From 7577d2d3ae8fa203604cc3b09e2ed81678163d46 Mon Sep 17 00:00:00 2001
From: John Ericson <John.Ericson@Obsidian.Systems>
Date: Mon, 26 May 2025 13:54:04 -0400
Subject: [PATCH] Deprecate hacky way of making structured attrs

The method tested for in the previous commit is now deprecated.

Co-authored-by: Eelco Dolstra <edolstra@gmail.com>
---
 doc/manual/rl-next/deprecate__json.md | 11 +++++++++++
 src/libexpr/eval.cc                   |  1 +
 src/libexpr/include/nix/expr/eval.hh  |  2 +-
 src/libexpr/primops.cc                |  2 ++
 tests/functional/structured-attrs.sh  |  3 +++
 5 files changed, 18 insertions(+), 1 deletion(-)
 create mode 100644 doc/manual/rl-next/deprecate__json.md

diff --git a/doc/manual/rl-next/deprecate__json.md b/doc/manual/rl-next/deprecate__json.md
new file mode 100644
index 000000000..7fc05832f
--- /dev/null
+++ b/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
diff --git a/src/libexpr/eval.cc b/src/libexpr/eval.cc
index 1a067e75c..ce35901b1 100644
--- a/src/libexpr/eval.cc
+++ b/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"))
diff --git a/src/libexpr/include/nix/expr/eval.hh b/src/libexpr/include/nix/expr/eval.hh
index a4347b3e1..27294d114 100644
--- a/src/libexpr/include/nix/expr/eval.hh
+++ b/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,
diff --git a/src/libexpr/primops.cc b/src/libexpr/primops.cc
index f3e3e1290..01cfb3b0e 100644
--- a/src/libexpr/primops.cc
+++ b/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);
                 }
 
             }
diff --git a/tests/functional/structured-attrs.sh b/tests/functional/structured-attrs.sh
index fe9bcc0ee..465676b41 100755
--- a/tests/functional/structured-attrs.sh
+++ b/tests/functional/structured-attrs.sh
@@ -45,6 +45,9 @@ test "$(<<<"$jsonOut" jq '.variables.outputs.value.out' -r)" = "$(<<<"$jsonOut"
 
 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}'