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Merge pull request #13273 from NixOS/deprecate-structured-attrs-hack

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Deprecate hacky way of making structured attrs
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John Ericson 2025-05-28 13:40:23 -04:00 • committed by GitHub
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5 changed files with 26 additions and 1 deletions
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11
doc/manual/rl-next/deprecate__json.md Normal file
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@ -0,0 +1,11 @@
---
synopsis: Deprecate manually making structured attrs with `__json = ...;`
prs: [13220]
---
The proper way to create a derivation using [structured attrs] in the Nix language is by using `__structuredAttrs = true` with [`builtins.derivation`].
However, by exploiting how structured attrs are implementated, it has also been possible to create them by setting the `__json` environment variable to a serialized JSON string.
This sneaky alternative method is now deprecated, and may be disallowed in future versions of Nix.
[structured attrs]: @docroot@/language/advanced-attributes.md#adv-attr-structuredAttrs
[`builtins.derivation`]: @docroot@/language/builtins.html#builtins-derivation

1
src/libexpr/eval.cc
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@ -212,6 +212,7 @@ EvalState::EvalState(
, sRight(symbols.create("right"))
, sWrong(symbols.create("wrong"))
, sStructuredAttrs(symbols.create("__structuredAttrs"))
, sJson(symbols.create("__json"))
, sAllowedReferences(symbols.create("allowedReferences"))
, sAllowedRequisites(symbols.create("allowedRequisites"))
, sDisallowedReferences(symbols.create("disallowedReferences"))

2
src/libexpr/include/nix/expr/eval.hh
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@ -213,7 +213,7 @@ public:
const Symbol sWith, sOutPath, sDrvPath, sType, sMeta, sName, sValue,
sSystem, sOverrides, sOutputs, sOutputName, sIgnoreNulls,
sFile, sLine, sColumn, sFunctor, sToString,
sRight, sWrong, sStructuredAttrs,
sRight, sWrong, sStructuredAttrs, sJson,
sAllowedReferences, sAllowedRequisites, sDisallowedReferences, sDisallowedRequisites,
sMaxSize, sMaxClosureSize,
sBuilder, sArgs,

2
src/libexpr/primops.cc
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@ -1427,6 +1427,8 @@ static void derivationStrictInternal(
else if (i->name == state.sOutputHashMode) handleHashMode(s);
else if (i->name == state.sOutputs)
handleOutputs(tokenizeString<Strings>(s));
else if (i->name == state.sJson)
warn("In derivation '%s': setting structured attributes via '__json' is deprecated, and may be disallowed in future versions of Nix. Set '__structuredAttrs = true' instead.", drvName);
}
}

11
tests/functional/structured-attrs.sh
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@ -40,3 +40,14 @@ jsonOut="$(nix print-dev-env -f structured-attrs-shell.nix --json)"
test "$(<<<"$jsonOut" jq '.structuredAttrs|keys|.[]' -r)" = "$(printf ".attrs.json\n.attrs.sh")"
test "$(<<<"$jsonOut" jq '.variables.outputs.value.out' -r)" = "$(<<<"$jsonOut" jq '.structuredAttrs.".attrs.json"' -r | jq -r '.outputs.out')"
# Hacky way of making structured attrs. We should preserve for now for back compat, but also deprecate.
hackyExpr='derivation { name = "a"; system = "foo"; builder = "/bin/sh"; __json = builtins.toJSON { a = 1; }; }'
# Check for deprecation message
expectStderr 0 nix-instantiate --expr "$hackyExpr" --eval --strict | grepQuiet "In derivation 'a': setting structured attributes via '__json' is deprecated, and may be disallowed in future versions of Nix. Set '__structuredAttrs = true' instead."
# Check it works with the expected structured attrs
hacky=$(nix-instantiate --expr "$hackyExpr")
nix derivation show "$hacky" | jq --exit-status '."'"$hacky"'".env.__json | fromjson | . == {"a": 1}'