x86/rust: support RETHUNK

When support for `-Zfunction-return` lands in Rust [1], this patch may
be used to enable RETHUNK support on top of the previous patch.

Link: rust-lang/rust#116892 [1]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

arch/x86/Makefile

@@ -24,11 +24,15 @@ RETPOLINE_CFLAGS	+= $(call cc-option,-mindirect-branch-cs-prefix)
 
 ifdef CONFIG_RETHUNK
 RETHUNK_CFLAGS		:= -mfunction-return=thunk-extern
+RETHUNK_RUSTFLAGS	:= -Zfunction-return=thunk-extern
 RETPOLINE_CFLAGS	+= $(RETHUNK_CFLAGS)
+RETPOLINE_RUSTFLAGS	+= $(RETHUNK_RUSTFLAGS)
 endif
 
 export RETHUNK_CFLAGS
+export RETHUNK_RUSTFLAGS
 export RETPOLINE_CFLAGS
+export RETPOLINE_RUSTFLAGS
 export RETPOLINE_VDSO_CFLAGS
 
 # For gcc stack alignment is specified with -mpreferred-stack-boundary,
@@ -193,6 +197,7 @@ KBUILD_CFLAGS += -fno-asynchronous-unwind-tables
 # Avoid indirect branches in kernel to deal with Spectre
 ifdef CONFIG_RETPOLINE
   KBUILD_CFLAGS += $(RETPOLINE_CFLAGS)
+  KBUILD_RUSTFLAGS += $(RETPOLINE_RUSTFLAGS)
   # Additionally, avoid generating expensive indirect jumps which
   # are subject to retpolines for small number of switch cases.
   # LLVM turns off jump table generation by default when under

init/Kconfig

@@ -1889,7 +1889,6 @@ config RUST
 	depends on !GCC_PLUGINS
 	depends on !RANDSTRUCT
 	depends on !DEBUG_INFO_BTF || PAHOLE_HAS_LANG_EXCLUDE
-	depends on !RETHUNK
 	select CONSTRUCTORS
 	help
 	  Enables Rust support in the kernel.