ci: test libc++ instead of libstdc++ in one job

Author: ryanofsky

ci/configs/llvm.sh

@@ -1,8 +1,16 @@
-CI_DESC="CI job using LLVM-based tools (clang, clang-tidy, iwyu) and testing Ninja"
+CI_DESC="CI job using LLVM-based libraries and tools (clang, libc++, clang-tidy, iwyu) and testing Ninja"
 CI_DIR=build-llvm
+NIX_ARGS=(
+  --arg libcxx true
+)
+
+# Note: CMAKE_CXX_STANDARD_LIBRARY is still an undocumented, experimental switch
+# (present in CMake ≥3.29) that toggles libc++/libstdc++; we use it for now, but
+# expect breakage if CMake renames or removes it in a future release.
 CMAKE_ARGS=(
   -G Ninja
   -DCMAKE_CXX_COMPILER=clang++
+  -DCMAKE_CXX_STANDARD_LIBRARY=libc++
   -DCMAKE_CXX_FLAGS="-Werror -Wall -Wextra -Wpedantic -Wthread-safety-analysis -Wno-unused-parameter"
   -DCMAKE_EXPORT_COMPILE_COMMANDS=ON
   -DMP_ENABLE_CLANG_TIDY=ON

ci/scripts/run.sh

@@ -1,4 +1,6 @@
 #!/usr/bin/env bash
 set -o errexit -o nounset -o pipefail -o xtrace
 
-nix-shell --pure --keep CI_CONFIG --run ci/scripts/ci.sh shell.nix
+[ "${CI_CONFIG+x}" ] && source "$CI_CONFIG"
+
+nix-shell --pure --keep CI_CONFIG "${NIX_ARGS[@]+"${NIX_ARGS[@]}"}" --run ci/scripts/ci.sh shell.nix

shell.nix

@@ -1,14 +1,22 @@
-{ pkgs ? import <nixpkgs> {} }:
+{ pkgs ? import <nixpkgs> {}
+, libcxx ? false # Whether to use libc++ toolchain and libraries instead of libstdc++
+}:
 
-pkgs.mkShell {
-  buildInputs = with pkgs; [
+let
+  lib  = pkgs.lib;
+  llvm = pkgs.llvmPackages_20;
+  mkShell = pkgs.mkShell.override (lib.optionalAttrs libcxx { stdenv = llvm.libcxxStdenv; });
+  capnproto = pkgs.capnproto.override (lib.optionalAttrs libcxx { clangStdenv = llvm.libcxxStdenv; });
+in mkShell {
+  buildInputs = [
     capnproto
+    llvm.libcxx
   ];
   nativeBuildInputs = with pkgs; [
     cmake
     include-what-you-use
-    llvmPackages_20.clang
-    llvmPackages_20.clang-tools
+    llvm.clang
+    llvm.clang-tools
     ninja
   ];
 }