Deprecate hacky way of making structured attrs

doc/manual/rl-next/deprecate__json.md

@@ -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

src/libexpr/eval.cc

@@ -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"))

src/libexpr/include/nix/expr/eval.hh

@@ -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,

src/libexpr/primops.cc

@@ -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);
                 }
 
             }

tests/functional/structured-attrs.sh

@@ -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}'