[CMake] Add error to clarify that lldb requires libcxx

This adds a specific error message to clarify that lldb requires libcxx when
built together with clang on macOS. In addition, the lldb building docs are also

Fixes https://bugs.llvm.org/show_bug.cgi?id=41866

Reviewers: sgraenitz, JDevlieghere, EricWF

Reviewed By: sgraenitz

Subscribers: mgorny, lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D61877

llvm-svn: 360756
diff --git a/lldb/CMakeLists.txt b/lldb/CMakeLists.txt
index 64807da..af7c312 100644
--- a/lldb/CMakeLists.txt
+++ b/lldb/CMakeLists.txt
@@ -124,6 +124,24 @@
           message(WARNING "LLDB test suite requires libc++ in llvm/projects/libcxx or an existing build symlinked to ${cxx_dir}")
+        # We require libcxx for the test suite, so if we aren't building it,
+        # try to provide a helpful error about how to resolve the situation.
+        if(NOT TARGET cxx)
+            # If `LLVM_ENABLE_PROJECTS` is not being used (implying that we are
+            # using the old layout), suggest checking it out.
+            message(FATAL_ERROR
+              "LLDB test suite requires libc++, but it is currently disabled. "
+              "Please checkout `libcxx` in `llvm/projects` or disable tests "
+              "via `LLDB_INCLUDE_TESTS=OFF`.")
+          else()
+            # If `LLVM_ENABLE_PROJECTS` is being used, suggest adding it.
+            message(FATAL_ERROR
+              "LLDB test suite requires libc++, but it is currently disabled. "
+              "Please add `libcxx` to `LLVM_ENABLE_PROJECTS` or disable tests "
+              "via `LLDB_INCLUDE_TESTS=OFF`.")
+          endif()
+        endif()
         list(APPEND LLDB_TEST_DEPS cxx)
diff --git a/lldb/docs/resources/build.rst b/lldb/docs/resources/build.rst
index e8d99f7..4c8d242 100644
--- a/lldb/docs/resources/build.rst
+++ b/lldb/docs/resources/build.rst
@@ -151,6 +151,12 @@
 LLVM, then you can pass LLVM-specific CMake variables to cmake when building
+If you are building both Clang and LLDB together, be sure to also add libc++,
+which is currently required for testing on macOS:
+  > cmake -D LLVM_ENABLE_PROJECTS='clang;lldb;libcxx' $PATH_TO_LLVM -G Ninja
 Here are some commonly used LLDB-specific CMake variables: