_KW; }
                    (...|\$[^\{\"\\]|\\.|\$\\.)+ would have triggered.
                    This is technically invalid, but we leave the problem to the
                    parser who fails with exact location. */
-                return STR;
+                return EOF;
               }
 
 \'\'(\ *\n)?     { PUSH_STATE(IND_STRING); return IND_STRING_OPEN; }

src/libexpr/nixexpr.cc

@@ -6,10 +6,8 @@
 
 #include <cstdlib>
 
-
 namespace nix {
 
-
 /* Displaying abstract syntax trees. */
 
 static void showString(std::ostream & str, std::string_view s)
@@ -294,35 +292,46 @@ std::string showAttrPath(const SymbolTable & symbols, const AttrPath & attrPath)
 
 /* Computing levels/displacements for variables. */
 
-void Expr::bindVars(const EvalState & es, const StaticEnv & env)
+void Expr::bindVars(EvalState & es, const std::shared_ptr<const StaticEnv> & env)
 {
     abort();
 }
 
-void ExprInt::bindVars(const EvalState & es, const StaticEnv & env)
+void ExprInt::bindVars(EvalState & es, const std::shared_ptr<const StaticEnv> & env)
 {
+    if (es.debugRepl)
+        es.exprEnvs.insert(std::make_pair(this, env));
 }
 
-void ExprFloat::bindVars(const EvalState & es, const StaticEnv & env)
+void ExprFloat::bindVars(EvalState & es, const std::shared_ptr<const StaticEnv> & env)
 {
+    if (es.debugRepl)
+        es.exprEnvs.insert(std::make_pair(this, env));
 }
 
-void ExprString::bindVars(const EvalState & es, const StaticEnv & env)
+void ExprString::bindVars(EvalState & es, const std::shared_ptr<const StaticEnv> & env)
 {
+    if (es.debugRepl)
+        es.exprEnvs.insert(std::make_pair(this, env));
 }
 
-void ExprPath::bindVars(const EvalState & es, const StaticEnv & env)
+void ExprPath::bindVars(EvalState & es, const std::shared_ptr<const StaticEnv> & env)
 {
+    if (es.debugRepl)
+        es.exprEnvs.insert(std::make_pair(this, env));
 }
 
-void ExprVar::bindVars(const EvalState & es, const StaticEnv & env)
+void ExprVar::bindVars(EvalState & es, const std::shared_ptr<const StaticEnv> & env)
 {
+    if (es.debugRepl)
+        es.exprEnvs.insert(std::make_pair(this, env));
+
     /* Check whether the variable appears in the environment.  If so,
        set its level and displacement. */
     const StaticEnv * curEnv;
     Level level;
     int withLevel = -1;
-    for (curEnv = &env, level = 0; curEnv; curEnv = curEnv->up, level++) {
+    for (curEnv = env.get(), level = 0; curEnv; curEnv = curEnv->up, level++) {
         if (curEnv->isWith) {
             if (withLevel == -1) withLevel = level;
         } else {
@@ -348,8 +357,11 @@ void ExprVar::bindVars(const EvalState & es, const StaticEnv & env)
     this->level = withLevel;
 }
 
-void ExprSelect::bindVars(const EvalState & es, const StaticEnv & env)
+void ExprSelect::bindVars(EvalState & es, const std::shared_ptr<const StaticEnv> & env)
 {
+    if (es.debugRepl)
+        es.exprEnvs.insert(std::make_pair(this, env));
+
     e->bindVars(es, env);
     if (def) def->bindVars(es, env);
     for (auto & i : attrPath)
@@ -357,64 +369,78 @@ void ExprSelect::bindVars(const EvalState & es, const StaticEnv & env)
             i.expr->bindVars(es, env);
 }
 
-void ExprOpHasAttr::bindVars(const EvalState & es, const StaticEnv & env)
+void ExprOpHasAttr::bindVars(EvalState & es, const std::shared_ptr<const StaticEnv> & env)
 {
+    if (es.debugRepl)
+        es.exprEnvs.insert(std::make_pair(this, env));
+
     e->bindVars(es, env);
     for (auto & i : attrPath)
         if (!i.symbol)
             i.expr->bindVars(es, env);
 }
 
-void ExprAttrs::bindVars(const EvalState & es, const StaticEnv & env)
+void ExprAttrs::bindVars(EvalState & es, const std::shared_ptr<const StaticEnv> & env)
 {
-    const StaticEnv * dynamicEnv = &env;
+    if (es.debugRepl)
-    StaticEnv newEnv(false, &env, recursive ? attrs.size() : 0);
+        es.exprEnvs.insert(std::make_pair(this, env));
 
     if (recursive) {
-        dynamicEnv = &newEnv;
+        auto newEnv = std::make_shared<StaticEnv>(false, env.get(), recursive ? attrs.size() : 0);
 
         Displacement displ = 0;
         for (auto & i : attrs)
-            newEnv.vars.emplace_back(i.first, i.second.displ = displ++);
+            newEnv->vars.emplace_back(i.first, i.second.displ = displ++);
 
         // No need to sort newEnv since attrs is in sorted order.
 
         for (auto & i : attrs)
             i.second.e->bindVars(es, i.second.inherited ? env : newEnv);
-    }
 
-    else
+        for (auto & i : dynamicAttrs) {
+            i.nameExpr->bindVars(es, newEnv);
+            i.valueExpr->bindVars(es, newEnv);
+        }
+    }
+    else {
         for (auto & i : attrs)
             i.second.e->bindVars(es, env);
 
-    for (auto & i : dynamicAttrs) {
+        for (auto & i : dynamicAttrs) {
-        i.nameExpr->bindVars(es, *dynamicEnv);
+            i.nameExpr->bindVars(es, env);
-        i.valueExpr->bindVars(es, *dynamicEnv);
+            i.valueExpr->bindVars(es, env);
+        }
     }
 }
 
-void ExprList::bindVars(const EvalState & es, const StaticEnv & env)
+void ExprList::bindVars(EvalState & es, const std::shared_ptr<const StaticEnv> & env)
 {
+    if (es.debugRepl)
+        es.exprEnvs.insert(std::make_pair(this, env));
+
     for (auto & i : elems)
         i->bindVars(es, env);
 }
 
-void ExprLambda::bindVars(const EvalState & es, const StaticEnv & env)
+void ExprLambda::bindVars(EvalState & es, const std::shared_ptr<const StaticEnv> & env)
 {
-    StaticEnv newEnv(
+    if (es.debugRepl)
-        false, &env,
+        es.exprEnvs.insert(std::make_pair(this, env));
+
+    auto newEnv = std::make_shared<StaticEnv>(
+        false, env.get(),
         (hasFormals() ? formals->formals.size() : 0) +
         (!arg ? 0 : 1));
 
     Displacement displ = 0;
 
-    if (arg) newEnv.vars.emplace_back(arg, displ++);
+    if (arg) newEnv->vars.emplace_back(arg, displ++);
 
     if (hasFormals()) {
         for (auto & i : formals->formals)
-            newEnv.vars.emplace_back(i.name, displ++);
+            newEnv->vars.emplace_back(i.name, displ++);
 
-        newEnv.sort();
+        newEnv->sort();
 
         for (auto & i : formals->formals)
             if (i.def) i.def->bindVars(es, newEnv);
@@ -423,20 +449,26 @@ void ExprLambda::bindVars(const EvalState & es, const StaticEnv & env)
     body->bindVars(es, newEnv);
 }
 
-void ExprCall::bindVars(const EvalState & es, const StaticEnv & env)
+void ExprCall::bindVars(EvalState & es, const std::shared_ptr<const StaticEnv> & env)
 {
+    if (es.debugRepl)
+        es.exprEnvs.insert(std::make_pair(this, env));
+
     fun->bindVars(es, env);
     for (auto e : args)
         e->bindVars(es, env);
 }
 
-void ExprLet::bindVars(const EvalState & es, const StaticEnv & env)
+void ExprLet::bindVars(EvalState & es, const std::shared_ptr<const StaticEnv> & env)
 {
-    StaticEnv newEnv(false, &env, attrs->attrs.size());
+    if (es.debugRepl)
+        es.exprEnvs.insert(std::make_pair(this, env));
+
+    auto newEnv = std::make_shared<StaticEnv>(false, env.get(), attrs->attrs.size());
 
     Displacement displ = 0;
     for (auto & i : attrs->attrs)
-        newEnv.vars.emplace_back(i.first, i.second.displ = displ++);
+        newEnv->vars.emplace_back(i.first, i.second.displ = displ++);
 
     // No need to sort newEnv since attrs->attrs is in sorted order.
 
@@ -446,51 +478,71 @@ void ExprLet::bindVars(const EvalState & es, const StaticEnv & env)
     body->bindVars(es, newEnv);
 }
 
-void ExprWith::bindVars(const EvalState & es, const StaticEnv & env)
+void ExprWith::bindVars(EvalState & es, const std::shared_ptr<const StaticEnv> & env)
 {
+    if (es.debugRepl)
+        es.exprEnvs.insert(std::make_pair(this, env));
+
     /* Does this `with' have an enclosing `with'?  If so, record its
        level so that `lookupVar' can look up variables in the previous
        `with' if this one doesn't contain the desired attribute. */
     const StaticEnv * curEnv;
     Level level;
     prevWith = 0;
-    for (curEnv = &env, level = 1; curEnv; curEnv = curEnv->up, level++)
+    for (curEnv = env.get(), level = 1; curEnv; curEnv = curEnv->up, level++)
         if (curEnv->isWith) {
             prevWith = level;
             break;
         }
 
+    if (es.debugRepl)
+        es.exprEnvs.insert(std::make_pair(this, env));
+
     attrs->bindVars(es, env);
-    StaticEnv newEnv(true, &env);
+    auto newEnv = std::make_shared<StaticEnv>(true, env.get());
     body->bindVars(es, newEnv);
 }
 
-void ExprIf::bindVars(const EvalState & es, const StaticEnv & env)
+void ExprIf::bindVars(EvalState & es, const std::shared_ptr<const StaticEnv> & env)
 {
+    if (es.debugRepl)
+        es.exprEnvs.insert(std::make_pair(this, env));
+
     cond->bindVars(es, env);
     then->bindVars(es, env);
     else_->bindVars(es, env);
 }
 
-void ExprAssert::bindVars(const EvalState & es, const StaticEnv & env)
+void ExprAssert::bindVars(EvalState & es, const std::shared_ptr<const StaticEnv> & env)
 {
+    if (es.debugRepl)
+        es.exprEnvs.insert(std::make_pair(this, env));
+
     cond->bindVars(es, env);
     body->bindVars(es, env);
 }
 
-void ExprOpNot::bindVars(const EvalState & es, const StaticEnv & env)
+void ExprOpNot::bindVars(EvalState & es, const std::shared_ptr<const StaticEnv> & env)
 {
+    if (es.debugRepl)
+        es.exprEnvs.insert(std::make_pair(this, env));
+
     e->bindVars(es, env);
 }
 
-void ExprConcatStrings::bindVars(const EvalState & es, const StaticEnv & env)
+void ExprConcatStrings::bindVars(EvalState & es, const std::shared_ptr<const StaticEnv> & env)
 {
+    if (es.debugRepl)
+        es.exprEnvs.insert(std::make_pair(this, env));
+
     for (auto & i : *this->es)
         i.second->bindVars(es, env);
 }
 
-void ExprPos::bindVars(const EvalState & es, const StaticEnv & env)
+void ExprPos::bindVars(EvalState & es, const std::shared_ptr<const StaticEnv> & env)
 {
+    if (es.debugRepl)
+        es.exprEnvs.insert(std::make_pair(this, env));
 }
 
 

src/libexpr/nixexpr.hh

@@ -22,7 +22,6 @@ MakeError(UndefinedVarError, Error);
 MakeError(MissingArgumentError, EvalError);
 MakeError(RestrictedPathError, Error);
 
-
 /* Position objects. */
 
 struct Pos
@@ -143,24 +142,25 @@ struct Expr
 {
     virtual ~Expr() { };
     virtual void show(const SymbolTable & symbols, std::ostream & str) const;
-    virtual void bindVars(const EvalState & es, const StaticEnv & env);
+    virtual void bindVars(EvalState & es, const std::shared_ptr<const StaticEnv> & env);
     virtual void eval(EvalState & state, Env & env, Value & v);
     virtual Value * maybeThunk(EvalState & state, Env & env);
     virtual void setName(Symbol name);
+    virtual PosIdx getPos() const { return noPos; }
 };
 
 #define COMMON_METHODS \
     void show(const SymbolTable & symbols, std::ostream & str) const;    \
     void eval(EvalState & state, Env & env, Value & v); \
-    void bindVars(const EvalState & es, const StaticEnv & env);
+    void bindVars(EvalState & es, const std::shared_ptr<const StaticEnv> & env);
 
 struct ExprInt : Expr
 {
     NixInt n;
     Value v;
     ExprInt(NixInt n) : n(n) { v.mkInt(n); };
-    COMMON_METHODS
     Value * maybeThunk(EvalState & state, Env & env);
+    COMMON_METHODS
 };
 
 struct ExprFloat : Expr
@@ -168,8 +168,8 @@ struct ExprFloat : Expr
     NixFloat nf;
     Value v;
     ExprFloat(NixFloat nf) : nf(nf) { v.mkFloat(nf); };
-    COMMON_METHODS
     Value * maybeThunk(EvalState & state, Env & env);
+    COMMON_METHODS
 };
 
 struct ExprString : Expr
@@ -177,8 +177,8 @@ struct ExprString : Expr
     std::string s;
     Value v;
     ExprString(std::string s) : s(std::move(s)) { v.mkString(this->s.data()); };
-    COMMON_METHODS
     Value * maybeThunk(EvalState & state, Env & env);
+    COMMON_METHODS
 };
 
 struct ExprPath : Expr
@@ -186,8 +186,8 @@ struct ExprPath : Expr
     std::string s;
     Value v;
     ExprPath(std::string s) : s(std::move(s)) { v.mkPath(this->s.c_str()); };
-    COMMON_METHODS
     Value * maybeThunk(EvalState & state, Env & env);
+    COMMON_METHODS
 };
 
 typedef uint32_t Level;
@@ -213,8 +213,9 @@ struct ExprVar : Expr
 
     ExprVar(Symbol name) : name(name) { };
     ExprVar(const PosIdx & pos, Symbol name) : pos(pos), name(name) { };
-    COMMON_METHODS
     Value * maybeThunk(EvalState & state, Env & env);
+    PosIdx getPos() const override { return pos; }
+    COMMON_METHODS
 };
 
 struct ExprSelect : Expr
@@ -224,6 +225,7 @@ struct ExprSelect : Expr
     AttrPath attrPath;
     ExprSelect(const PosIdx & pos, Expr * e, const AttrPath & attrPath, Expr * def) : pos(pos), e(e), def(def), attrPath(attrPath) { };
     ExprSelect(const PosIdx & pos, Expr * e, Symbol name) : pos(pos), e(e), def(0) { attrPath.push_back(AttrName(name)); };
+    PosIdx getPos() const override { return pos; }
     COMMON_METHODS
 };
 
@@ -232,6 +234,7 @@ struct ExprOpHasAttr : Expr
     Expr * e;
     AttrPath attrPath;
     ExprOpHasAttr(Expr * e, const AttrPath & attrPath) : e(e), attrPath(attrPath) { };
+    PosIdx getPos() const override { return e->getPos(); }
     COMMON_METHODS
 };
 
@@ -260,6 +263,7 @@ struct ExprAttrs : Expr
     DynamicAttrDefs dynamicAttrs;
     ExprAttrs(const PosIdx &pos) : recursive(false), pos(pos) { };
     ExprAttrs() : recursive(false) { };
+    PosIdx getPos() const override { return pos; }
     COMMON_METHODS
 };
 
@@ -268,6 +272,11 @@ struct ExprList : Expr
     std::vector<Expr *> elems;
     ExprList() { };
     COMMON_METHODS
+
+    PosIdx getPos() const override
+    {
+        return elems.empty() ? noPos : elems.front()->getPos();
+    }
 };
 
 struct Formal
@@ -320,6 +329,7 @@ struct ExprLambda : Expr
     void setName(Symbol name);
     std::string showNamePos(const EvalState & state) const;
     inline bool hasFormals() const { return formals != nullptr; }
+    PosIdx getPos() const override { return pos; }
     COMMON_METHODS
 };
 
@@ -331,6 +341,7 @@ struct ExprCall : Expr
     ExprCall(const PosIdx & pos, Expr * fun, std::vector<Expr *> && args)
         : fun(fun), args(args), pos(pos)
     { }
+    PosIdx getPos() const override { return pos; }
     COMMON_METHODS
 };
 
@@ -348,6 +359,7 @@ struct ExprWith : Expr
     Expr * attrs, * body;
     size_t prevWith;
     ExprWith(const PosIdx & pos, Expr * attrs, Expr * body) : pos(pos), attrs(attrs), body(body) { };
+    PosIdx getPos() const override { return pos; }
     COMMON_METHODS
 };
 
@@ -356,6 +368,7 @@ struct ExprIf : Expr
     PosIdx pos;
     Expr * cond, * then, * else_;
     ExprIf(const PosIdx & pos, Expr * cond, Expr * then, Expr * else_) : pos(pos), cond(cond), then(then), else_(else_) { };
+    PosIdx getPos() const override { return pos; }
     COMMON_METHODS
 };
 
@@ -364,6 +377,7 @@ struct ExprAssert : Expr
     PosIdx pos;
     Expr * cond, * body;
     ExprAssert(const PosIdx & pos, Expr * cond, Expr * body) : pos(pos), cond(cond), body(body) { };
+    PosIdx getPos() const override { return pos; }
     COMMON_METHODS
 };
 
@@ -385,11 +399,12 @@ struct ExprOpNot : Expr
         { \
             str << "("; e1->show(symbols, str); str << " " s " "; e2->show(symbols, str); str << ")"; \
         } \
-        void bindVars(const EvalState & es, const StaticEnv & env)    \
+        void bindVars(EvalState & es, const std::shared_ptr<const StaticEnv> & env)    \
         { \
             e1->bindVars(es, env); e2->bindVars(es, env);    \
         } \
         void eval(EvalState & state, Env & env, Value & v); \
+        PosIdx getPos() const override { return pos; } \
     };
 
 MakeBinOp(ExprOpEq, "==")
@@ -404,9 +419,10 @@ struct ExprConcatStrings : Expr
 {
     PosIdx pos;
     bool forceString;
-    std::vector<std::pair<PosIdx, Expr *> > * es;
+    std::vector<std::pair<PosIdx, Expr *>> * es;
-    ExprConcatStrings(const PosIdx & pos, bool forceString, std::vector<std::pair<PosIdx, Expr *> > * es)
+    ExprConcatStrings(const PosIdx & pos, bool forceString, std::vector<std::pair<PosIdx, Expr *>> * es)
         : pos(pos), forceString(forceString), es(es) { };
+    PosIdx getPos() const override { return pos; }
     COMMON_METHODS
 };
 
