callCabal2nix: Fix filtering for non-cleanSourceable sources.

What was here before wasn't correct anyway, and now it works in
restricted mode.

Fixes #35207

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

@@ -1,14 +0,0 @@
-# | 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

@@ -148,14 +148,13 @@ in package-set { inherit pkgs stdenv callPackage; } self // {
     callCabal2nix = name: src: args:
       overrideCabal (self.callPackage (haskellSrc2nix {
         inherit name;
-        src = pkgs.lib.cleanSourceWith
-          { src = if pkgs.lib.canCleanSource src
-                    then src
-                    else pkgs.safeDiscardStringContext src;
-            filter = path: type:
-              pkgs.lib.hasSuffix "${name}.cabal" path ||
-              pkgs.lib.hasSuffix "package.yaml" path;
-          };
+        src =
+          let filter = path: type:
+                pkgs.lib.hasSuffix "${name}.cabal" path ||
+                baseNameOf path == "package.yaml";
+          in if pkgs.lib.canCleanSource src
+               then pkgs.lib.cleanSourceWith { inherit src filter; }
+             else src;
       }) args) (_: { inherit src; });
 
     # : { root : Path

pkgs/top-level/all-packages.nix

@@ -20842,8 +20842,6 @@ with pkgs;
 
   tlwg = callPackage ../data/fonts/tlwg { };
 
-  safeDiscardStringContext = callPackage ../build-support/safe-discard-string-context.nix { };
-
   simplehttp2server = callPackage ../servers/simplehttp2server { };
 
   diceware = callPackage ../tools/security/diceware { };