[llvm][cmake] Error when a runtime is in LLVM_ENABLE_PROJECTS and LLVM_ENABLE_RUNTIMES

[DavidSpickett]

The documentation tells you not to do this:
https://llvm.org/docs/CMake.html#llvm-related-variables

But until now we did not enforce it.

$ cmake ../llvm-project/llvm/ -DCMAKE_BUILD_TYPE=Release -DLLVM_ENABLE_PROJECTS="pstl" -DLLVM_ENABLE_RUNTIMES="libcxx;pstl"

CMake Error at CMakeLists.txt:166 (message):
  Runtime project "pstl" found in LLVM_ENABLE_PROJECTS and
  LLVM_ENABLE_RUNTIMES.  It must only appear in one of them and that one
  should almost always be LLVM_ENABLE_RUNTIMES.

[jhuber6]

If you use LLVM_ENABLE_RUNTIMES=libc in full build mode it will automatically put LLVM_ENABLE_PROJECTS=libc, we use this to bootstrap some libc specific utilities. This patch breaks any runtimes build of the libc project. See https://github.com/llvm/llvm-project/blob/main/llvm/CMakeLists.txt#L204.

[jhuber6]

That being said I do think it's a good warning to have in general, since I've seen users attempt to set both unknowingly.

[llvm-ci]

LLVM Buildbot has detected a new failure on builder lld-x86_64-win running on as-worker-93 while building llvm at step 7 "test-build-unified-tree-check-all".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/146/builds/1235

Here is the relevant piece of the build log for the reference

Step 7 (test-build-unified-tree-check-all) failure: test (failure)
******************** TEST 'LLVM-Unit :: Support/./SupportTests.exe/35/87' FAILED ********************
Script(shard):
--
GTEST_OUTPUT=json:C:\a\lld-x86_64-win\build\unittests\Support\.\SupportTests.exe-LLVM-Unit-15624-35-87.json GTEST_SHUFFLE=0 GTEST_TOTAL_SHARDS=87 GTEST_SHARD_INDEX=35 C:\a\lld-x86_64-win\build\unittests\Support\.\SupportTests.exe
--

Script:
--
C:\a\lld-x86_64-win\build\unittests\Support\.\SupportTests.exe --gtest_filter=ProgramEnvTest.CreateProcessLongPath
--
C:\a\lld-x86_64-win\llvm-project\llvm\unittests\Support\ProgramTest.cpp(160): error: Expected equality of these values:
  0
  RC
    Which is: -2

C:\a\lld-x86_64-win\llvm-project\llvm\unittests\Support\ProgramTest.cpp(163): error: fs::remove(Twine(LongPath)): did not return errc::success.
error number: 13
error message: permission denied



C:\a\lld-x86_64-win\llvm-project\llvm\unittests\Support\ProgramTest.cpp:160
Expected equality of these values:
  0
  RC
    Which is: -2

C:\a\lld-x86_64-win\llvm-project\llvm\unittests\Support\ProgramTest.cpp:163
fs::remove(Twine(LongPath)): did not return errc::success.
error number: 13
error message: permission denied




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