From 8dc663ea1895309d910e008cbf5076910b6e9dc6 Mon Sep 17 00:00:00 2001
From: Peter Collingbourne <pcc@google.com>
Date: Thu, 26 Jun 2025 15:54:58 -0700
Subject: [PATCH] llvmPackages.clang: embed resource directory in Clang
 libraries

When Clang is statically linked against other programs they are unable to
find the headers in Clang's resource directory. Typically the resource
directory is found by searching a path relative to argv[0] but this
would only really work for Clang itself due to each binary having a
separate prefix (and not in Nix because of the full resource directory
being split between multiple derivations and assembled in the wrapper).

Because users of Clang as a library typically only need include in order
for parsing to succeed, let's set that as the resource directory. The LLVM
patch to make this work was sent upstream in llvm/llvm-project#145996,
I intend to land it upstream and drop it from this PR.
---
 .../compilers/llvm/common/clang/default.nix          | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/pkgs/development/compilers/llvm/common/clang/default.nix b/pkgs/development/compilers/llvm/common/clang/default.nix
index 03e8aaec9b54..f671bf370aa5 100644
--- a/pkgs/development/compilers/llvm/common/clang/default.nix
+++ b/pkgs/development/compilers/llvm/common/clang/default.nix
@@ -158,6 +158,9 @@ stdenv.mkDerivation (
         (lib.cmakeFeature "LLVM_TABLEGEN_EXE" "${buildLlvmTools.tblgen}/bin/llvm-tblgen")
         (lib.cmakeFeature "CLANG_TABLEGEN" "${buildLlvmTools.tblgen}/bin/clang-tblgen")
       ]
+      ++ lib.optionals (lib.versionAtLeast release_version "21") [
+        (lib.cmakeFeature "CLANG_RESOURCE_DIR" "${placeholder "lib"}/lib/clang/${lib.versions.major release_version}")
+      ]
       ++ lib.optionals (lib.versionAtLeast release_version "17") [
         (lib.cmakeBool "LLVM_INCLUDE_TESTS" false)
       ]
@@ -211,9 +214,12 @@ stdenv.mkDerivation (
         mkdir -p $lib/lib/clang
         mv $lib/lib/17 $lib/lib/clang/17
       '')
-      + (lib.optionalString (lib.versionAtLeast release_version "19") ''
-        mv $out/lib/clang $lib/lib/clang
-      '')
+      + (lib.optionalString
+        ((lib.versionAtLeast release_version "19") && !(lib.versionAtLeast release_version "21"))
+        ''
+          mv $out/lib/clang $lib/lib/clang
+        ''
+      )
       + ''
 
         # Move libclang to 'lib' output