[PATCH 6.12.y] rust: kbuild: treat `build_error` and `rustdoc` as kernel objects

[Sasha Levin <sashal@kernel.org>]

From: Miguel Ojeda <ojeda@kernel.org>

[ Upstream commit 16c43a56b79e2c3220b043236369a129d508c65a ]

Even if normally `build_error` isn't a kernel object, it should still
be treated as such so that we pass the same flags. Similarly, `rustdoc`
targets are never kernel objects, but we need to treat them as such.

Otherwise, starting with Rust 1.91.0 (released 2025-10-30), `rustc`
will complain about missing sanitizer flags since `-Zsanitizer` is a
target modifier too [1]:

    error: mixing `-Zsanitizer` will cause an ABI mismatch in crate `build_error`
     --> rust/build_error.rs:3:1
      |
    3 | //! Build-time error.
      | ^
      |
      = help: the `-Zsanitizer` flag modifies the ABI so Rust crates compiled with different values of this flag cannot be used together safely
      = note: unset `-Zsanitizer` in this crate is incompatible with `-Zsanitizer=kernel-address` in dependency `core`
      = help: set `-Zsanitizer=kernel-address` in this crate or unset `-Zsanitizer` in `core`
      = help: if you are sure this will not cause problems, you may use `-Cunsafe-allow-abi-mismatch=sanitizer` to silence this error

Thus explicitly mark them as kernel objects.

Cc: stable@vger.kernel.org # Needed in 6.12.y and later (Rust is pinned in older LTSs).
Link: https://github.com/rust-lang/rust/pull/138736 [1]
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Tested-by: Justin M. Forbes <jforbes@fedoraproject.org>
Link: https://patch.msgid.link/20251102212853.1505384-1-ojeda@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
[ Adjust context ]
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 rust/Makefile | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/rust/Makefile b/rust/Makefile
index 07c13100000cd..6bcc478900af2 100644
--- a/rust/Makefile
+++ b/rust/Makefile
@@ -107,12 +107,18 @@ rustdoc-core: private rustc_target_flags = --edition=$(core-edition) $(core-cfgs
 rustdoc-core: $(RUST_LIB_SRC)/core/src/lib.rs rustdoc-clean FORCE
 	+$(call if_changed,rustdoc)
 
+# Even if `rustdoc` targets are not kernel objects, they should still be
+# treated as such so that we pass the same flags. Otherwise, for instance,
+# `rustdoc` will complain about missing sanitizer flags causing an ABI mismatch.
+rustdoc-compiler_builtins: private is-kernel-object := y
 rustdoc-compiler_builtins: $(src)/compiler_builtins.rs rustdoc-core FORCE
 	+$(call if_changed,rustdoc)
 
+rustdoc-ffi: private is-kernel-object := y
 rustdoc-ffi: $(src)/ffi.rs rustdoc-core FORCE
 	+$(call if_changed,rustdoc)
 
+rustdoc-kernel: private is-kernel-object := y
 rustdoc-kernel: private rustc_target_flags = --extern ffi \
     --extern build_error --extern macros=$(objtree)/$(obj)/libmacros.so \
     --extern bindings --extern uapi
@@ -433,6 +439,10 @@ $(obj)/compiler_builtins.o: private rustc_objcopy = -w -W '__*'
 $(obj)/compiler_builtins.o: $(src)/compiler_builtins.rs $(obj)/core.o FORCE
 	+$(call if_changed_rule,rustc_library)
 
+# Even if normally `build_error` is not a kernel object, it should still be
+# treated as such so that we pass the same flags. Otherwise, for instance,
+# `rustc` will complain about missing sanitizer flags causing an ABI mismatch.
+$(obj)/build_error.o: private is-kernel-object := y
 $(obj)/build_error.o: $(src)/build_error.rs $(obj)/compiler_builtins.o FORCE
 	+$(call if_changed_rule,rustc_library)
 
-- 
2.51.0