haskellPackages.llvm-ffi: 20.0 -> 21.0

We keep using LLVM 20, though, since LLVM 21 is still just a release
candidate on our branch and llvm-ffi doesn't yet supported a released
LLVM 21, just the git version. We can use the older version by passing
a specific cabal flag.

The evaluation warning reminding us to upgrade now also checks whether
LLVM is a release candidate before triggering.

pkgs/development/haskell-modules/configuration-hackage2nix/main.yaml

@@ -37,8 +37,6 @@ default-package-overrides:
   - hsc3 < 0.21
   # 2024-12-31: last version that's compatible with GHC < 9.9
   - htree < 0.2.0.0
-  # 2025-07-10: use latest released version of LLVM
-  - llvm-ffi == 20.*
   # 2025-06-11: last version that supports pandoc == 3.6.* which is prescribed by LTS 23
   - pandoc-crossref < 0.3.20
 # keep-sorted end

pkgs/development/haskell-modules/configuration-nix.nix

@@ -508,19 +508,24 @@ builtins.intersectAttrs super {
   # LLVM input that llvm-ffi declares.
   llvm-ffi =
     let
-      matchingLlvmVersion = lib.strings.toInt (lib.versions.major super.llvm-ffi.version);
-      nextLlvmVersion = matchingLlvmVersion + 1;
+      chosenLlvmVersion = 20;
+      nextLlvmAttr = "llvmPackages_${toString (chosenLlvmVersion + 1)}";
+      shouldUpgrade =
+        pkgs ? ${nextLlvmAttr} && (lib.strings.match ".+rc.+" pkgs.${nextLlvmAttr}.llvm.version) == null;
     in
-    lib.warnIf (pkgs ? "llvmPackages_${toString nextLlvmVersion}")
-      # This package can be updated by changing the version constraint in
-      # configuration-hackage2nix/main.yaml and regenerating the haskellPackages set.
-      "haskellPackages.llvm-ffi: LLVM ${toString nextLlvmVersion} is available in Nixpkgs, consider updating."
-      addBuildDepends
+    lib.warnIf shouldUpgrade
+      "haskellPackages.llvm-ffi: ${nextLlvmAttr} is available in Nixpkgs, consider updating."
+      lib.pipe
+      super.llvm-ffi
       [
-        pkgs."llvmPackages_${toString matchingLlvmVersion}".llvm.lib
-        pkgs."llvmPackages_${toString matchingLlvmVersion}".llvm.dev
-      ]
-      super.llvm-ffi;
+        # ATTN: There is no matching flag for the latest supported LLVM version,
+        # so you may need to remove this when updating chosenLlvmVersion
+        (enableCabalFlag "LLVM${toString chosenLlvmVersion}00")
+        (addBuildDepends [
+          pkgs."llvmPackages_${toString chosenLlvmVersion}".llvm.lib
+          pkgs."llvmPackages_${toString chosenLlvmVersion}".llvm.dev
+        ])
+      ];
 
   # Needs help finding LLVM.
   spaceprobe = addBuildTool self.buildHaskellPackages.llvmPackages.llvm super.spaceprobe;

pkgs/development/haskell-modules/hackage-packages.nix

@@ -425715,30 +425715,6 @@ self: {
   ) { };
 
   "llvm-ffi" = callPackage (
-    {
-      mkDerivation,
-      base,
-      enumset,
-      LLVM,
-    }:
-    mkDerivation {
-      pname = "llvm-ffi";
-      version = "20.0";
-      sha256 = "1vd9rzqq9fcml9zgrxrvplcgxd925xyy7ixprcxb8j4vc0a8hgzc";
-      isLibrary = true;
-      isExecutable = true;
-      libraryHaskellDepends = [
-        base
-        enumset
-      ];
-      librarySystemDepends = [ LLVM ];
-      description = "FFI bindings to the LLVM compiler toolkit";
-      license = lib.licenses.bsd3;
-      maintainers = [ lib.maintainers.thielema ];
-    }
-  ) { LLVM = null; };
-
-  "llvm-ffi_21_0" = callPackage (
     {
       mkDerivation,
       base,
@@ -425758,7 +425734,6 @@ self: {
       librarySystemDepends = [ LLVM-21git ];
       description = "FFI bindings to the LLVM compiler toolkit";
       license = lib.licenses.bsd3;
-      hydraPlatforms = lib.platforms.none;
       maintainers = [ lib.maintainers.thielema ];
     }
   ) { LLVM-21git = null; };