[PATCH v3] rust: Makefile: bound rustdoc workaround to affected versions

[HeeSu Kim <mlksvender@gmail.com>]

The `-Cunsafe-allow-abi-mismatch=fixed-x18` workaround was added to
handle a rustdoc bug where target modifiers were not properly saved [1].

This bug was fixed in Rust 1.90.0 [2]. Restrict the workaround to only
apply for Rust 1.88.x and 1.89.x versions that are affected by the
bug, preserving ABI compatibility checks on newer compiler versions.

Link: https://github.com/rust-lang/rust/issues/144521 [1]
Link: https://github.com/rust-lang/rust/pull/144523 [2]
Suggested-by: Gary Guo <gary@garyguo.net>
Link: https://lore.kernel.org/rust-for-linux/DG4JM9PU51M0.1YRGM9HVTY24U@garyguo.net/
Cc: stable@vger.kernel.org # Useful in 6.18.y and later.
Signed-off-by: HeeSu Kim <mlksvender@gmail.com>
---
Changes in v3:
- Remove Fixes: tag (this is a feature, not a fix)
- Use full URLs with Link: tags instead of GitHub-style references
- Add Link: to lore.kernel.org for Suggested-by attribution
- Add Cc: stable for potential backporting to 6.18.y

Changes in v2:
- Change approach: bound to affected Rust versions instead of ARM64-only
  (the flag is simply ignored on non-ARM64 architectures)

 rust/Makefile | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/rust/Makefile b/rust/Makefile
index 5c0155b83454..55e2dc865207 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 bug was fixed in Rust 1.90.0, so only apply for 1.88.x and 1.89.x.
+rustdoc_modifiers_workaround := $(if $(call rustc-min-version,108800),$(if $(call test-lt,$(CONFIG_RUSTC_VERSION),109000),-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