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

[Justin Forbes <jmforbes@linuxtx.org>]

On Mon, Nov 3, 2025 at 3:28 AM Alice Ryhl <aliceryhl@google.com> wrote:
>
> 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>

I can verify that this fixes kernel builds with rust 1.91 in Fedora.

Tested-by: Justin M. Forbes <jforbes@fedoraproject.org>