[libcxx][CMake] `LIBCXX_ENABLE_FILESYSTEM=OFF` is not respected on Windows for freestanding runtimes build

[moleium]

When configuring a freestanding libcxx build on Windows using the runtimes build entrypoint, setting -DLIBCXX_ENABLE_FILESYSTEM=OFF does not prevent filesystem source files from being added to the build target. This results in a compilation failure, as these sources have dependencies (e.g, Unicode, wide characters) that are typically disabled in such a configuration.

This issue does not occur on Linux with an identical configruation.

Environment

• OS: Windows 10 22H2
• Compiler: Clang 21.1.1 (From MSYS2 clang64 toolchain)
• Build System: Ninja
• LLVM commit: main branch

Reproduction

1. From the root of llvm-project, run the following CMake command:

cmake -G Ninja -S runtimes -B build-freestanding 
-DLLVM_ENABLE_RUNTIMES=libcxx
-DCMAKE_BUILD_TYPE=Release
-DCMAKE_INSTALL_PREFIX=install
-DCMAKE_C_COMPILER=clang
-DCMAKE_CXX_COMPILER=clang++
-DLIBCXX_ENABLE_SHARED=OFF
-DLIBCXX_ENABLE_STATIC=ON
-DLIBCXX_CXX_ABI=none
-DLIBCXX_ENABLE_EXCEPTIONS=OFF
-DLIBCXX_ENABLE_RTTI=OFF
-DLIBCXX_ENABLE_FILESYSTEM=OFF
-DLIBCXX_ENABLE_LOCALIZATION=OFF
-DLIBCXX_ENABLE_UNICODE=OFF
-DLIBCXX_ENABLE_WIDE_CHARACTERS=OFF
-DLIBCXX_ENABLE_THREADS=OFF
-DLIBCXX_ENABLE_MONOTONIC_CLOCK=OFF
-DLIBCXX_ENABLE_TIME_ZONE_DATABASE=OFF
-DLIBCXX_ENABLE_RANDOM_DEVICE=OFF
-DLIBCXX_ENABLE_NEW_DELETE_DEFINITIONS=OFF
-DLIBCXX_INCLUDE_TESTS=OFF
-DLIBCXX_INCLUDE_BENCHMARKS=OFF
-DLIBCXX_USE_COMPILER_RT=ON

2. Go to the build directoryt and run ninja:

cd build-freestanding
ninja

Actual Behavior

The build fails with compilation errors in filesystem-related source files.

... llvm-project/build-freestanding/include/c++/v1/__filesystem/path.h:186:9: error: unknown type name 'wstring'; did you mean 'string'?
  186 | typedef wstring __path_string;
      |         ^~~~~~~
      |         string
... llvm-project/build-freestanding/include/c++/v1/__fwd/string.h:45:7: note: 'string' declared here
   45 | using string = basic_string<char>;
      |       ^

Expected Behavior

The build should succeed. Source files from the libcxx/src/filesystem directory should not be part of the build when LIBCXX_ENABLE_FILESYSTEM is OFF.

Speculation

Probable root cause is in libcxx/src/CMakeLists.txt. Filesystem source files (filesystem_*.cpp) are unconditionally added to the LIBCXX_SOURCES variable.

These files should be moved inside the guard for LIBCXX_ENABLE_FILESYSTEM block.