lib/cpp: Add support for building libstdc++ as a module

Provide a mechanism to build libstdc++ as a module, instead of
requiring that it be included in the toolchain.

Signed-off-by: Keith Packard <keithp@keithp.com>

Author: keith-packard

MAINTAINERS.yml

@@ -5304,6 +5304,15 @@ West:
   labels:
     - "area: Audio"
 
+"West project: libstdcxx":
+  status: maintained
+  maintainers:
+    - Keith Packard
+  files:
+    - modules/Kconfig.libstdc++
+  labels:
+    - "area: C++"
+
 "West project: littlefs":
   status: odd fixes
   files:

lib/cpp/Kconfig

@@ -118,6 +118,25 @@ config GLIBCXX_LIBCPP
 	  Build with GNU C++ Standard Library (libstdc++) provided by the GNU
 	  Compiler Collection (GCC)-based toolchain.
 
+choice GLIBCXX_LIBCPP_SOURCE
+	prompt "Source of libstdc++"
+	depends on GLIBCXX_LIBCPP
+	default GLIBCXX_LIBCPP_USE_TOOLCHAIN
+
+config GLIBCXX_LIBCPP_USE_MODULE
+	bool "libstdc++ from module"
+	depends on ZEPHYR_LIBSTDCXX_MODULE
+	help
+	  Use libstdc++ module instead of libstdc++ included with toolchain.
+
+config GLIBCXX_LIBCPP_USE_TOOLCHAIN
+	bool "libstdc++ from toolchain"
+	depends on NEWLIB_LIBC || PICOLIBC_USE_TOOLCHAIN
+	help
+	  Use libstdc++ included with toolchain.
+
+endchoice
+
 config LIBCXX_LIBCPP
 	bool "LLVM C++ Standard Library"
 	depends on !NATIVE_APPLICATION

lib/libc/Kconfig

@@ -45,7 +45,7 @@ config NEWLIB_LIBC_SUPPORTED
 config PICOLIBC_SUPPORTED
 	bool
 	depends on !NATIVE_APPLICATION
-	depends on ("$(TOOLCHAIN_HAS_PICOLIBC)" = "y") || (ZEPHYR_PICOLIBC_MODULE && !REQUIRES_FULL_LIBCPP)
+	depends on ("$(TOOLCHAIN_HAS_PICOLIBC)" = "y") || ZEPHYR_PICOLIBC_MODULE
 	default y
 	select FULL_LIBC_SUPPORTED
 	help

lib/libc/picolibc/Kconfig

@@ -5,13 +5,12 @@ if PICOLIBC
 
 choice PICOLIBC_SOURCE
 	prompt "Source of Picolibc"
-	default PICOLIBC_USE_TOOLCHAIN if REQUIRES_FULL_LIBCPP || "$(TOOLCHAIN_HAS_PICOLIBC)" = "y"
+	default PICOLIBC_USE_TOOLCHAIN if "$(TOOLCHAIN_HAS_PICOLIBC)" = "y"
 	default PICOLIBC_USE_MODULE
 
 config PICOLIBC_USE_MODULE
 	bool "Picolibc from module"
 	depends on ZEPHYR_PICOLIBC_MODULE
-	depends on !GLIBCXX_LIBCPP
 	help
 	  Use picolibc module instead of picolibc included with toolchain.
 	  This is enabled by default for toolchains other than the Zephyr
@@ -24,7 +23,6 @@ config PICOLIBC_USE_TOOLCHAIN
 	select THREAD_LOCAL_STORAGE if ARCH_HAS_THREAD_LOCAL_STORAGE && TOOLCHAIN_SUPPORTS_THREAD_LOCAL_STORAGE
 	help
 	  Use picolibc included with the toolchain.
-	  This is required when using a full C++ standard library (`REQUIRES_FULL_LIBCPP=y`).
 
 endchoice # PICOLIBC_SOURCE
 

modules/Kconfig

@@ -49,6 +49,7 @@ source "modules/zcbor/Kconfig"
 source "modules/Kconfig.mcuboot"
 source "modules/Kconfig.intel"
 source "modules/hostap/Kconfig"
+source "modules/Kconfig.libstdc++"
 
 comment "Unavailable modules, please install those via the project manifest."
 

modules/Kconfig.libstdc++

@@ -0,0 +1,5 @@
+# Copyright 2024 Google LLC
+# SPDX-License-Identifier: Apache-2.0
+
+config ZEPHYR_LIBSTDCXX_MODULE
+	bool

west.yml

@@ -23,6 +23,8 @@ manifest:
       url-base: https://github.com/zephyrproject-rtos
     - name: babblesim
       url-base: https://github.com/BabbleSim
+    - name: keithp
+      url-base: https://github.com/keith-packard
 
   group-filter: [-babblesim, -optional]
 
@@ -293,6 +295,11 @@ manifest:
       path: modules/hal/libmetal
       groups:
         - hal
+    - name: libstdcxx
+      remote: keithp
+      repo-path: libstdcxx-module
+      revision: 9ac70a1eec3bec2bc158eb4b384cdc97631cdd77
+      path: modules/lib/libstdcxx
     - name: littlefs
       path: modules/fs/littlefs
       groups: