[libcxxabi] Unbreak `LLVM_ENABLE_PROJECTS=libcxxabi` build

[Ericson2314]

Summary

D132324 was soft-reverted, this this wouldn't be by fiat prohibited
anymore.

However, builds were still failing as reported in https://reviews.llvm.org/D132324#3739383.
The issue was that D132298 still relied on this case not happening.

Since it in fact does happen with the soft-revert, I need to accommodate
that possibility in the if condition.

Additionally make the soft-revert closer to a real revert with
SEND_ERROR -> WARNING so CMake doesn't have a non-zero exit status
after competing configuring.

[aaron.ballman]

I don't have a particularly good way to test this, but the doc publication bot is still red and we need to address that ASAP, so I'm okay landing this speculatively and watching the bots to ensure the expected ones go back to green. That said, from inspection, I think this won't fully address the failures from the libunwind sphinx builder: https://lab.llvm.org/buildbot/#/builders/31/builds/27142 (it does make it a warning rather than an error which should hopefully be sufficient to get that bot back to green, but we may still want a libubwind exception in the CMake).

[kadircet]

libcxxabi/CMakeLists.txt

136–137

i think we should have a fixme here (and in the relevant test) to drop the check for PROJECTS once libcxx/abi can't be built through it.

libcxxabi/test/CMakeLists.txt

64–65

isn't the first NOT here, before libcxx is wrong? (same for below)

i think we want libcxx to be present in RUNTIMES but not in PROJECTS.

llvm/CMakeLists.txt

112

should we also drop libcxx(abi) from this list? probably as a follow up patch.

[Ericson2314]

Fix more things for posterity

(I will close this in a second)

[Ericson2314]

libcxxabi/test/CMakeLists.txt

64–65

Yeah it is wrong. Stupid copy past mistake. More reason the revert proposed rather than me failing to ninja fixes is the right approach.

[Ericson2314]

Abandoning because the plan is to revert and because @ldionne is skeptical building libc++ without libc++abi ought to be supported anyways.

(Updated before this because I at least wanted some record of what the correct code would look like were we to go ahead with this.)

libcxxabi/CMakeLists.txt

 avoid demangling" OFF)
 
 if (NOT LIBCXXABI_ENABLE_SHARED AND NOT LIBCXXABI_ENABLE_STATIC)
   message(FATAL_ERROR "libc++abi must be built as either a shared or static library.")
 endif()
 
 set(LIBCXXABI_LIBCXX_INCLUDES "" CACHE PATH
     "Specify path to libc++ includes.")
-if (NOT libcxx IN_LIST LLVM_ENABLE_RUNTIMES)
+# FIXME checking two lists to support different sorts of builds is bad.
+if ((NOT libcxx IN_LIST LLVM_ENABLE_RUNTIMES) AND (NOT libcxx IN_LIST LLVM_ENABLE_PROJECTS))
   if (NOT IS_DIRECTORY ${LIBCXXABI_LIBCXX_INCLUDES})
     message(FATAL_ERROR
       "LIBCXXABI_LIBCXX_INCLUDES=${LIBCXXABI_LIBCXX_INCLUDES} is not a valid directory. "
       "Please provide the path to where the libc++ headers have been installed.")
   endif()
   add_library(cxx-headers INTERFACE)
   target_include_directories(cxx-headers INTERFACE "${LIBCXXABI_LIBCXX_INCLUDES}")
 endif()

libcxxabi/test/CMakeLists.txt

 else()
   set(LIBCXXABI_TEST_DEPS cxxabi_static)
 endif()
 
 if (LIBCXXABI_BUILD_EXTERNAL_THREAD_LIBRARY)
   list(APPEND LIBCXXABI_TEST_DEPS cxx_external_threads)
 endif()
 
-if(libcxx IN_LIST LLVM_ENABLE_RUNTIMES)
+# FIXME checking two lists to support different sorts of builds is bad.
+if (libcxx IN_LIST LLVM_ENABLE_RUNTIMES OR libcxx IN_LIST LLVM_ENABLE_PROJECTS)
   list(APPEND LIBCXXABI_TEST_DEPS cxx)
 endif()
-if(libunwind IN_LIST LLVM_ENABLE_RUNTIMES)
+if (libunwind IN_LIST LLVM_ENABLE_RUNTIMES OR libunwind IN_LIST LLVM_ENABLE_PROJECTS)
   if (LIBCXXABI_USE_LLVM_UNWINDER AND TARGET unwind)
     list(APPEND LIBCXXABI_TEST_DEPS unwind)
   endif()
 endif()
 
 set(AUTO_GEN_COMMENT "## Autogenerated by libcxxabi configuration.\n# Do not edit!")
 set(SERIALIZED_LIT_PARAMS "# Lit parameters serialized here for llvm-lit to pick them up\n")
 

llvm/CMakeLists.txt

 Learn more about these options in our documentation at https://llvm.org/docs/CMake.html#cmake-build-type
 ")
 endif()
 
 # Side-by-side subprojects layout: automatically set the
 # LLVM_EXTERNAL_${project}_SOURCE_DIR using LLVM_ALL_PROJECTS
 # This allows an easy way of setting up a build directory for llvm and another
 # one for llvm+clang+... using the same sources.
 set(LLVM_ALL_PROJECTS "bolt;clang;clang-tools-extra;compiler-rt;cross-project-tests;libc;libclc;libcxx;libcxxabi;libunwind;lld;lldb;mlir;openmp;polly;pstl")
 # The flang project is not yet part of "all" projects (see C++ requirements)
 set(LLVM_EXTRA_PROJECTS "flang")
 # List of all known projects in the mono repo
 set(LLVM_KNOWN_PROJECTS "${LLVM_ALL_PROJECTS};${LLVM_EXTRA_PROJECTS}")
 set(LLVM_ENABLE_PROJECTS "" CACHE STRING
     "Semicolon-separated list of projects to build (${LLVM_KNOWN_PROJECTS}), or \"all\".")
 foreach(proj ${LLVM_ENABLE_PROJECTS})
   if (NOT proj STREQUAL "all" AND NOT proj STREQUAL "llvm" AND NOT "${proj}" IN_LIST LLVM_KNOWN_PROJECTS)
      MESSAGE(FATAL_ERROR "${proj} isn't a known project: ${LLVM_KNOWN_PROJECTS}")
   endif()
 endforeach()
 foreach(proj "libcxx" "libcxxabi" "libunwind")
   if (${proj} IN_LIST LLVM_ENABLE_PROJECTS)
     # TODO(Ericson2314): Making non-fatal for now because of out-of-tree docs
     # CI jobs use this.
-    message(SEND_ERROR
+    message(WARNING
       "Using LLVM_ENABLE_PROJECTS=${proj} is incorrect. Please use -DLLVM_ENABLE_RUNTIMES=${proj} or "
       "see the instructions at https://libcxx.llvm.org/BuildingLibcxx.html for building the runtimes.")
   endif()
 endforeach()
 
 if( LLVM_ENABLE_PROJECTS STREQUAL "all" )
   set( LLVM_ENABLE_PROJECTS ${LLVM_ALL_PROJECTS})
 endif()