Add a flag that controls if clang-tidy and clang-include-fixer are built into libclang.

[thakis]

Summary

Based on the discussion on D55415, also make the flag default to false.
Having libclang depend on clang-tools-extra means check-clang builds all
of clang-tools-extra, which besides being a layering violation takes
quite some time, since clang-tools-extra has many files that are slow
to compile.

Longer term, we likely will want to remove this flag completely. If
people need this functionality, maybe there could be a
libclang-tools-extra that's libclang + clang-tidy and
clang-includes-fixer linked in.

[hans]

Looks very reasonable to be. Maybe worth a release note in case this is surprising to any libclang users? And maybe cc the other folks from D55415?

clang/tools/libclang/CMakeLists.txt

65

Isn't it LLVM_ENABLE_PROJECTS?

[thakis]

Thanks! Added a release note. I had cc'd everyone who had commented on D55415 -- did I miss anyone?

clang/tools/libclang/CMakeLists.txt

65

Err, yes. Done.

clang/docs/ReleaseNotes.rst

 
 
 Build System Changes
 --------------------
 
 These are major changes to the build system that have happened since the 10.0.0
 release of Clang. Users of the build system should adjust accordingly.
 
-- ...
+- clang-tidy and clang-include-fixer are no longer compiled into libclang by
+  default. You can set ``LIBCLANG_INCLUDE_CLANG_TOOLS_EXTRA=ON`` to undo that,
+  but it's expected that that setting will go away eventually. If this is
+  something you need, please reach out to the mailing list to discuss possible
+  ways forward.
 
 AST Matchers
 ------------
 
 - ...
 
 clang-format
 ------------

clang/tools/libclang/CMakeLists.txt

set(LIBS
   clangSerialization
   clangTooling
 )
 
 if (CLANG_ENABLE_ARCMT)
   list(APPEND LIBS clangARCMigrate)
 endif ()
 
+option(LIBCLANG_INCLUDE_CLANG_TOOLS_EXTRA
+  "Include code from clang-tools-extra in libclang." OFF)
+
+if (LIBCLANG_INCLUDE_CLANG_TOOLS_EXTRA)
   if (TARGET clangTidyPlugin)
     add_definitions(-DCLANG_TOOL_EXTRA_BUILD)
     list(APPEND LIBS clangTidyPlugin)
     list(APPEND LIBS clangIncludeFixerPlugin)
     if(LLVM_ENABLE_MODULES)
       list(APPEND LLVM_COMPILE_FLAGS "-fmodules-ignore-macro=CLANG_TOOL_EXTRA_BUILD")
     endif()
+  else ()
+    message(FATAL_ERROR "LIBCLANG_INCLUDE_CLANG_TOOLS_EXTRA needs clang-tools-extra in LLVM_ENABLE_PROJECTS")
+  endif ()
 endif ()
 
 find_library(DL_LIBRARY_PATH dl)
 if (DL_LIBRARY_PATH)
   list(APPEND LIBS dl)
 endif()
 
 option(LIBCLANG_BUILD_STATIC

llvm/utils/gn/secondary/clang/tools/libclang/BUILD.gn

 import("//clang/lib/ARCMigrate/enable.gni")
 import("//llvm/version.gni")
 
 # This build file is just enough to get check-clang to pass, it's missing
 # several things from the CMake build:
 # - using libclang.exports
 # - a build target copying the Python bindings
 # - the GN linux build always builds without -fPIC (as if LLVM_ENABLE_PIC=OFF
 #   in the CMake build), so libclang is always a static library on linux
 # - the GN build doesn't have LIBCLANG_BUILD_STATIC
 
+declare_args() {
+  # Whether to include code from clang-tools-extra in libclang.
+  libclang_include_clang_tools_extra = false
+}
+
 libclang_target_type = "shared_library"
 if (host_os != "win" && host_os != "mac") {
   # ELF targets need -fPIC to build shared libs but they aren't on by default.
   # For now, make libclang a static lib there.
   libclang_target_type = "static_library"
 }
 
 target(libclang_target_type, "libclang") {
target(libclang_target_type, "libclang") {
   if (clang_enable_arcmt) {
     deps += [ "//clang/lib/ARCMigrate" ]
   }
 
   defines = []
 
   # FIXME: Once the GN build has a way to select which bits to build,
   # only include this dependency if clang-tools-extra is part of the build.
-  # FIXME: libclang depending on anything in clang-tools-extra seems like
-  # a layering violation.
-  if (true) {
+  if (libclang_include_clang_tools_extra) {
     defines += [ "CLANG_TOOL_EXTRA_BUILD" ]
     deps += [
       "//clang-tools-extra/clang-include-fixer/plugin",
       "//clang-tools-extra/clang-tidy/plugin",
     ]
   }
 
   if (host_os == "win") {