callCabal2nix: Fix calling with a path in the store.

lib/default.nix

···
       hiPrioSet;
     inherit (sources) pathType pathIsDirectory cleanSourceFilter
       cleanSource sourceByRegex sourceFilesBySuffices
-      commitIdFromGitRepo cleanSourceWith;
+      commitIdFromGitRepo cleanSourceWith pathHasContext canCleanSource;
     inherit (modules) evalModules closeModules unifyModuleSyntax
       applyIfFunction unpackSubmodule packSubmodule mergeModules
       mergeModules' mergeOptionDecls evalOptionValue mergeDefinitions

lib/sources.nix

···
                 else lib.head matchRef
            else throw ("Not a .git directory: " + path);
     in lib.flip readCommitFromFile "HEAD";
+
+  pathHasContext = builtins.hasContext or (lib.hasPrefix builtins.storeDir);
+
+  canCleanSource = src: src ? _isLibCleanSourceWith || !(pathHasContext (toString src));
 }

pkgs/build-support/safe-discard-string-context.nix

···
+# | Discard the context of a string while ensuring that expected path
+# validity invariants hold.
+#
+# This relies on import-from-derivation, but it is only useful in
+# contexts where the string is going to be used in an
+# import-from-derivation anyway.
+#
+# safeDiscardStringContext : String → String
+{ writeText }: s:
+  builtins.seq
+    (import (writeText
+               "discard.nix"
+               "${builtins.substring 0 0 s}null\n"))
+    (builtins.unsafeDiscardStringContext s)

pkgs/development/haskell-modules/make-package-set.nix

···
       overrideCabal (self.callPackage (haskellSrc2nix {
         inherit name;
         src = pkgs.lib.cleanSourceWith
-          { inherit src;
+          { src = if pkgs.lib.canCleanSource src
+                    then src
+                    else pkgs.safeDiscardStringContext src;
             filter = path: type:
               pkgs.lib.hasSuffix "${name}.cabal" path ||
               pkgs.lib.hasSuffix "package.yaml" path;

pkgs/top-level/all-packages.nix

···
   wal-g = callPackage ../tools/backup/wal-g {};
 
   tlwg = callPackage ../data/fonts/tlwg { };
+
+  safeDiscardStringContext = callPackage ../build-support/safe-discard-string-context.nix { };
 }