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

Also add IWYU to the default job, since using libc++ instead of libstdc++
causes IWYU to give slightly different recommendations. Better to comply with
both sets of recommendations so IWYU is usable on more platforms.

Author: ryanofsky

CMakeLists.txt

@@ -40,6 +40,9 @@ if(MP_ENABLE_IWYU)
     message(FATAL_ERROR "MP_ENABLE_IWYU is ON but include-what-you-use was not found.")
   endif()
   set(CMAKE_CXX_INCLUDE_WHAT_YOU_USE "${IWYU_EXECUTABLE};-Xiwyu;--error")
+  if(DEFINED ENV{IWYU_MAPPING_FILE})
+    list(APPEND CMAKE_CXX_INCLUDE_WHAT_YOU_USE "-Xiwyu" "--mapping_file=$ENV{IWYU_MAPPING_FILE}")
+  endif()
 endif()
 
 include("cmake/compat_config.cmake")

ci/configs/default.sh

@@ -1,4 +1,5 @@
-CI_DESC="CI job using default libraries and tools"
+CI_DESC="CI job using default libraries and tools, and running IWYU"
 CI_DIR=build-default
 export CXXFLAGS="-Werror -Wall -Wextra -Wpedantic -Wno-unused-parameter"
+CMAKE_ARGS=(-DMP_ENABLE_IWYU=ON)
 BUILD_ARGS=(-k)

ci/configs/llvm.sh

@@ -1,5 +1,6 @@
-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 enableLibcxx true)
 export CXX=clang++
 export CXXFLAGS="-Werror -Wall -Wextra -Wpedantic -Wthread-safety-analysis -Wno-unused-parameter"
 CMAKE_ARGS=(

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 --keep CI_CLEAN --run ci/scripts/ci.sh shell.nix
+[ "${CI_CONFIG+x}" ] && source "$CI_CONFIG"
+
+nix-shell --pure --keep CI_CONFIG --keep CI_CLEAN "${NIX_ARGS[@]+"${NIX_ARGS[@]}"}" --run ci/scripts/ci.sh shell.nix

shell.nix

@@ -1,14 +1,26 @@
-{ pkgs ? import <nixpkgs> {} }:
+{ pkgs ? import <nixpkgs> {}
+, enableLibcxx ? false # Whether to use libc++ toolchain and libraries instead of libstdc++
+}:
 
-pkgs.mkShell {
-  buildInputs = with pkgs; [
+let
+  lib  = pkgs.lib;
+  llvm = pkgs.llvmPackages_20;
+  capnproto = pkgs.capnproto.override (lib.optionalAttrs enableLibcxx { clangStdenv = llvm.libcxxStdenv; });
+  clang = if enableLibcxx then llvm.libcxxClang else llvm.clang;
+  clang-tools = llvm.clang-tools.override { inherit enableLibcxx; };
+in pkgs.mkShell {
+  buildInputs = [
     capnproto
+    llvm.libcxx
   ];
   nativeBuildInputs = with pkgs; [
+    clang
+    clang-tools
     cmake
     include-what-you-use
-    llvmPackages_20.clang
-    llvmPackages_20.clang-tools
     ninja
   ];
+
+  # Tell IWYU where its libc++ mapping lives
+  IWYU_MAPPING_FILE = if enableLibcxx then "${llvm.libcxx.dev}/include/c++/v1/libcxx.imp" else null;
 }