cmake: toolchain: Introduce TOOLCHAIN_HAS_LIBCXX

This commit introduces `TOOLCHAIN_HAS_LIBCXX` CMake variable, which is set
to `y` when LLVM C++ Standard Library aka. libc++ is available.

This helps filter libc++-specific Kconfig and tests in a more refined
manner.

Signed-off-by: Stephanos Ioannidis <[email protected]>

Author: stephanosio

cmake/modules/kconfig.cmake

@@ -141,6 +141,12 @@ else()
   set(_local_TOOLCHAIN_HAS_GLIBCXX n)
 endif()
 
+if(TOOLCHAIN_HAS_LIBCXX)
+  set(_local_TOOLCHAIN_HAS_LIBCXX y)
+else()
+  set(_local_TOOLCHAIN_HAS_LIBCXX n)
+endif()
+
 # APP_DIR: Path to the main image (sysbuild) or synonym for APPLICATION_SOURCE_DIR (non-sysbuild)
 zephyr_get(APP_DIR VAR APP_DIR APPLICATION_SOURCE_DIR)
 
@@ -166,6 +172,7 @@ set(COMMON_KCONFIG_ENV_SETTINGS
   TOOLCHAIN_HAS_NEWLIB=${_local_TOOLCHAIN_HAS_NEWLIB}
   TOOLCHAIN_HAS_PICOLIBC=${_local_TOOLCHAIN_HAS_PICOLIBC}
   TOOLCHAIN_HAS_GLIBCXX=${_local_TOOLCHAIN_HAS_GLIBCXX}
+  TOOLCHAIN_HAS_LIBCXX=${_local_TOOLCHAIN_HAS_LIBCXX}
   EDT_PICKLE=${EDT_PICKLE}
   # Export all Zephyr modules to Kconfig
   ${ZEPHYR_KCONFIG_MODULES_DIR}

cmake/toolchain/llvm/generic.cmake

@@ -41,4 +41,6 @@ if(NOT LLVM_TOOLCHAIN_PATH STREQUAL "")
   endif()
 endif()
 
+set(TOOLCHAIN_HAS_LIBCXX ON CACHE BOOL "True if toolchain supports libc++")
+
 message(STATUS "Found toolchain: llvm (clang/ld)")

cmake/toolchain/xt-clang/generic.cmake

@@ -16,5 +16,6 @@ set(LINKER xt-ld)
 # obtain license information from remote licensing servers. So here
 # forces the assembler ID to be GNU to speed things up a bit.
 set(CMAKE_ASM_COMPILER_ID "GNU")
+set(TOOLCHAIN_HAS_LIBCXX ON CACHE BOOL "True if toolchain supports libc++")
 
 message(STATUS "Found toolchain: xt-clang (${XTENSA_TOOLCHAIN_PATH})")

lib/cpp/Kconfig

@@ -95,7 +95,7 @@ config FULL_LIBCPP_SUPPORTED
 choice LIBCPP_IMPLEMENTATION
 	prompt "C++ Standard Library Implementation"
 	default EXTERNAL_LIBCPP if REQUIRES_FULL_LIBCPP && NATIVE_BUILD
-	default LIBCXX_LIBCPP if REQUIRES_FULL_LIBCPP && "$(ZEPHYR_TOOLCHAIN_VARIANT)" = "llvm"
+	default LIBCXX_LIBCPP if REQUIRES_FULL_LIBCPP && "$(TOOLCHAIN_HAS_LIBCXX)" = "y"
 	default GLIBCXX_LIBCPP if REQUIRES_FULL_LIBCPP && "$(TOOLCHAIN_HAS_GLIBCXX)" = "y"
 	default MINIMAL_LIBCPP
 
@@ -120,8 +120,8 @@ config GLIBCXX_LIBCPP
 
 config LIBCXX_LIBCPP
 	bool "LLVM C++ Standard Library"
+	depends on "$(TOOLCHAIN_HAS_LIBCXX)" = "y"
 	depends on NEWLIB_LIBC
-	depends on "$(ZEPHYR_TOOLCHAIN_VARIANT)" = "llvm"
 	select FULL_LIBCPP_SUPPORTED
 	help
 	  Build with LLVM C++ Standard Library (libc++) provided by LLVM