@@ -414,6 +430,7 @@ struct ExprPos : Expr
 {
     PosIdx pos;
     ExprPos(const PosIdx & pos) : pos(pos) { };
+    PosIdx getPos() const override { return pos; }
     COMMON_METHODS
 };
 

src/libexpr/parser.y

@@ -193,7 +193,7 @@ static Formals * toFormals(ParseData & data, ParserFormals * formals,
 
 
 static Expr * stripIndentation(const PosIdx pos, SymbolTable & symbols,
-    std::vector<std::pair<PosIdx, std::variant<Expr *, StringToken> > > & es)
+    std::vector<std::pair<PosIdx, std::variant<Expr *, StringToken>>> & es)
 {
     if (es.empty()) return new ExprString("");
 
@@ -233,7 +233,7 @@ static Expr * stripIndentation(const PosIdx pos, SymbolTable & symbols,
     }
 
     /* Strip spaces from each line. */
-    auto * es2 = new std::vector<std::pair<PosIdx, Expr *> >;
+    auto * es2 = new std::vector<std::pair<PosIdx, Expr *>>;
     atStartOfLine = true;
     size_t curDropped = 0;
     size_t n = es.size();
@@ -320,8 +320,8 @@ void yyerror(YYLTYPE * loc, yyscan_t scanner, ParseData * data, const char * err
   StringToken uri;
   StringToken str;
   std::vector<nix::AttrName> * attrNames;
-  std::vector<std::pair<nix::PosIdx, nix::Expr *> > * string_parts;
+  std::vector<std::pair<nix::PosIdx, nix::Expr *>> * string_parts;
-  std::vector<std::pair<nix::PosIdx, std::variant<nix::Expr *, StringToken> > > * ind_string_parts;
+  std::vector<std::pair<nix::PosIdx, std::variant<nix::Expr *, StringToken>>> * ind_string_parts;
 }
 
 %type <e> start expr expr_function expr_if expr_op
@@ -415,7 +415,7 @@ expr_op
   | expr_op UPDATE expr_op { $$ = new ExprOpUpdate(CUR_POS, $1, $3); }
   | expr_op '?' attrpath { $$ = new ExprOpHasAttr($1, *$3); }
   | expr_op '+' expr_op
-    { $$ = new ExprConcatStrings(CUR_POS, false, new std::vector<std::pair<PosIdx, Expr *> >({{makeCurPos(@1, data), $1}, {makeCurPos(@3, data), $3}})); }
+    { $$ = 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 { $$ = new ExprCall(CUR_POS, new ExprVar(data->symbols.create("__div")), {$1, $3}); }
@@ -503,9 +503,9 @@ string_parts_interpolated
   : string_parts_interpolated STR
   { $$ = $1; $1->emplace_back(makeCurPos(@2, data), new ExprString(std::string($2))); }
   | string_parts_interpolated DOLLAR_CURLY expr '}' { $$ = $1; $1->emplace_back(makeCurPos(@2, data), $3); }
-  | DOLLAR_CURLY expr '}' { $$ = new std::vector<std::pair<PosIdx, Expr *> >; $$->emplace_back(makeCurPos(@1, data), $2); }
+  | DOLLAR_CURLY expr '}' { $$ = new std::vector<std::pair<PosIdx, Expr *>>; $$->emplace_back(makeCurPos(@1, data), $2); }
   | STR DOLLAR_CURLY expr '}' {
-      $$ = new std::vector<std::pair<PosIdx, Expr *> >;
+      $$ = new std::vector<std::pair<PosIdx, Expr *>>;
       $$->emplace_back(makeCurPos(@1, data), new ExprString(std::string($1)));
       $$->emplace_back(makeCurPos(@2, data), $3);
     }
@@ -528,7 +528,7 @@ path_start
 ind_string_parts
   : ind_string_parts IND_STR { $$ = $1; $1->emplace_back(makeCurPos(@2, data), $2); }
   | ind_string_parts DOLLAR_CURLY expr '}' { $$ = $1; $1->emplace_back(makeCurPos(@2, data), $3); }
-  | { $$ = new std::vector<std::pair<PosIdx, std::variant<Expr *, StringToken> > >; }
+  | { $$ = new std::vector<std::pair<PosIdx, std::variant<Expr *, StringToken>>>; }
   ;
 
 binds
@@ -643,7 +643,7 @@ namespace nix {
 
 
 Expr * EvalState::parse(char * text, size_t length, FileOrigin origin,
-    const PathView path, const PathView basePath, StaticEnv & staticEnv)
+    const PathView path, const PathView basePath, std::shared_ptr<StaticEnv> & staticEnv)
 {
     yyscan_t scanner;
     std::string file;
@@ -706,7 +706,7 @@ Expr * EvalState::parseExprFromFile(const Path & path)
 }
 
 
-Expr * EvalState::parseExprFromFile(const Path & path, StaticEnv & staticEnv)
+Expr * EvalState::parseExprFromFile(const Path & path, std::shared_ptr<StaticEnv> & staticEnv)
 {
     auto buffer = readFile(path);
     // readFile should have left some extra space for terminators
@@ -715,7 +715,7 @@ Expr * EvalState::parseExprFromFile(const Path & path, StaticEnv & staticEnv)
 }
 
 
-Expr * EvalState::parseExprFromString(std::string s, const Path & basePath, StaticEnv & staticEnv)
+Expr * EvalState::parseExprFromString(std::string s, const Path & basePath, std::shared_ptr<StaticEnv> & staticEnv)
 {
     s.append("\0\0", 2);
     return parse(s.data(), s.size(), foString, "", basePath, staticEnv);
@@ -782,13 +782,13 @@ Path EvalState::findFile(SearchPath & searchPath, const std::string_view path, c
     if (hasPrefix(path, "nix/"))
         return concatStrings(corepkgsPrefix, path.substr(4));
 
-    throw ThrownError({
+    debugThrowLastTrace(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]
-    });
+    }));
 }
 
 

src/libexpr/primops.cc

@@ -46,7 +46,7 @@ StringMap EvalState::realiseContext(const PathSet & context)
         auto [ctx, outputName] = decodeContext(*store, i);
         auto ctxS = store->printStorePath(ctx);
         if (!store->isValidPath(ctx))
-            throw InvalidPathError(store->printStorePath(ctx));
+            debugThrowLastTrace(InvalidPathError(store->printStorePath(ctx)));
         if (!outputName.empty() && ctx.isDerivation()) {
             drvs.push_back({ctx, {outputName}});
         } else {
@@ -57,9 +57,9 @@ StringMap EvalState::realiseContext(const PathSet & context)
     if (drvs.empty()) return {};
 
     if (!evalSettings.enableImportFromDerivation)
-        throw Error(
+        debugThrowLastTrace(Error(
             "cannot build '%1%' during evaluation because the option 'allow-import-from-derivation' is disabled",
-            store->printStorePath(drvs.begin()->drvPath));
+            store->printStorePath(drvs.begin()->drvPath)));
 
     /* Build/substitute the context. */
     std::vector<DerivedPath> buildReqs;
@@ -72,8 +72,8 @@ StringMap EvalState::realiseContext(const PathSet & context)
         for (auto & outputName : outputs) {
             auto outputPath = get(outputPaths, outputName);
             if (!outputPath)
-                throw Error("derivation '%s' does not have an output named '%s'",
+                debugThrowLastTrace(Error("derivation '%s' does not have an output named '%s'",
-                        store->printStorePath(drvPath), outputName);
+                    store->printStorePath(drvPath), outputName));
             res.insert_or_assign(
                 downstreamPlaceholder(*store, drvPath, outputName),
                 store->printStorePath(*outputPath)
@@ -216,11 +216,11 @@ static void import(EvalState & state, const PosIdx pos, Value & vPath, Value * v
             Env * env = &state.allocEnv(vScope->attrs->size());
             env->up = &state.baseEnv;
 
-            StaticEnv staticEnv(false, &state.staticBaseEnv, vScope->attrs->size());
+            auto staticEnv = std::make_shared<StaticEnv>(false, state.staticBaseEnv.get(), vScope->attrs->size());
 
             unsigned int displ = 0;
             for (auto & attr : *vScope->attrs) {
-                staticEnv.vars.emplace_back(attr.name, displ);
+                staticEnv->vars.emplace_back(attr.name, displ);
                 env->values[displ++] = attr.value;
             }
 
@@ -319,17 +319,16 @@ void prim_importNative(EvalState & state, const PosIdx pos, Value * * args, Valu
 
     void *handle = dlopen(path.c_str(), RTLD_LAZY | RTLD_LOCAL);
     if (!handle)
-        throw EvalError("could not open '%1%': %2%", path, dlerror());
+        state.debugThrowLastTrace(EvalError("could not open '%1%': %2%", path, dlerror()));
 
     dlerror();
     ValueInitializer func = (ValueInitializer) dlsym(handle, sym.c_str());
     if(!func) {
         char *message = dlerror();
         if (message)
-            throw EvalError("could not load symbol '%1%' from '%2%': %3%", sym, path, message);
+            state.debugThrowLastTrace(EvalError("could not load symbol '%1%' from '%2%': %3%", sym, path, message));
         else
-            throw EvalError("symbol '%1%' from '%2%' resolved to NULL when a function pointer was expected",
+            state.debugThrowLastTrace(EvalError("symbol '%1%' from '%2%' resolved to NULL when a function pointer was expected", sym, path));
-                sym, path);
     }
 
