rust: workaround rustdoc target modifiers bug

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: [email protected] # Needed in 6.12.y and later (Rust is pinned in older LTSs).
Reported-by: Konrad Dybcio <[email protected]>
Closes: https://lore.kernel.org/rust-for-linux/[email protected]/
Link: rust-lang/rust#144521 [1]
Link: rust-lang/rust#144523 [2]
Reviewed-by: Alice Ryhl <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Miguel Ojeda <[email protected]>

Author: ojeda

rust/Makefile

@@ -65,6 +65,10 @@ 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)
+
 # `rustc` recognizes `--remap-path-prefix` since 1.26.0, but `rustdoc` only
 # since Rust 1.81.0. Moreover, `rustdoc` ICEs on out-of-tree builds since Rust
 # 1.82.0 (https://github.com/rust-lang/rust/issues/138520). Thus workaround both
@@ -77,6 +81,7 @@ 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 $<
 
@@ -215,6 +220,7 @@ 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