diff --git a/clang/runtime/CMakeLists.txt b/clang/runtime/CMakeLists.txt
--- a/clang/runtime/CMakeLists.txt
+++ b/clang/runtime/CMakeLists.txt
@@ -34,6 +34,9 @@
 endif()
 
 if(LLVM_BUILD_EXTERNAL_COMPILER_RT AND EXISTS ${COMPILER_RT_SRC_ROOT}/)
+  message(WARNING "LLVM_BUILD_EXTERNAL_COMPILER_RT is deprecated and will be "
+                  "removed in LLVM 16, please migrate to "
+                  "LLVM_ENABLE_RUNTIMES=compiler-rt")
 
   # Add compiler-rt as an external project.
   set(COMPILER_RT_PREFIX ${CMAKE_BINARY_DIR}/projects/compiler-rt)