[libc++abi] Provide an explicit error when trying to build for MSVC

[ldionne]

Fixes #119322

[llvmbot]

@llvm/pr-subscribers-libunwind

@llvm/pr-subscribers-libcxxabi

Author: Louis Dionne (ldionne)

Changes

Fixes #119322

---

Full diff: https://github.com/llvm/llvm-project/pull/119370.diff

1 Files Affected:

• (modified) libcxxabi/CMakeLists.txt (+5)

diff --git a/libcxxabi/CMakeLists.txt b/libcxxabi/CMakeLists.txt
index da0e8b286cddc1..158d61ff4fcb25 100644
--- a/libcxxabi/CMakeLists.txt
+++ b/libcxxabi/CMakeLists.txt
@@ -34,6 +34,11 @@ MACRO_ENSURE_OUT_OF_SOURCE_BUILD(
  build directory and run 'cmake /path/to/${PROJECT_NAME} [options]' there."
  )
 
+if (MSVC)
+  message(FATAL_ERROR "Libc++abi can't be built for MSVC targets, and doing so is pointless anyway because such "
+                      "targets must use the MS C++ ABI, and libc++abi provides the Itanium C++ ABI.")
+endif()
+
 #===============================================================================
 # Setup CMake Options
 #===============================================================================

[mstorsjo]

Looks resonable.

We could also consider doing the same for libunwind. While most of libunwind also is pointless as it is tied to the Itanium ABI, one could in theory want to use an API like _Unwind_Backtrace. But libunwind doesn't build in this configuration right now, and would require some more tweaks. (This was suggested in https://reviews.llvm.org/D86612 earlier.) So until that's actually fixed up so that libunwind compiles in that configuration, libunwind probably also would benefit from such an early message to users who try to enable it without knowing why they would need it.

[mstorsjo]

LGTM, thanks!