From de1e068b1f49a8b2b3abf9e2d1ad0b987e75841e Mon Sep 17 00:00:00 2001
From: sternenseemann <sternenseemann@systemli.org>
Date: Tue, 9 Sep 2025 15:39:30 +0200
Subject: [PATCH 1/3] ghcWithPackages: expose targetPrefix

---
 pkgs/development/haskell-modules/with-packages-wrapper.nix | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/pkgs/development/haskell-modules/with-packages-wrapper.nix b/pkgs/development/haskell-modules/with-packages-wrapper.nix
index 6fb552d22c75..b683febeaf6c 100644
--- a/pkgs/development/haskell-modules/with-packages-wrapper.nix
+++ b/pkgs/development/haskell-modules/with-packages-wrapper.nix
@@ -178,7 +178,7 @@ else
     + postBuild;
     preferLocalBuild = true;
     passthru = {
-      inherit (ghc) version meta;
+      inherit (ghc) version meta targetPrefix;
 
       hoogle = hoogleWithPackages';
 

From d1bc568af24346f8af5d50277c58679c9e40c71c Mon Sep 17 00:00:00 2001
From: sternenseemann <sternenseemann@systemli.org>
Date: Tue, 9 Sep 2025 15:40:03 +0200
Subject: [PATCH 2/3] tests.haskell.ghcWithPackages: verify -fllvm works with
 useLLVM

Note that this test also succeeds in pkgsCross though using a stdenv
with cc is crucial, since its setup hooks are required for -lgmp to work.

Maybe this is something we should look at for GHC again, maybe we can
add extra entries to the package db, so it passes the necessary -L flags
by itself?
---
 pkgs/test/haskell/ghcWithPackages/default.nix | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/pkgs/test/haskell/ghcWithPackages/default.nix b/pkgs/test/haskell/ghcWithPackages/default.nix
index c31721b9531e..e1140ae0b482 100644
--- a/pkgs/test/haskell/ghcWithPackages/default.nix
+++ b/pkgs/test/haskell/ghcWithPackages/default.nix
@@ -1,6 +1,7 @@
 {
   lib,
   runCommand,
+  runCommandCC,
   haskellPackages,
 }:
 
@@ -45,4 +46,21 @@ lib.recurseIntoAttrs {
 
         touch $out
       '';
+
+  use-llvm =
+    let
+      ghc = (haskellPackages.ghcWithPackages.override { useLLVM = true; }) (_: [ ]);
+    in
+    runCommandCC "ghc-with-packages-use-llvm"
+      {
+        nativeBuildInputs = [ ghc ];
+      }
+      ''
+        echo 'main = pure ()' > test.hs
+        # -ddump-llvm is unnecessary, but nice for visual feedback in the build log
+        ${ghc.targetPrefix}ghc --make -fllvm -keep-llvm-files -ddump-llvm test.hs
+        # Did we actually use the LLVM backend?
+        test -f test.ll
+        touch $out
+      '';
 }

From 49ac23d75d420c9f9d27dcfbeb30971f1a760de4 Mon Sep 17 00:00:00 2001
From: sternenseemann <sternenseemann@systemli.org>
Date: Tue, 9 Sep 2025 15:43:29 +0200
Subject: [PATCH 3/3] tests.haskell.ghcWithPackages.regression-224542: make
 cross aware

Obviously, we can't run GHCi of a cross-compiler, but respecting targetPrefix
will give a more helpful error message:

    <command line>: not built for interactive use
---
 pkgs/test/haskell/ghcWithPackages/default.nix | 13 ++++++++-----
 1 file changed, 8 insertions(+), 5 deletions(-)

diff --git a/pkgs/test/haskell/ghcWithPackages/default.nix b/pkgs/test/haskell/ghcWithPackages/default.nix
index e1140ae0b482..aedcac0d2c91 100644
--- a/pkgs/test/haskell/ghcWithPackages/default.nix
+++ b/pkgs/test/haskell/ghcWithPackages/default.nix
@@ -22,16 +22,19 @@ lib.recurseIntoAttrs {
 
   # See: https://github.com/NixOS/nixpkgs/pull/224542
   regression-224542 =
+    let
+      ghc = haskellPackages.ghcWithPackages (hsPkgs: [
+        hsPkgs.hspec
+      ]);
+    in
     runCommand "regression-224542"
       {
-        buildInputs = [
-          (haskellPackages.ghcWithPackages (hsPkgs: [
-            hsPkgs.hspec
-          ]))
+        nativeBuildInputs = [
+          ghc
         ];
       }
       ''
-        ghc --interactive \
+        ${ghc.targetPrefix}ghc --interactive \
           -Werror=unrecognised-warning-flags \
           -Werror=missed-extra-shared-lib \
           2>&1 \