     (func)(state, v);
@@ -344,12 +343,11 @@ void prim_exec(EvalState & state, const PosIdx pos, Value * * args, Value & v)
     state.forceList(*args[0], pos);
     auto elems = args[0]->listElems();
     auto count = args[0]->listSize();
-    if (count == 0) {
+    if (count == 0)
-        throw EvalError({
+        state.debugThrowLastTrace(EvalError({
             .msg = hintfmt("at least one argument to 'exec' required"),
             .errPos = state.positions[pos]
-        });
+        }));
-    }
     PathSet context;
     auto program = state.coerceToString(pos, *elems[0], context, false, false).toOwned();
     Strings commandArgs;
@@ -359,11 +357,11 @@ void prim_exec(EvalState & state, const PosIdx pos, Value * * args, Value & v)
     try {
         auto _ = state.realiseContext(context); // FIXME: Handle CA derivations
     } catch (InvalidPathError & e) {
-        throw EvalError({
+        state.debugThrowLastTrace(EvalError({
             .msg = hintfmt("cannot execute '%1%', since path '%2%' is not valid",
                 program, e.path),
             .errPos = state.positions[pos]
-        });
+        }));
     }
 
     auto output = runProgram(program, true, commandArgs);
@@ -547,7 +545,7 @@ struct CompareValues
         if (v1->type() == nInt && v2->type() == nFloat)
             return v1->integer < v2->fpoint;
         if (v1->type() != v2->type())
-            throw EvalError("cannot compare %1% with %2%", showType(*v1), showType(*v2));
+            state.debugThrowLastTrace(EvalError("cannot compare %1% with %2%", showType(*v1), showType(*v2)));
         switch (v1->type()) {
             case nInt:
                 return v1->integer < v2->integer;
@@ -569,14 +567,14 @@ struct CompareValues
                     }
                 }
             default:
-                throw EvalError("cannot compare %1% with %2%", showType(*v1), showType(*v2));
+                state.debugThrowLastTrace(EvalError("cannot compare %1% with %2%", showType(*v1), showType(*v2)));
         }
     }
 };
 
 
 #if HAVE_BOEHMGC
-typedef std::list<Value *, gc_allocator<Value *> > ValueList;
+typedef std::list<Value *, gc_allocator<Value *>> ValueList;
 #else
 typedef std::list<Value *> ValueList;
 #endif
@@ -599,10 +597,10 @@ static Bindings::iterator getAttr(
 
         auto aPos = attrSet->pos;
         if (!aPos) {
-            throw TypeError({
+            state.debugThrowLastTrace(TypeError({
                 .msg = errorMsg,
                 .errPos = state.positions[pos],
-            });
+            }));
         } else {
             auto e = TypeError({
                 .msg = errorMsg,
@@ -612,7 +610,7 @@ static Bindings::iterator getAttr(
             // Adding another trace for the function name to make it clear
             // which call received wrong arguments.
             e.addTrace(state.positions[pos], hintfmt("while invoking '%s'", funcName));
-            throw e;
+            state.debugThrowLastTrace(e);
         }
     }
 
@@ -666,10 +664,10 @@ static void prim_genericClosure(EvalState & state, const PosIdx pos, Value * * a
         Bindings::iterator key =
             e->attrs->find(state.sKey);
         if (key == e->attrs->end())
-            throw EvalError({
+            state.debugThrowLastTrace(EvalError({
                 .msg = hintfmt("attribute 'key' required"),
                 .errPos = state.positions[pos]
-            });
+            }));
         state.forceValue(*key->value, pos);
 
         if (!doneKeys.insert(key->value).second) continue;
@@ -725,6 +723,41 @@ static RegisterPrimOp primop_genericClosure(RegisterPrimOp::Info {
     .fun = prim_genericClosure,
 });
 
+
+static RegisterPrimOp primop_break({
+    .name = "break",
+    .args = {"v"},
+    .doc = R"(
+      In debug mode (enabled using `--debugger`), pause Nix expression evaluation and enter the REPL.
+      Otherwise, return the argument `v`.
+    )",
+    .fun = [](EvalState & state, const PosIdx pos, Value * * args, Value & v)
+    {
+        if (state.debugRepl && !state.debugTraces.empty()) {
+            auto error = Error(ErrorInfo {
+                .level = lvlInfo,
+                .msg = hintfmt("breakpoint reached"),
+                .errPos = state.positions[pos],
+            });
+
+            auto & dt = state.debugTraces.front();
+            state.runDebugRepl(&error, dt.env, dt.expr);
+
+            if (state.debugQuit) {
+                // If the user elects to quit the repl, throw an exception.
+                throw Error(ErrorInfo{
+                    .level = lvlInfo,
+                    .msg = hintfmt("quit the debugger"),
+                    .errPos = state.positions[noPos],
+                });
+            }
+        }
+
+        // Return the value we were passed.
+        v = *args[0];
+    }
+});
+
 static RegisterPrimOp primop_abort({
     .name = "abort",
     .args = {"s"},
@@ -735,7 +768,7 @@ static RegisterPrimOp primop_abort({
     {
         PathSet context;
         auto s = state.coerceToString(pos, *args[0], context).toOwned();
-        throw Abort("evaluation aborted with the following error message: '%1%'", s);
+        state.debugThrowLastTrace(Abort("evaluation aborted with the following error message: '%1%'", s));
     }
 });
 
@@ -753,7 +786,7 @@ static RegisterPrimOp primop_throw({
     {
       PathSet context;
       auto s = state.coerceToString(pos, *args[0], context).toOwned();
-      throw ThrownError(s);
+      state.debugThrowLastTrace(ThrownError(s));
     }
 });
 
@@ -1008,37 +1041,37 @@ static void prim_derivationStrict(EvalState & state, const PosIdx pos, Value * *
             if (s == "recursive") ingestionMethod = FileIngestionMethod::Recursive;
             else if (s == "flat") ingestionMethod = FileIngestionMethod::Flat;
             else
-                throw EvalError({
+                state.debugThrowLastTrace(EvalError({
                     .msg = hintfmt("invalid value '%s' for 'outputHashMode' attribute", s),
                     .errPos = state.positions[posDrvName]
-                });
+                }));
         };
 
         auto handleOutputs = [&](const Strings & ss) {
             outputs.clear();
             for (auto & j : ss) {
                 if (outputs.find(j) != outputs.end())
-                    throw EvalError({
+                    state.debugThrowLastTrace(EvalError({
                         .msg = hintfmt("duplicate derivation output '%1%'", j),
                         .errPos = state.positions[posDrvName]
-                    });
+                    }));
                 /* !!! Check whether j is a valid attribute
                    name. */
                 /* Derivations cannot be named ‘drv’, because
                    then we'd have an attribute ‘drvPath’ in
                    the resulting set. */
                 if (j == "drv")
-                    throw EvalError({
+                    state.debugThrowLastTrace(EvalError({
                         .msg = hintfmt("invalid derivation output name 'drv'" ),
                         .errPos = state.positions[posDrvName]
-                    });
+                    }));
                 outputs.insert(j);
             }
             if (outputs.empty())
-                throw EvalError({
+                state.debugThrowLastTrace(EvalError({
                     .msg = hintfmt("derivation cannot have an empty set of outputs"),
                     .errPos = state.positions[posDrvName]
-                });
+                }));
         };
 
         try {
@@ -1163,23 +1196,23 @@ static void prim_derivationStrict(EvalState & state, const PosIdx pos, Value * *
 
     /* Do we have all required attributes? */
     if (drv.builder == "")
-        throw EvalError({
+        state.debugThrowLastTrace(EvalError({
             .msg = hintfmt("required attribute 'builder' missing"),
             .errPos = state.positions[posDrvName]
-        });
+        }));
 
     if (drv.platform == "")
-        throw EvalError({
+        state.debugThrowLastTrace(EvalError({
             .msg = hintfmt("required attribute 'system' missing"),
             .errPos = state.positions[posDrvName]
-        });
+        }));
 
     /* Check whether the derivation name is valid. */
     if (isDerivation(drvName))
-        throw EvalError({
+        state.debugThrowLastTrace(EvalError({
             .msg = hintfmt("derivation names are not allowed to end in '%s'", drvExtension),
             .errPos = state.positions[posDrvName]
-        });
+        }));
 
     if (outputHash) {
         /* Handle fixed-output derivations.
@@ -1187,10 +1220,10 @@ static void prim_derivationStrict(EvalState & state, const PosIdx pos, Value * *
            Ignore `__contentAddressed` because fixed output derivations are
            already content addressed. */
         if (outputs.size() != 1 || *(outputs.begin()) != "out")
-            throw Error({
+            state.debugThrowLastTrace(Error({
                 .msg = hintfmt("multiple outputs are not supported in fixed-output derivations"),
                 .errPos = state.positions[posDrvName]
-            });
+            }));
 
         auto h = newHashAllowEmpty(*outputHash, parseHashTypeOpt(outputHashAlgo));
 
@@ -1358,10 +1391,10 @@ static RegisterPrimOp primop_toPath({
 static void prim_storePath(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
     if (evalSettings.pureEval)
-        throw EvalError({
+        state.debugThrowLastTrace(EvalError({
             .msg = hintfmt("'%s' is not allowed in pure evaluation mode", "builtins.storePath"),
             .errPos = state.positions[pos]
-        });
+        }));
 
