stdenv: assert that env is an attrset

doc/release-notes/rl-2511.section.md

@@ -27,6 +27,8 @@
 
 - `gnome-keyring` no longer ships with an SSH agent anymore because it has been deprecated upstream. You should use `gcr_4` instead, which provides the same features. More information on why this was done can be found on [the relevant GCR upstream PR](https://gitlab.gnome.org/GNOME/gcr/-/merge_requests/67).
 
+- `stdenv.mkDerivation` and other derivation builders that use it no longer allow the value of `env` to be anything but an attribute set, for the purpose of setting environment variables that are available to the [builder](https://nix.dev/manual/nix/latest/store/derivation/#builder) process. An environment variable called `env` can still be provided by means of `mkDerivation { env.env = ...; }`, though we recommend to use a more specific name than "env".
+
 - `conftest` since `0.60.0` has moved to use rego `v1` as default. To continue using `v0` use `--rego-version v0`. For more information about upgrading to Rego v1 syntax, see the [upstream docs](https://www.openpolicyagent.org/docs/latest/v0-upgrade/).
 
 - `tooling-language-server` has been renamed to `deputy` (both the package and binary), following the rename of the upstream project.

pkgs/stdenv/generic/make-derivation.nix

@@ -748,18 +748,15 @@ let
     let
       mainProgram = meta.mainProgram or null;
       env' = env // lib.optionalAttrs (mainProgram != null) { NIX_MAIN_PROGRAM = mainProgram; };
-      envIsExportable = isAttrs env' && !isDerivation env';
 
       derivationArg = makeDerivationArgument (
-        removeAttrs attrs (
-          [
-            "meta"
-            "passthru"
-            "pos"
-          ]
-          ++ optional (__structuredAttrs || envIsExportable) "env"
-        )
-        // optionalAttrs __structuredAttrs { env = checkedEnv; }
+        removeAttrs attrs ([
+          "meta"
+          "passthru"
+          "pos"
+          "env"
+        ])
+        // lib.optionalAttrs __structuredAttrs { env = checkedEnv; }
         // {
           cmakeFlags = makeCMakeFlags attrs;
           mesonFlags = makeMesonFlags attrs;
@@ -787,8 +784,8 @@ let
               }";
           errors = lib.concatMapStringsSep "\n" makeError overlappingNames;
         in
-        assert assertMsg envIsExportable
-          "When using structured attributes, `env` must be an attribute set of environment variables.";
+        assert assertMsg (isAttrs env && !isDerivation env)
+          "`env` must be an attribute set of environment variables. Set `env.env` or pick a more specific name.";
         assert assertMsg (overlappingNames == [ ])
           "The `env` attribute set cannot contain any attributes passed to derivation. The following attributes are overlapping:\n${errors}";
         mapAttrs (
@@ -882,7 +879,7 @@ let
         # should be made available to Nix expressions using the
         # derivation (e.g., in assertions).
         passthru
-    ) (derivation (derivationArg // optionalAttrs envIsExportable checkedEnv));
+    ) (derivation (derivationArg // checkedEnv));
 
 in
 {