Starting with Rust 1.88.0 (released 2025-06-26), `rustdoc` complains about a target modifier mismatch in configurations where `-Zfixed-x18` is passed:
error: mixing `-Zfixed-x18` will cause an ABI mismatch in crate `rust_out` | = help: the `-Zfixed-x18` flag modifies the ABI so Rust crates compiled with different values of this flag cannot be used together safely = note: unset `-Zfixed-x18` in this crate is incompatible with `-Zfixed-x18=` in dependency `core` = help: set `-Zfixed-x18=` in this crate or unset `-Zfixed-x18` in `core` = help: if you are sure this will not cause problems, you may use `-Cunsafe-allow-abi-mismatch=fixed-x18` to silence this error
The reason is that `rustdoc` was not passing the target modifiers when configuring the session options, and thus it would report a mismatch that did not exist as soon as a target modifier is used in a dependency.
We did not notice it in the kernel until now because `-Zfixed-x18` has been a target modifier only since 1.88.0 (and it is the only one we use so far).
The issue has been reported upstream [1] and a fix has been submitted [2], including a test similar to the kernel case.
[ This is now fixed upstream (thanks Guillaume for the quick review), so it will be fixed in Rust 1.90.0 (expected 2025-09-18).
- Miguel ]
Meanwhile, conditionally pass `-Cunsafe-allow-abi-mismatch=fixed-x18` to workaround the issue on our side.
Cc: stable@vger.kernel.org # Needed in 6.12.y and later (Rust is pinned in older LTSs). Reported-by: Konrad Dybcio konrad.dybcio@oss.qualcomm.com Closes: https://lore.kernel.org/rust-for-linux/36cdc798-524f-4910-8b77-d7b9fac08d77@... Link: https://github.com/rust-lang/rust/issues/144521 [1] Link: https://github.com/rust-lang/rust/pull/144523 [2] Reviewed-by: Alice Ryhl aliceryhl@google.com Link: https://lore.kernel.org/r/20250727092317.2930617-1-ojeda@kernel.org Signed-off-by: Miguel Ojeda ojeda@kernel.org (cherry picked from commit abbf9a44944171ca99c150adad9361a2f517d3b6) --- rust/Makefile | 6 ++++++ 1 file changed, 6 insertions(+)
diff --git a/rust/Makefile b/rust/Makefile index 17491d8229a4..dc1ec2e077e1 100644 --- a/rust/Makefile +++ b/rust/Makefile @@ -55,6 +55,10 @@ core-cfgs = \
core-edition := $(if $(call rustc-min-version,108700),2024,2021)
+# `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) + quiet_cmd_rustdoc = RUSTDOC $(if $(rustdoc_host),H, ) $< cmd_rustdoc = \ OBJTREE=$(abspath $(objtree)) \ @@ -63,6 +67,7 @@ quiet_cmd_rustdoc = RUSTDOC $(if $(rustdoc_host),H, ) $< -Zunstable-options --generate-link-to-definition \ --output $(rustdoc_output) \ --crate-name $(subst rustdoc-,,$@) \ + $(rustdoc_modifiers_workaround) \ $(if $(rustdoc_host),,--sysroot=/dev/null) \ @$(objtree)/include/generated/rustc_cfg $<
@@ -175,6 +180,7 @@ quiet_cmd_rustdoc_test_kernel = RUSTDOC TK $< --extern bindings --extern uapi \ --no-run --crate-name kernel -Zunstable-options \ --sysroot=/dev/null \ + $(rustdoc_modifiers_workaround) \ --test-builder $(objtree)/scripts/rustdoc_test_builder \ $< $(rustdoc_test_kernel_quiet); \ $(objtree)/scripts/rustdoc_test_gen