     PathSet context;
     Path path = state.checkSourcePath(state.coerceToPath(pos, *args[0], context));
@@ -1370,10 +1403,10 @@ static void prim_storePath(EvalState & state, const PosIdx pos, Value * * args,
        e.g. nix-push does the right thing. */
     if (!state.store->isStorePath(path)) path = canonPath(path, true);
     if (!state.store->isInStore(path))
-        throw EvalError({
+        state.debugThrowLastTrace(EvalError({
             .msg = hintfmt("path '%1%' is not in the Nix store", path),
             .errPos = state.positions[pos]
-        });
+        }));
     auto path2 = state.store->toStorePath(path).first;
     if (!settings.readOnlyMode)
         state.store->ensurePath(path2);
@@ -1476,7 +1509,7 @@ static void prim_readFile(EvalState & state, const PosIdx pos, Value * * args, V
     auto path = realisePath(state, pos, *args[0]);
     auto s = readFile(path);
     if (s.find((char) 0) != std::string::npos)
-        throw Error("the contents of the file '%1%' cannot be represented as a Nix string", path);
+        state.debugThrowLastTrace(Error("the contents of the file '%1%' cannot be represented as a Nix string", path));
     StorePathSet refs;
     if (state.store->isInStore(path)) {
         try {
@@ -1528,13 +1561,12 @@ static void prim_findFile(EvalState & state, const PosIdx pos, Value * * args, V
             auto rewrites = state.realiseContext(context);
             path = rewriteStrings(path, rewrites);
         } catch (InvalidPathError & e) {
-            throw EvalError({
+            state.debugThrowLastTrace(EvalError({
                 .msg = hintfmt("cannot find '%1%', since path '%2%' is not valid", path, e.path),
                 .errPos = state.positions[pos]
-            });
+            }));
         }
 
-
         searchPath.emplace_back(prefix, path);
     }
 
@@ -1555,10 +1587,10 @@ static void prim_hashFile(EvalState & state, const PosIdx pos, Value * * args, V
     auto type = state.forceStringNoCtx(*args[0], pos);
     std::optional<HashType> ht = parseHashType(type);
     if (!ht)
-        throw Error({
+        state.debugThrowLastTrace(Error({
             .msg = hintfmt("unknown hash type '%1%'", type),
             .errPos = state.positions[pos]
-        });
+        }));
 
     auto path = realisePath(state, pos, *args[1]);
 
@@ -1795,13 +1827,13 @@ static void prim_toFile(EvalState & state, const PosIdx pos, Value * * args, Val
 
     for (auto path : context) {
         if (path.at(0) != '/')
-            throw EvalError( {
+            state.debugThrowLastTrace(EvalError({
                 .msg = hintfmt(
                     "in 'toFile': the file named '%1%' must not contain a reference "
                     "to a derivation but contains (%2%)",
                     name, path),
                 .errPos = state.positions[pos]
-            });
+            }));
         refs.insert(state.store->parseStorePath(path));
     }
 
@@ -1959,7 +1991,7 @@ static void addPath(
                 ? state.store->computeStorePathForPath(name, path, method, htSHA256, filter).first
                 : state.store->addToStore(name, path, method, htSHA256, filter, state.repair, refs);
             if (expectedHash && expectedStorePath != dstPath)
-                throw Error("store path mismatch in (possibly filtered) path added from '%s'", path);
+                state.debugThrowLastTrace(Error("store path mismatch in (possibly filtered) path added from '%s'", path));
             state.allowAndSetStorePathString(dstPath, v);
         } else
             state.allowAndSetStorePathString(*expectedStorePath, v);
@@ -1977,12 +2009,12 @@ static void prim_filterSource(EvalState & state, const PosIdx pos, Value * * arg
 
     state.forceValue(*args[0], pos);
     if (args[0]->type() != nFunction)
-        throw TypeError({
+        state.debugThrowLastTrace(TypeError({
             .msg = hintfmt(
                 "first argument in call to 'filterSource' is not a function but %1%",
                 showType(*args[0])),
             .errPos = state.positions[pos]
-        });
+        }));
 
     addPath(state, pos, std::string(baseNameOf(path)), path, args[0], FileIngestionMethod::Recursive, std::nullopt, v, context);
 }
@@ -2066,16 +2098,16 @@ static void prim_path(EvalState & state, const PosIdx pos, Value * * args, Value
         else if (n == "sha256")
             expectedHash = newHashAllowEmpty(state.forceStringNoCtx(*attr.value, attr.pos), htSHA256);
         else
-            throw EvalError({
+            state.debugThrowLastTrace(EvalError({
                 .msg = hintfmt("unsupported argument '%1%' to 'addPath'", state.symbols[attr.name]),
                 .errPos = state.positions[attr.pos]
-            });
+            }));
     }
     if (path.empty())
-        throw EvalError({
+        state.debugThrowLastTrace(EvalError({
             .msg = hintfmt("'path' required"),
             .errPos = state.positions[pos]
-        });
+        }));
     if (name.empty())
         name = baseNameOf(path);
 
@@ -2447,10 +2479,10 @@ static void prim_functionArgs(EvalState & state, const PosIdx pos, Value * * arg
         return;
     }
     if (!args[0]->isLambda())
-        throw TypeError({
+        state.debugThrowLastTrace(TypeError({
             .msg = hintfmt("'functionArgs' requires a function"),
             .errPos = state.positions[pos]
-        });
+        }));
 
     if (!args[0]->lambda.fun->hasFormals()) {
         v.mkAttrs(&state.emptyBindings);
@@ -2538,7 +2570,7 @@ static void prim_zipAttrsWith(EvalState & state, const PosIdx pos, Value * * arg
                 attrsSeen[attr.name].first++;
         } catch (TypeError & e) {
             e.addTrace(state.positions[pos], hintfmt("while invoking '%s'", "zipAttrsWith"));
-            throw;
+            state.debugThrowLastTrace(e);
         }
     }
 
@@ -2625,10 +2657,10 @@ static void elemAt(EvalState & state, const PosIdx pos, Value & list, int n, Val
 {
     state.forceList(list, pos);
     if (n < 0 || (unsigned int) n >= list.listSize())
-        throw Error({
+        state.debugThrowLastTrace(Error({
             .msg = hintfmt("list index %1% is out of bounds", n),
             .errPos = state.positions[pos]
-        });
+        }));
     state.forceValue(*list.listElems()[n], pos);
     v = *list.listElems()[n];
 }
@@ -2673,10 +2705,10 @@ static void prim_tail(EvalState & state, const PosIdx pos, Value * * args, Value
 {
     state.forceList(*args[0], pos);
     if (args[0]->listSize() == 0)
-        throw Error({
+        state.debugThrowLastTrace(Error({
             .msg = hintfmt("'tail' called on an empty list"),
             .errPos = state.positions[pos]
-        });
+        }));
 
     state.mkList(v, args[0]->listSize() - 1);
     for (unsigned int n = 0; n < v.listSize(); ++n)
@@ -2911,10 +2943,10 @@ static void prim_genList(EvalState & state, const PosIdx pos, Value * * args, Va
     auto len = state.forceInt(*args[1], pos);
 
     if (len < 0)
-        throw EvalError({
+        state.debugThrowLastTrace(EvalError({
             .msg = hintfmt("cannot create list of size %1%", len),
             .errPos = state.positions[pos]
-        });
+        }));
 
     state.mkList(v, len);
 
@@ -3123,7 +3155,7 @@ static void prim_concatMap(EvalState & state, const PosIdx pos, Value * * args,
             state.forceList(lists[n], lists[n].determinePos(args[0]->determinePos(pos)));
         } catch (TypeError &e) {
             e.addTrace(state.positions[pos], hintfmt("while invoking '%s'", "concatMap"));
-            throw;
+            state.debugThrowLastTrace(e);
         }
         len += lists[n].listSize();
     }
@@ -3218,10 +3250,10 @@ static void prim_div(EvalState & state, const PosIdx pos, Value * * args, Value
 
     NixFloat f2 = state.forceFloat(*args[1], pos);
     if (f2 == 0)
-        throw EvalError({
+        state.debugThrowLastTrace(EvalError({
             .msg = hintfmt("division by zero"),
             .errPos = state.positions[pos]
-        });
+        }));
 
     if (args[0]->type() == nFloat || args[1]->type() == nFloat) {
         v.mkFloat(state.forceFloat(*args[0], pos) / state.forceFloat(*args[1], pos));
@@ -3230,10 +3262,10 @@ static void prim_div(EvalState & state, const PosIdx pos, Value * * args, Value
         NixInt i2 = state.forceInt(*args[1], pos);
         /* Avoid division overflow as it might raise SIGFPE. */
         if (i1 == std::numeric_limits<NixInt>::min() && i2 == -1)
-            throw EvalError({
+            state.debugThrowLastTrace(EvalError({
                 .msg = hintfmt("overflow in integer division"),
                 .errPos = state.positions[pos]
-            });
+            }));
 
         v.mkInt(i1 / i2);
     }
@@ -3361,10 +3393,10 @@ static void prim_substring(EvalState & state, const PosIdx pos, Value * * args,
     auto s = state.coerceToString(pos, *args[2], context);
 
     if (start < 0)
-        throw EvalError({
+        state.debugThrowLastTrace(EvalError({
             .msg = hintfmt("negative start position in 'substring'"),
             .errPos = state.positions[pos]
-        });
+        }));
 
     v.mkString((unsigned int) start >= s->size() ? "" : s->substr(start, len), context);
 }
@@ -3412,10 +3444,10 @@ static void prim_hashString(EvalState & state, const PosIdx pos, Value * * args,
     auto type = state.forceStringNoCtx(*args[0], pos);
     std::optional<HashType> ht = parseHashType(type);
     if (!ht)
-        throw Error({
+        state.debugThrowLastTrace(Error({
             .msg = hintfmt("unknown hash type '%1%'", type),
             .errPos = state.positions[pos]
-        });
+        }));
 
