nix: dev-shell fixes (#9437)

Add the packages required for prebuilt LLVM to `LD_LIBRARY_PATH`.

Also, switch from `stdenvNoCC` to a proper clang/llvm stdenv.

---------

Co-authored-by: Ellie Hermaszewska <[email protected]>

Author: rkoser-nv

flake.nix

@@ -14,35 +14,44 @@
       system:
       let
         pkgs = import nixpkgs { inherit system; };
+        llvmPackages = pkgs.llvmPackages_21;
+        # We want to use Clang instead of GCC because it seems to behave better
+        # with LLDB, so we use `mkShell` with the LLVM stdenv
+        mkShell = pkgs.mkShell.override { stdenv = llvmPackages.stdenv; };
+        # We must use the clangd from `clang-tools` package so that it is
+        # wrapped properly. This is harder than it seems becase there is a
+        # clangd in clang-unwrapped, which would normally come first thanks to
+        # the cc-wrapper/setup-hook adding ${clang-unwrapped}/bin to PATH very
+        # early in `mkDerivation` setup. We work around this using a shell hook
+        # (below) as that executes very late in shell instantiation and can
+        # therefore override cc-wrapper.
+        #
+        # See https://github.com/NixOS/nixpkgs/issues/76486 for the upstream bug.
+        clangd-only = (
+          pkgs.linkFarm "clangd-only" [
+            {
+              name = "bin/clangd";
+              # New enough to support `HeaderInsertion: Never` in `.clangd`.
+              path = "${llvmPackages.clang-tools}/bin/clangd";
+            }
+          ]
+        );
       in
       {
-        # We want to use Clang instead of GCC because it seems to behave better
-        # with LLDB, so we use `mkShellNoCC` here instead of `mkShell` because
-        # the latter brings in GCC by default on Linux.
-        devShell = pkgs.mkShellNoCC {
+        devShell = mkShell {
           buildInputs = [
-            # We must list clangd before the `clang` package to make sure it
-            # comes earlier on the `PATH`, and we must get it from the
-            # `clang-tools` package so that it is wrapped properly.
-            (pkgs.linkFarm "clangd-21" [
-              {
-                name = "bin/clangd";
-                # New enough to support `HeaderInsertion: Never` in `.clangd`.
-                path = "${pkgs.llvmPackages_21.clang-tools}/bin/clangd";
-              }
-            ])
+            # Pull in only clang-format from clang-tools 17. This matches the
+            # version used in CI.
             (pkgs.linkFarm "clang-format-17" [
               {
                 name = "bin/clang-format";
-                # Match the clang-format version used in CI.
                 path = "${pkgs.llvmPackages_17.clang-tools}/bin/clang-format";
               }
             ])
 
-            pkgs.clang
             pkgs.cmake
             pkgs.gersemi
-            pkgs.lldb
+            llvmPackages.lldb
             pkgs.ninja
             pkgs.nixfmt-rfc-style
             pkgs.prettier
@@ -51,12 +60,26 @@
             pkgs.vulkan-loader # Ensure this gets built to use in library path.
             pkgs.xorg.libX11
           ];
+
           LD_LIBRARY_PATH = pkgs.lib.makeLibraryPath [
             # In addition to this, running the Vulkan tests on Linux distros
             # other than NixOS may require the use of nixGL:
             # https://github.com/nix-community/nixGL
             pkgs.vulkan-loader
+            # Needed for the prebuilt LLVM
+            pkgs.libz
+            pkgs.zstd
+            # Despite requiring this packages (slang) to be built with Clang,
+            # the prebuilt libslang-llvm.so is actually linked against GCC's
+            # libstdc++.so.6
+            pkgs.stdenv.cc.cc.lib
           ];
+
+          # Use a shell hook to make sure the wrapped clangd is in the path
+          # before the unwrapped one included by llvmPackages.stdenv
+          shellHook = ''
+            PATH="${clangd-only}/bin:$PATH"
+          '';
         };
       }
     );