[PATCH v4] rust: Makefile: bound rustdoc workaround to affected versions

[HeeSu Kim]

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.

Add `rustc-max-version` macro to `scripts/Makefile.compiler` for
version upper bound checks, mirroring the existing `rustc-min-version`.

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@xxxxxxxxxxx>
Link: https://lore.kernel.org/rust-for-linux/DG4JM9PU51M0.1YRGM9HVTY24U@xxxxxxxxxxx/
Suggested-by: Miguel Ojeda <ojeda@xxxxxxxxxx>
Link: https://lore.kernel.org/rust-for-linux/CANiq72n39eU9WE=Yh0_yJzmqMxo=QAaU2pN0UqP9jZ7bT7rhgA@xxxxxxxxxxxxxx/
Cc: stable@xxxxxxxxxxxxxxx # Useful in 6.18.y and later.
Signed-off-by: HeeSu Kim <mlksvender@xxxxxxxxx>
---
Changes in v4:
- Add rustc-max-version macro for cleaner version bounds
- Use rustc-max-version instead of test-lt for readability

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 ++-
 scripts/Makefile.compiler | 4 ++++
 2 files changed, 6 insertions(+), 1 deletion(-)

diff --git a/rust/Makefile b/rust/Makefile
index 5c0155b83454..1e8a75bc2878 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 rustc-max-version,108999),-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)
diff --git a/scripts/Makefile.compiler b/scripts/Makefile.compiler
index ef91910de265..85268f6f1494 100644
--- a/scripts/Makefile.compiler
+++ b/scripts/Makefile.compiler
@@ -71,6 +71,10 @@ clang-min-version = $(call test-ge, $(CONFIG_CLANG_VERSION), $1)
 # Usage: rustc-$(call rustc-min-version, 108500) += -Cfoo
 rustc-min-version = $(call test-ge, $(CONFIG_RUSTC_VERSION), $1)
 
+# rustc-max-version
+# Usage: rustc-$(call rustc-max-version, 109000) += -Cfoo
+rustc-max-version = $(call test-le, $(CONFIG_RUSTC_VERSION), $1)
+
 # ld-option
 # Usage: KBUILD_LDFLAGS += $(call ld-option, -X, -Y)
 ld-option = $(call try-run, $(LD) $(KBUILD_LDFLAGS) $(1) -v,$(1),$(2),$(3))
-- 
2.52.0