Re: [PATCH v3 0/4] s390: Enable Rust support and add required arch glue

[Heiko Carstens <hca@linux.ibm.com>]

On Wed, May 27, 2026 at 02:10:25PM +0200, Miguel Ojeda wrote:
> On Wed, May 27, 2026 at 12:38 PM Alice Ryhl <aliceryhl@google.com> wrote:
> >
> > Based on some very quick research, it looks like enabling the backchain
> > target feature does not affect the ABI in the sense that mixing together
> > object files with and without this option will still work. If that's the
> > case, we can add this to the list of known target features for this
> > target in rustc, which will silence this warning.
> >
> > (If it affects the ABI in a breaking manner, it needs to be added as a
> > separate flag rather than being added as part of the -Ctarget-feature
> > flag.)
> 
> Yes, from the different discussions what I saw is that the issue was
> mixing it with `-Zpacked-struct`, but that is already rejected by the
> compiler. They also discussed whether it should have been its own
> flag, though.

I guess you meant `-Zpacked-stack`? We need both `-Zpacked-stack`, as
well as `-Ctarget-feature=+backchain`. Otherwise the result would be
incorrect code, due to different stack layouts.