stdenv.mkDerivation: warn on nested lists

doc/release-notes/rl-2605.section.md

@@ -410,6 +410,8 @@ gnuradioMinimal.override {
 
 - `fetchFromSavannah` is now deprecated and is expected to be fully removed in a future release. From now on, use `fetchgit` or a Savannah releases mirror when applicable.
 
+- Using nested lists in build/runtime inputs in `mkDerivation` is now deprecated.
+
 ### Additions and Improvements {#sec-nixpkgs-release-26.05-lib-additions-improvements}
 
 - The builder `php.buildComposerProject2` for PHP applications has been improved for better reliability and stability.

pkgs/stdenv/generic/make-derivation.nix

@@ -52,6 +52,7 @@ let
     unique
     unsafeDiscardStringContext
     unsafeGetAttrPos
+    warn
     warnIf
     zipAttrsWith
     ;
@@ -482,7 +483,12 @@ let
             if isSingularDependency dep then
               index + 1
             else if isList dep then
-              seq (checkDependencyList' ([ index ] ++ positions) name dep) (index + 1)
+              warn
+                ''
+                  Dependency of package '${attrs.name or attrs.pname}' uses a nested list in attribute '${name}'.
+                  This is deprecated as of Nixpkgs release 26.05, and support will
+                  be removed in a future nixpkgs release.''
+                (seq (checkDependencyList' ([ index ] ++ positions) name dep) (index + 1))
             else
               throw "Dependency is not of a valid type: ${
                 concatMapStrings (ix: "element ${toString ix} of ") ([ index ] ++ positions)