Re: [PATCH v2 5/6] kbuild: rust: warn if the out-of-tree compiler differs from the kernel one

[Matthias Goergens <matthias.goergens@gmail.com>]

Hi Miguel,

When you applied the rest of this series you held this patch back "to
give people more time to look at it".  Nicolas reviewed and Alice
tested the series at the time, but nobody commented on this patch, and
it never went in.  Here is a case from the field where it would have
helped.

Arch Linux users building an out-of-tree Rust module with DKMS (the
bcachefs module, in this case) hit E0514 against the headers package:

  crate `core` compiled by rustc 1.98.1 (48a229cea 2026-09-01) (Arch Linux rust 1:1.98.1-2): /usr/lib/modules/6.18.53-1-lts/build/rust/libcore.rmeta
  = help: please recompile that crate using this compiler (rustc 1.98.1 (48a229cea 2026-09-01) (Arch Linux rust 1:1.98.1-1)) (consider running `cargo clean` first)

Both are 1.98.1 from the same commit, in two package releases: the
linux-lts headers were built with the second while users still had the
first.  (Arch has since rebuilt its kernels.)  The advice to recompile
`core` and run `cargo clean` does not apply to a headers package, and
the forum thread spent several posts on repository settings before the
cause was clear:

  https://bbs.archlinux.org/viewtopic.php?id=314993

The check in this patch compares `rustc --version`, which includes the
distribution's description, so it fires in exactly this case.  I
rebased it onto current mainline (the only conflict is the pahole
check added since; I put the Rust check after the C one) and tried it
the other way round, with the Arch 7.2.8.arch1-1 headers (now in the
Arch Linux Archive) and the newer rustc:

  warning: the Rust compiler differs from the one used to build the kernel
    The kernel was built by: rustc 1.98.1 (48a229cea 2026-09-01) (Arch Linux rust 1:1.98.1-1)
    You are using:           rustc 1.98.1 (48a229cea 2026-09-01) (Arch Linux rust 1:1.98.1-2)
    RUSTC [M] rust_oot.o
  error[E0514]: found crate `core` compiled by an incompatible version of rustc

With a matching rustc it prints nothing.  If you would like to take it
now, I can send the rebased version.

Thanks,
Matthias