[PATCH] rust: Makefile: apply fixed-x18 workaround only on arm64

[HeeSu Kim]

The `-Cunsafe-allow-abi-mismatch=fixed-x18` option is a workaround for
a rustdoc issue (rust-lang/rust#144521) where target modifiers were not
properly saved. This affects both rustdoc and doctests builds.

However, `fixed-x18` is an ARM64-specific ABI modifier related to the
x18 register reservation (used for Shadow Call Stack). Currently, this
workaround is applied unconditionally on all architectures, which is
incorrect and may cause issues on non-ARM64 builds.

Restrict the workaround to only apply when building for arm64
(SRCARCH=arm64).

Fixes: 68e8e79e1bdc ("rust: fix `rustdoc` target modifiers")
Signed-off-by: HeeSu Kim <mlksvender@xxxxxxxxx>
---
 rust/Makefile | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/rust/Makefile b/rust/Makefile
index 5c0155b83..2bd547611 100644
--- a/rust/Makefile
+++ b/rust/Makefile
@@ -136,7 +136,8 @@ pin_init-flags := \
 
 # `rustdoc` did not save the target modifiers, thus workaround for
 # the time being (https://github.com/rust-lang/rust/issues/144521).
-rustdoc_modifiers_workaround := $(if $(call rustc-min-version,108800),-Cunsafe-allow-abi-mismatch=fixed-x18)
+# The `fixed-x18` modifier is ARM64-specific, so only apply it there.
+rustdoc_modifiers_workaround := $(if $(filter arm64,$(SRCARCH)),$(if $(call rustc-min-version,108800),-Cunsafe-allow-abi-mismatch=fixed-x18))
 
 # Similarly, for doctests (https://github.com/rust-lang/rust/issues/146465).
 doctests_modifiers_workaround := $(rustdoc_modifiers_workaround)$(if $(call rustc-min-version,109100),$(comma)sanitizer)
-- 
2.52.0