Re: arm `rustdoc` Rust 1.85.0-only build error

[Fabian Grünbichler]

On Tue, Mar 31, 2026, at 9:00 PM, Miguel Ojeda wrote:
> Hi Christian, Russell, arm, Fabian,
>
> For Rust 1.85.0, for arm32, for the `rustdoc` target (i.e. all those
> combined), I am seeing:
>
>       RUSTDOC   
> .../1.85.0-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/lib.rs
>     error: target feature `fp-armv8` cannot be toggled with 
> `#[target_feature]`: Rust ties `fp-armv8` to `neon`
>         --> 
> .../1.85.0-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/../../stdarch/crates/core_arch/src/arm_shared/neon/generated.rs:7538:48
>          |
>     7538 | #[cfg_attr(target_arch = "arm", target_feature(enable = 
> "fp-armv8,v8"))]
>          |                                                
> ^^^^^^^^^^^^^^^^^^^^^
>
> The issue is [1], was introduced in Rust 1.85.0 and was fixed already in
> Rust 1.85.1 [2]:
>
>   Link: https://github.com/rust-lang/rust/issues/137366 [1]
>   Link: https://github.com/rust-lang/rust/pull/137632 [2]
>
> It is unfortunate since our minimum is going to be 1.85.0 since that is
> what Debian Stable has (even if patches may be on top) -- I generally
> test the latest patch versions for each minor, but I noticed this since
> I also test the actual minimum, and I am bumping it to 1.85.0.
>
> To be clear, it is likely almost no one actually cares about this, since
> nobody complained yet, and this can easily be fixed using the already
> released Rust 1.85.1.
>
> By the way, what is Debian's policy on upstream Rust patch versions?

In unstable, we pull them in usually by virtue of lagging behind a bit anyway.

In stable there is no policy per se - both importing a new smallish important
upstream release, or cherry-picking patches are options in general. A few
packages with clear upstream LTS policies are updated often (systemd, glibc,
the kernel itself, firefox-esr and chromium would be the most popular
examples). If there is no upstream stable release series that matches Debian
stable policies, the usual approach is to do a targeted backport of just the
fixes.

It's a bit unfortunate that the timing lined up like it did, because given the
lack of rustc LTS, usually there are no point releases for the version to be
included in Debian stable anyway.

It's up to the stable release managers how big of a delta is acceptable.

I will check how the full diff for 1.85.1 looks like compared to just picking
the rustdoc fix referenced above, and then file a stable update request. AFAIU
either option works for you?

The next Trixie point release is slated for May 16th, so we still have a bit of
time to discuss this with SRM.

Fabian