     PathSet context; // discarded
     auto s = state.forceString(*args[1], context, pos);
@@ -3482,19 +3514,18 @@ void prim_match(EvalState & state, const PosIdx pos, Value * * args, Value & v)
                 (v.listElems()[i] = state.allocValue())->mkString(match[i + 1].str());
         }
 
-    } catch (std::regex_error &e) {
+    } catch (std::regex_error & e) {
         if (e.code() == std::regex_constants::error_space) {
             // limit is _GLIBCXX_REGEX_STATE_LIMIT for libstdc++
-            throw EvalError({
+            state.debugThrowLastTrace(EvalError({
                 .msg = hintfmt("memory limit exceeded by regular expression '%s'", re),
                 .errPos = state.positions[pos]
-            });
+            }));
-        } else {
+        } else
-            throw EvalError({
+            state.debugThrowLastTrace(EvalError({
                 .msg = hintfmt("invalid regular expression '%s'", re),
                 .errPos = state.positions[pos]
-            });
+            }));
-        }
     }
 }
 
@@ -3587,19 +3618,18 @@ void prim_split(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 
         assert(idx == 2 * len + 1);
 
-    } catch (std::regex_error &e) {
+    } catch (std::regex_error & e) {
         if (e.code() == std::regex_constants::error_space) {
             // limit is _GLIBCXX_REGEX_STATE_LIMIT for libstdc++
-            throw EvalError({
+            state.debugThrowLastTrace(EvalError({
                 .msg = hintfmt("memory limit exceeded by regular expression '%s'", re),
                 .errPos = state.positions[pos]
-            });
+            }));
-        } else {
+        } else
-            throw EvalError({
+            state.debugThrowLastTrace(EvalError({
                 .msg = hintfmt("invalid regular expression '%s'", re),
                 .errPos = state.positions[pos]
-            });
+            }));
-        }
     }
 }
 
@@ -3675,10 +3705,10 @@ static void prim_replaceStrings(EvalState & state, const PosIdx pos, Value * * a
     state.forceList(*args[0], pos);
     state.forceList(*args[1], pos);
     if (args[0]->listSize() != args[1]->listSize())
-        throw EvalError({
+        state.debugThrowLastTrace(EvalError({
             .msg = hintfmt("'from' and 'to' arguments to 'replaceStrings' have different lengths"),
             .errPos = state.positions[pos]
-        });
+        }));
 
     std::vector<std::string> from;
     from.reserve(args[0]->listSize());
@@ -3931,7 +3961,7 @@ void EvalState::createBaseEnv()
        because attribute lookups expect it to be sorted. */
     baseEnv.values[0]->attrs->sort();
 
-    staticBaseEnv.sort();
+    staticBaseEnv->sort();
 
     /* Note: we have to initialize the 'derivation' constant *after*
        building baseEnv/staticBaseEnv because it uses 'builtins'. */

src/libexpr/primops/fetchTree.cc

@@ -108,16 +108,16 @@ static void fetchTree(
 
         if (auto aType = args[0]->attrs->get(state.sType)) {
             if (type)
-                throw Error({
+                state.debugThrowLastTrace(EvalError({
                     .msg = hintfmt("unexpected attribute 'type'"),
                     .errPos = state.positions[pos]
-                });
+                }));
             type = state.forceStringNoCtx(*aType->value, aType->pos);
         } else if (!type)
-            throw Error({
+            state.debugThrowLastTrace(EvalError({
                 .msg = hintfmt("attribute 'type' is missing in call to 'fetchTree'"),
                 .errPos = state.positions[pos]
-            });
+            }));
 
         attrs.emplace("type", type.value());
 
@@ -138,16 +138,16 @@ static void fetchTree(
             else if (attr.value->type() == nInt)
                 attrs.emplace(state.symbols[attr.name], uint64_t(attr.value->integer));
             else
-                throw TypeError("fetchTree argument '%s' is %s while a string, Boolean or integer is expected",
+                state.debugThrowLastTrace(TypeError("fetchTree argument '%s' is %s while a string, Boolean or integer is expected",
-                    state.symbols[attr.name], showType(*attr.value));
+                    state.symbols[attr.name], showType(*attr.value)));
         }
 
         if (!params.allowNameArgument)
             if (auto nameIter = attrs.find("name"); nameIter != attrs.end())
-                throw Error({
+                state.debugThrowLastTrace(EvalError({
-                    .msg = hintfmt("attribute 'name' isn't supported in call to 'fetchTree'"),
+                    .msg = hintfmt("attribute 'name' isn’t supported in call to 'fetchTree'"),
                     .errPos = state.positions[pos]
-                });
+                }));
 
         input = fetchers::Input::fromAttrs(std::move(attrs));
     } else {
@@ -167,7 +167,7 @@ static void fetchTree(
         input = lookupInRegistries(state.store, input).first;
 
     if (evalSettings.pureEval && !input.isLocked())
-        throw Error("in pure evaluation mode, 'fetchTree' requires a locked input, at %s", state.positions[pos]);
+        state.debugThrowLastTrace(EvalError("in pure evaluation mode, 'fetchTree' requires a locked input, at %s", state.positions[pos]));
 
     auto [tree, input2] = input.fetch(state.store);
 
@@ -206,17 +206,17 @@ static void fetch(EvalState & state, const PosIdx pos, Value * * args, Value & v
             else if (n == "name")
                 name = state.forceStringNoCtx(*attr.value, attr.pos);
             else
-                throw EvalError({
+                state.debugThrowLastTrace(EvalError({
                     .msg = hintfmt("unsupported argument '%s' to '%s'", n, who),
                     .errPos = state.positions[attr.pos]
-                });
+                }));
-            }
+        }
 
         if (!url)
-            throw EvalError({
+            state.debugThrowLastTrace(EvalError({
                 .msg = hintfmt("'url' argument required"),
                 .errPos = state.positions[pos]
-            });
+            }));
     } else
         url = state.forceStringNoCtx(*args[0], pos);
 
@@ -228,7 +228,7 @@ static void fetch(EvalState & state, const PosIdx pos, Value * * args, Value & v
         name = baseNameOf(*url);
 
     if (evalSettings.pureEval && !expectedHash)
-        throw Error("in pure evaluation mode, '%s' requires a 'sha256' argument", who);
+        state.debugThrowLastTrace(EvalError("in pure evaluation mode, '%s' requires a 'sha256' argument", who));
 
     // early exit if pinned and already in the store
     if (expectedHash && expectedHash->type == htSHA256) {
@@ -255,8 +255,8 @@ static void fetch(EvalState & state, const PosIdx pos, Value * * args, Value & v
             ? state.store->queryPathInfo(storePath)->narHash
             : hashFile(htSHA256, state.store->toRealPath(storePath));
         if (hash != *expectedHash)
-            throw Error((unsigned int) 102, "hash mismatch in file downloaded from '%s':\n  specified: %s\n  got:       %s",
+            state.debugThrowLastTrace(EvalError((unsigned int) 102, "hash mismatch in file downloaded from '%s':\n  specified: %s\n  got:       %s",
-                *url, expectedHash->to_string(Base32, true), hash.to_string(Base32, true));
+                *url, expectedHash->to_string(Base32, true), hash.to_string(Base32, true)));
     }
 
     state.allowAndSetStorePathString(storePath, v);

src/libexpr/value-to-json.cc

@@ -85,6 +85,7 @@ void printValueAsJSON(EvalState & state, bool strict,
                 .errPos = state.positions[v.determinePos(pos)]
             });
             e.addTrace(state.positions[pos], hintfmt("message for the trace"));
+            state.debugThrowLastTrace(e);
             throw e;
     }
 }
@@ -99,7 +100,7 @@ void printValueAsJSON(EvalState & state, bool strict,
 void ExternalValueBase::printValueAsJSON(EvalState & state, bool strict,
     JSONPlaceholder & out, PathSet & context) const
 {
-    throw TypeError("cannot convert %1% to JSON", showType());
+    state.debugThrowLastTrace(TypeError("cannot convert %1% to JSON", showType()));
 }
 
 

src/libexpr/value.hh

@@ -404,9 +404,9 @@ public:
 
 
 #if HAVE_BOEHMGC
-typedef std::vector<Value *, traceable_allocator<Value *> > ValueVector;
+typedef std::vector<Value *, traceable_allocator<Value *>> ValueVector;
-typedef std::map<Symbol, Value *, std::less<Symbol>, traceable_allocator<std::pair<const Symbol, Value *> > > ValueMap;
+typedef std::map<Symbol, Value *, std::less<Symbol>, traceable_allocator<std::pair<const Symbol, Value *>>> ValueMap;
-typedef std::map<Symbol, ValueVector, std::less<Symbol>, traceable_allocator<std::pair<const Symbol, ValueVector> > > ValueVectorMap;
+typedef std::map<Symbol, ValueVector, std::less<Symbol>, traceable_allocator<std::pair<const Symbol, ValueVector>>> ValueVectorMap;
 #else
 typedef std::vector<Value *> ValueVector;
 typedef std::map<Symbol, Value *> ValueMap;

src/libfetchers/git.cc

@@ -449,11 +449,10 @@ struct GitInputScheme : InputScheme
             }
         }
 
