FROMLIST: kbuild: rust: re-run Kconfig if the version text changes

Re-run Kconfig if we detect the Rust compiler has changed via the version
text, like it is done for C.

Unlike C, and unlike `RUSTC_VERSION`, the `RUSTC_VERSION_TEXT` is kept
under `depends on RUST`, since it should not be needed unless `RUST`
is enabled.

Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Link: https://lore.kernel.org/r/20240808221138.873750-4-ojeda@kernel.org
Change-Id: I3e7f8a3f9f5c0de49b4e3fc1bbd819203debbbe1
Signed-off-by: Alice Ryhl <aliceryhl@google.com>

Author: ojeda

Makefile

@@ -680,9 +680,10 @@ endif
 
 # The expansion should be delayed until arch/$(SRCARCH)/Makefile is included.
 # Some architectures define CROSS_COMPILE in arch/$(SRCARCH)/Makefile.
-# CC_VERSION_TEXT is referenced from Kconfig (so it needs export),
+# CC_VERSION_TEXT and RUSTC_VERSION_TEXT are referenced from Kconfig (so they need export),
 # and from include/config/auto.conf.cmd to detect the compiler upgrade.
 CC_VERSION_TEXT = $(subst $(pound),,$(shell LC_ALL=C $(CC) --version 2>/dev/null | head -n 1))
+RUSTC_VERSION_TEXT = $(subst $(pound),,$(shell LC_ALL=C $(RUSTC) --version 2>/dev/null | head -n 1))
 
 ifneq ($(findstring clang,$(CC_VERSION_TEXT)),)
 include $(srctree)/scripts/Makefile.clang
@@ -703,7 +704,7 @@ ifdef config-build
 # KBUILD_DEFCONFIG may point out an alternative default configuration
 # used for 'make defconfig'
 include $(srctree)/arch/$(SRCARCH)/Makefile
-export KBUILD_DEFCONFIG KBUILD_KCONFIG CC_VERSION_TEXT
+export KBUILD_DEFCONFIG KBUILD_KCONFIG CC_VERSION_TEXT RUSTC_VERSION_TEXT
 
 config: outputmakefile scripts_basic FORCE
 	$(Q)$(MAKE) $(build)=scripts/kconfig $@

init/Kconfig

@@ -1930,7 +1930,9 @@ config RUST
 config RUSTC_VERSION_TEXT
 	string
 	depends on RUST
-	default "$(shell,LC_ALL=C $(RUSTC) --version 2>/dev/null | head -n 1)"
+	default "$(RUSTC_VERSION_TEXT)"
+	help
+	  See `CC_VERSION_TEXT`.
 
 config BINDGEN_VERSION_TEXT
 	string