[compiler-rt] Deprecate LLVM_ENABLE_PROJECTS in favor of LLVM_ENABLE_RUNTIMES (#124016)

We plan to make this a hard error in the LLVM 21 release.

Link #124012

Author: petrhosek

llvm/CMakeLists.txt

@@ -157,6 +157,13 @@ if ("libc" IN_LIST LLVM_ENABLE_PROJECTS)
     "https://libc.llvm.org/ for building the runtimes.")
 endif()
 
+if ("compiler-rt" IN_LIST LLVM_ENABLE_PROJECTS)
+  message(WARNING "Using LLVM_ENABLE_PROJECTS=compiler-rt is deprecated now, and will "
+    "become a fatal error in the LLVM 21 release.  Please use "
+    "-DLLVM_ENABLE_RUNTIMES=compiler-rt or see the instructions at "
+    "https://compiler-rt.llvm.org/ for building the runtimes.")
+endif()
+
 # Select the runtimes to build
 #
 # As we migrate runtimes to using the bootstrapping build, the set of default runtimes

llvm/docs/ReleaseNotes.md

@@ -120,6 +120,12 @@ Changes to building LLVM
 ------------------------
 
 * Raised the minimum MSVC version to Visual Studio 2019 16.8.
+* Deprecated support for building compiler-rt with `LLVM_ENABLE_PROJECTS`.
+  Users should instead use `LLVM_ENABLE_RUNTIMES`, either through the
+  runtimes or the bootstrapping build.
+* Deprecated support for building libc with `LLVM_ENABLE_PROJECTS`.
+  Users should instead use `LLVM_ENABLE_RUNTIMES`, either through the
+  runtimes or the bootstrapping build.
 
 Changes to TableGen
 -------------------