-        const Attrs unlockedAttrs({
+        Attrs unlockedAttrs({
             {"type", cacheType},
             {"name", name},
             {"url", actualUrl},
-            {"ref", *input.getRef()},
         });
 
         Path repoDir;
@@ -466,6 +465,7 @@ struct GitInputScheme : InputScheme
                     head = "master";
                 }
                 input.attrs.insert_or_assign("ref", *head);
+                unlockedAttrs.insert_or_assign("ref", *head);
             }
 
             if (!input.getRev())
@@ -482,6 +482,7 @@ struct GitInputScheme : InputScheme
                     head = "master";
                 }
                 input.attrs.insert_or_assign("ref", *head);
+                unlockedAttrs.insert_or_assign("ref", *head);
             }
 
             if (auto res = getCache()->lookup(store, unlockedAttrs)) {

src/libfetchers/tarball.cc

@@ -6,6 +6,7 @@
 #include "archive.hh"
 #include "tarfile.hh"
 #include "types.hh"
+#include "split.hh"
 
 namespace nix::fetchers {
 
@@ -168,24 +169,34 @@ std::pair<Tree, time_t> downloadTarball(
     };
 }
 
-struct TarballInputScheme : InputScheme
+// An input scheme corresponding to a curl-downloadable resource.
+struct CurlInputScheme : InputScheme
 {
+    virtual const std::string inputType() const = 0;
+    const std::set<std::string> transportUrlSchemes = {"file", "http", "https"};
+
+    const bool hasTarballExtension(std::string_view path) const
+    {
+        return hasSuffix(path, ".zip") || hasSuffix(path, ".tar")
+            || hasSuffix(path, ".tgz") || hasSuffix(path, ".tar.gz")
+            || hasSuffix(path, ".tar.xz") || hasSuffix(path, ".tar.bz2")
+            || hasSuffix(path, ".tar.zst");
+    }
+
+    virtual bool isValidURL(const ParsedURL & url) const = 0;
+
     std::optional<Input> inputFromURL(const ParsedURL & url) override
     {
-        if (url.scheme != "file" && url.scheme != "http" && url.scheme != "https") return {};
+        if (!isValidURL(url))
-
+            return std::nullopt;
-        if (!hasSuffix(url.path, ".zip")
-            && !hasSuffix(url.path, ".tar")
-            && !hasSuffix(url.path, ".tgz")
-            && !hasSuffix(url.path, ".tar.gz")
-            && !hasSuffix(url.path, ".tar.xz")
-            && !hasSuffix(url.path, ".tar.bz2")
-            && !hasSuffix(url.path, ".tar.zst"))
-            return {};
 
         Input input;
-        input.attrs.insert_or_assign("type", "tarball");
+
-        input.attrs.insert_or_assign("url", url.to_string());
+        auto urlWithoutApplicationScheme = url;
+        urlWithoutApplicationScheme.scheme = parseUrlScheme(url.scheme).transport;
+
+        input.attrs.insert_or_assign("type", inputType());
+        input.attrs.insert_or_assign("url", urlWithoutApplicationScheme.to_string());
         auto narHash = url.query.find("narHash");
         if (narHash != url.query.end())
             input.attrs.insert_or_assign("narHash", narHash->second);
@@ -194,14 +205,17 @@ struct TarballInputScheme : InputScheme
 
     std::optional<Input> inputFromAttrs(const Attrs & attrs) override
     {
-        if (maybeGetStrAttr(attrs, "type") != "tarball") return {};
+        auto type = maybeGetStrAttr(attrs, "type");
+        if (type != inputType()) return {};
 
+        std::set<std::string> allowedNames = {"type", "url", "narHash", "name", "unpack"};
         for (auto & [name, value] : attrs)
-            if (name != "type" && name != "url" && /* name != "hash" && */ name != "narHash" && name != "name")
+            if (!allowedNames.count(name))
-                throw Error("unsupported tarball input attribute '%s'", name);
+                throw Error("unsupported %s input attribute '%s'", *type, name);
 
         Input input;
         input.attrs = attrs;
+
         //input.locked = (bool) maybeGetStrAttr(input.attrs, "hash");
         return input;
     }
@@ -209,14 +223,9 @@ struct TarballInputScheme : InputScheme
     ParsedURL toURL(const Input & input) override
     {
         auto url = parseURL(getStrAttr(input.attrs, "url"));
-        // NAR hashes are preferred over file hashes since tar/zip files
+        // NAR hashes are preferred over file hashes since tar/zip files        // don't have a canonical representation.
-        // don't have a canonical representation.
         if (auto narHash = input.getNarHash())
             url.query.insert_or_assign("narHash", narHash->to_string(SRI, true));
-        /*
-        else if (auto hash = maybeGetStrAttr(input.attrs, "hash"))
-            url.query.insert_or_assign("hash", Hash(*hash).to_string(SRI, true));
-        */
         return url;
     }
 
@@ -225,6 +234,42 @@ struct TarballInputScheme : InputScheme
         return true;
     }
 
+};
+
+struct FileInputScheme : CurlInputScheme
+{
+    const std::string inputType() const override { return "file"; }
+
+    bool isValidURL(const ParsedURL & url) const override
+    {
+        auto parsedUrlScheme = parseUrlScheme(url.scheme);
+        return transportUrlSchemes.count(std::string(parsedUrlScheme.transport))
+            && (parsedUrlScheme.application
+                    ? parsedUrlScheme.application.value() == inputType()
+                    : !hasTarballExtension(url.path));
+    }
+
+    std::pair<StorePath, Input> fetch(ref<Store> store, const Input & input) override
+    {
+        auto file = downloadFile(store, getStrAttr(input.attrs, "url"), input.getName(), false);
+        return {std::move(file.storePath), input};
+    }
+};
+
+struct TarballInputScheme : CurlInputScheme
+{
+    const std::string inputType() const override { return "tarball"; }
+
+    bool isValidURL(const ParsedURL & url) const override
+    {
+        auto parsedUrlScheme = parseUrlScheme(url.scheme);
+
+        return transportUrlSchemes.count(std::string(parsedUrlScheme.transport))
+            && (parsedUrlScheme.application
+                    ? parsedUrlScheme.application.value() == inputType()
+                    : hasTarballExtension(url.path));
+    }
+
     std::pair<StorePath, Input> fetch(ref<Store> store, const Input & input) override
     {
         auto tree = downloadTarball(store, getStrAttr(input.attrs, "url"), input.getName(), false).first;
@@ -233,5 +278,6 @@ struct TarballInputScheme : InputScheme
 };
 
 static auto rTarballInputScheme = OnStartup([] { registerInputScheme(std::make_unique<TarballInputScheme>()); });
+static auto rFileInputScheme = OnStartup([] { registerInputScheme(std::make_unique<FileInputScheme>()); });
 
 }

src/libstore/http-binary-cache-store.cc

@@ -161,7 +161,12 @@ protected:
     void getFile(const std::string & path,
         Callback<std::optional<std::string>> callback) noexcept override
     {
-        checkEnabled();
+        try {
+            checkEnabled();
+        } catch (...) {
+            callback.rethrow();
+            return;
+        }
 
         auto request(makeRequest(path));
 

src/libstore/remote-store.cc

@@ -718,36 +718,34 @@ void RemoteStore::registerDrvOutput(const Realisation & info)
 void RemoteStore::queryRealisationUncached(const DrvOutput & id,
     Callback<std::shared_ptr<const Realisation>> callback) noexcept
 {
-    auto conn(getConnection());
-
-    if (GET_PROTOCOL_MINOR(conn->daemonVersion) < 27) {
-        warn("the daemon is too old to support content-addressed derivations, please upgrade it to 2.4");
-        try {
-            callback(nullptr);
-        } catch (...) { return callback.rethrow(); }
-    }
-
-    conn->to << wopQueryRealisation;
-    conn->to << id.to_string();
-    conn.processStderr();
-
-    auto real = [&]() -> std::shared_ptr<const Realisation> {
-        if (GET_PROTOCOL_MINOR(conn->daemonVersion) < 31) {
-            auto outPaths = worker_proto::read(
-                *this, conn->from, Phantom<std::set<StorePath>> {});
-            if (outPaths.empty())
-                return nullptr;
-            return std::make_shared<const Realisation>(Realisation { .id = id, .outPath = *outPaths.begin() });
-        } else {
-            auto realisations = worker_proto::read(
-                *this, conn->from, Phantom<std::set<Realisation>> {});
-            if (realisations.empty())
-                return nullptr;
-            return std::make_shared<const Realisation>(*realisations.begin());
-        }
-    }();
-
     try {
+        auto conn(getConnection());
+
+        if (GET_PROTOCOL_MINOR(conn->daemonVersion) < 27) {
+            warn("the daemon is too old to support content-addressed derivations, please upgrade it to 2.4");
+            return callback(nullptr);
+        }
+
+        conn->to << wopQueryRealisation;
+        conn->to << id.to_string();
+        conn.processStderr();
+
+        auto real = [&]() -> std::shared_ptr<const Realisation> {
+            if (GET_PROTOCOL_MINOR(conn->daemonVersion) < 31) {
+                auto outPaths = worker_proto::read(
+                    *this, conn->from, Phantom<std::set<StorePath>> {});
+                if (outPaths.empty())
+                    return nullptr;
+                return std::make_shared<const Realisation>(Realisation { .id = id, .outPath = *outPaths.begin() });
+            } else {
+                auto realisations = worker_proto::read(
+                    *this, conn->from, Phantom<std::set<Realisation>> {});
+                if (realisations.empty())
+                    return nullptr;
+                return std::make_shared<const Realisation>(*realisations.begin());
+            }
+        }();
+
         callback(std::shared_ptr<const Realisation>(real));
     } catch (...) { return callback.rethrow(); }
 }

src/libutil/error.hh

@@ -98,6 +98,15 @@ struct ErrPos {
     }
 };
 
