Re: [PATCH 2/2] rust: kbuild: workaround `rustdoc` doctests modifier bug

[Alice Ryhl <aliceryhl@google.com>]

On Sun, Nov 02, 2025 at 10:28:53PM +0100, Miguel Ojeda wrote:
> The `rustdoc` modifiers bug [1] was fixed in Rust 1.90.0 [2], for which
> we added a workaround in commit abbf9a449441 ("rust: workaround `rustdoc`
> target modifiers bug").
> 
> However, `rustdoc`'s doctest generation still has a similar issue [3],
> being fixed at [4], which does not affect us because we apply the
> workaround to both, and now, starting with Rust 1.91.0 (released
> 2025-10-30), `-Zsanitizer` is a target modifier too [5], which means we
> fail with:
> 
>       RUSTDOC TK rust/kernel/lib.rs
>     error: mixing `-Zsanitizer` will cause an ABI mismatch in crate `kernel`
>      --> rust/kernel/lib.rs:3:1
>       |
>     3 | //! The `kernel` crate.
>       | ^
>       |
>       = 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
> 
> A simple way around is to add the sanitizer to the list in the existing
> workaround (especially if we had not started to pass the sanitizer
> flags in the previous commit, since in that case that would not be
> necessary). However, that still applies the workaround in more cases
> than necessary.
> 
> Instead, only modify the doctests flags to ignore the check for
> sanitizers, so that it is more local (and thus the compiler keeps checking
> it for us in the normal `rustdoc` calls). Since the previous commit
> already treated the `rustdoc` calls as kernel objects, this should allow
> us in the future to easily remove this workaround when the time comes.
> 
> By the way, the `-Cunsafe-allow-abi-mismatch` flag overwrites previous
> ones rather than appending, so it needs to be all done in the same flag.
> Moreover, unknown modifiers are rejected, and thus we have to gate based
> on the version too.

Ah .. we may want to file a bug for that.

> Finally, `-Zsanitizer-cfi-normalize-integers` is not affected, so it is
> not needed in the workaround.
> 
> 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/issues/144521 [1]
> Link: https://github.com/rust-lang/rust/pull/144523 [2]
> Link: https://github.com/rust-lang/rust/issues/146465 [3]
> Link: https://github.com/rust-lang/rust/pull/148068 [4]
> Link: https://github.com/rust-lang/rust/pull/138736 [5]
> Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Reviewed-by: Alice Ryhl <aliceryhl@google.com>