+std::optional<LinesOfCode> getCodeLines(const ErrPos & errPos);
+
+void printCodeLines(std::ostream & out,
+    const std::string & prefix,
+    const ErrPos & errPos,
+    const LinesOfCode & loc);
+
+void printAtPos(const ErrPos & pos, std::ostream & out);
+
 struct Trace {
     std::optional<ErrPos> pos;
     hintformat hint;

src/libutil/json.cc

@@ -1,6 +1,7 @@
 #include "json.hh"
 
 #include <iomanip>
+#include <cstdint>
 #include <cstring>
 
 namespace nix {

src/libutil/ref.hh

@@ -7,7 +7,7 @@
 namespace nix {
 
 /* A simple non-nullable reference-counted pointer. Actually a wrapper
-   around std::shared_ptr that prevents non-null constructions. */
+   around std::shared_ptr that prevents null constructions. */
 template<typename T>
 class ref
 {

src/libutil/url.cc

@@ -1,6 +1,7 @@
 #include "url.hh"
 #include "url-parts.hh"
 #include "util.hh"
+#include "split.hh"
 
 namespace nix {
 
@@ -136,4 +137,21 @@ bool ParsedURL::operator ==(const ParsedURL & other) const
         && fragment == other.fragment;
 }
 
+/**
+ * Parse a URL scheme of the form '(applicationScheme\+)?transportScheme'
+ * into a tuple '(applicationScheme, transportScheme)'
+ *
+ * > parseUrlScheme("http") == ParsedUrlScheme{ {}, "http"}
+ * > parseUrlScheme("tarball+http") == ParsedUrlScheme{ {"tarball"}, "http"}
+ */
+ParsedUrlScheme parseUrlScheme(std::string_view scheme)
+{
+    auto application = splitPrefixTo(scheme, '+');
+    auto transport = scheme;
+    return ParsedUrlScheme {
+        .application = application,
+        .transport = transport,
+    };
+}
+
 }

src/libutil/url.hh

@@ -27,4 +27,19 @@ std::map<std::string, std::string> decodeQuery(const std::string & query);
 
 ParsedURL parseURL(const std::string & url);
 
+/*
+ * Although that’s not really standardized anywhere, an number of tools
+ * use a scheme of the form 'x+y' in urls, where y is the “transport layer”
+ * scheme, and x is the “application layer” scheme.
+ *
+ * For example git uses `git+https` to designate remotes using a Git
+ * protocol over http.
+ */
+struct ParsedUrlScheme {
+    std::optional<std::string_view> application;
+    std::string_view transport;
+};
+
+ParsedUrlScheme parseUrlScheme(std::string_view scheme);
+
 }

src/libutil/util.cc

@@ -1818,7 +1818,7 @@ AutoCloseFD createUnixDomainSocket(const Path & path, mode_t mode)
     if (chmod(path.c_str(), mode) == -1)
         throw SysError("changing permissions on '%1%'", path);
 
-    if (listen(fdSocket.get(), 5) == -1)
+    if (listen(fdSocket.get(), 100) == -1)
         throw SysError("cannot listen on socket '%1%'", path);
 
     return fdSocket;

src/nix/flake.md

@@ -181,9 +181,17 @@ Currently the `type` attribute can be one of the following:
 * `tarball`: Tarballs. The location of the tarball is specified by the
   attribute `url`.
 
-  In URL form, the schema must be `http://`, `https://` or `file://`
+  In URL form, the schema must be `tarball+http://`, `tarball+https://` or `tarball+file://`.
-  URLs and the extension must be `.zip`, `.tar`, `.tgz`, `.tar.gz`,
+  If the extension corresponds to a known archive format (`.zip`, `.tar`,
-  `.tar.xz`, `.tar.bz2` or `.tar.zst`.
+  `.tgz`, `.tar.gz`, `.tar.xz`, `.tar.bz2` or `.tar.zst`), then the `tarball+`
+  can be dropped.
+
+* `file`: Plain files or directory tarballs, either over http(s) or from the local
+  disk.
+
+  In URL form, the schema must be `file+http://`, `file+https://` or `file+file://`.
+  If the extension doesn’t correspond to a known archive format (as defined by the
+  `tarball` fetcher), then the `file+` prefix can be dropped.
 
 * `github`: A more efficient way to fetch repositories from
   GitHub. The following attributes are required:

src/nix/profile.cc

@@ -263,7 +263,8 @@ builtPathsPerInstallable(
 
 struct CmdProfileInstall : InstallablesCommand, MixDefaultProfile
 {
-    std::optional<int> priority;
+    std::optional<int64_t> priority;
+
     CmdProfileInstall() {
         addFlag({
             .longName = "priority",

tests/fetchTree-file.sh

@@ -0,0 +1,105 @@
+source common.sh
+
+clearStore
+
+cd "$TEST_ROOT"
+
+test_fetch_file () {
+    echo foo > test_input
+
+    input_hash="$(nix hash path test_input)"
+
+    nix eval --impure --file - <<EOF
+    let
+        tree = builtins.fetchTree { type = "file"; url = "file://$PWD/test_input"; };
+    in
+    assert (tree.narHash == "$input_hash");
+    tree
+EOF
+}
+
+# Make sure that `http(s)` and `file` flake inputs are properly extracted when
+# they should be, and treated as opaque files when they should be
+test_file_flake_input () {
+    rm -fr "$TEST_ROOT/testFlake";
+    mkdir "$TEST_ROOT/testFlake";
+    pushd testFlake
+
+    mkdir inputs
+    echo foo > inputs/test_input_file
+    tar cfa test_input.tar.gz inputs
+    cp test_input.tar.gz test_input_no_ext
+    input_tarball_hash="$(nix hash path test_input.tar.gz)"
+    input_directory_hash="$(nix hash path inputs)"
+
+    cat <<EOF > flake.nix
+    {
+        inputs.no_ext_default_no_unpack = {
+            url = "file://$PWD/test_input_no_ext";
+            flake = false;
+        };
+        inputs.no_ext_explicit_unpack = {
+            url = "tarball+file://$PWD/test_input_no_ext";
+            flake = false;
+        };
+        inputs.tarball_default_unpack = {
+            url = "file://$PWD/test_input.tar.gz";
+            flake = false;
+        };
+        inputs.tarball_explicit_no_unpack = {
+            url = "file+file://$PWD/test_input.tar.gz";
+            flake = false;
+        };
+        outputs = { ... }: {};
+    }
+EOF
+
+    nix flake update
+    nix eval --file - <<EOF
+    with (builtins.fromJSON (builtins.readFile ./flake.lock));
+
+    # Url inputs whose extension doesn’t match a know archive format should
+    # not be unpacked by default
+    assert (nodes.no_ext_default_no_unpack.locked.type == "file");
+    assert (nodes.no_ext_default_no_unpack.locked.unpack or false == false);
+    assert (nodes.no_ext_default_no_unpack.locked.narHash == "$input_tarball_hash");
+
+    # For backwards compatibility, flake inputs that correspond to the
+    # old 'tarball' fetcher should still have their type set to 'tarball'
+    assert (nodes.tarball_default_unpack.locked.type == "tarball");
+    # Unless explicitely specified, the 'unpack' parameter shouldn’t appear here
+    # because that would break older Nix versions
+    assert (!nodes.tarball_default_unpack.locked ? unpack);
+    assert (nodes.tarball_default_unpack.locked.narHash == "$input_directory_hash");
+
+    # Explicitely passing the unpack parameter should enforce the desired behavior
+    assert (nodes.no_ext_explicit_unpack.locked.narHash == nodes.tarball_default_unpack.locked.narHash);
+    assert (nodes.tarball_explicit_no_unpack.locked.narHash == nodes.no_ext_default_no_unpack.locked.narHash);
+    true
+EOF
+    popd
+
+    [[ -z "${NIX_DAEMON_PACKAGE}" ]] && return 0
+
+    # Ensure that a lockfile generated by the current Nix for tarball inputs
+    # can still be read by an older Nix
+
+    cat <<EOF > flake.nix
+    {
+        inputs.tarball = {
+            url = "file://$PWD/test_input.tar.gz";
+            flake = false;
+        };
+        outputs = { self, tarball }: {
+            foo = builtins.readFile "${tarball}/test_input_file";
+        };
+    }
+    nix flake update
+
+    clearStore
+    "$NIX_DAEMON_PACKAGE/bin/nix" eval .#foo
+EOF
+}
+
+test_fetch_file
+test_file_flake_input

tests/lang/parse-fail-eof-in-string.nix

@@ -0,0 +1,3 @@
+# https://github.com/NixOS/nix/issues/6562
+# Note that this file must not end with a newline.
+a 1"$

tests/local.mk

@@ -23,6 +23,7 @@ nix_tests = \
   fetchGit.sh \
   fetchurl.sh \
   fetchPath.sh \
+  fetchTree-file.sh \
   simple.sh \
   referrers.sh \
   optimise-store.sh \