[PATCH v13 2/5] rust: support formatting of foreign types

[Tamir Duberstein]

On Fri, Jul 4, 2025 at 6:09 AM Benno Lossin <lossin at kernel.org> wrote:
>
> On Fri Jul 4, 2025 at 1:23 AM CEST, Tamir Duberstein wrote:
> > On Thu, Jul 3, 2025 at 6:41 PM Tamir Duberstein <tamird at gmail.com> wrote:
> >> On Thu, Jul 3, 2025 at 4:36 PM Benno Lossin <lossin at kernel.org> wrote:
> >> >
> >> > I don't understand, can't you just do:
> >> >
> >> > * add `rust/kernel/fmt.rs`,
> >> > * add `rust/macros/fmt.rs`,
> >> > * change all occurrences of `core::fmt` to `kernel::fmt` and
> >> >   `format_args!` to `fmt!`.
> >>
> >> Yes, such a split could be done - I will do so in the next spin
> >>
> >>
> >> > The last one could be split by subsystem, no? Some subsystems might
> >> > interact and thus need simultaneous splitting, but there should be some
> >> > independent ones.
> >>
> >> Yes, it probably can. As you say, some subsystems might interact - the
> >> claimed benefit of doing this subsystem-by-subsystem split is that it
> >> avoids conflicts with ongoing work that will conflict with a large
> >> patch, but this is also the downside; if ongoing work changes the set
> >> of interactions between subsystems then a maintainer may find
> >> themselves unable to emit the log message they want (because one
> >> subsystem is using kernel::fmt while another is still on core::fmt).
> >
> > I gave this a try. I ran into the problem that `format_args!` (and,
> > after this patch, `fmt!`) is at the center of `print_macro!`, which
> > itself underpins various other formatting macros. This means we'd have
> > to bifurcate the formatting infrastructure to support an incremental
> > migration. That's quite a bit of code, and likely quite a mess in the
> > resulting git history -- and that's setting aside the toil required to
> > figure out the correct combinations of subsystems that must migrate
> > together.
>
> So here is what we can do without duplicating the logic, though it
> requires multiple cycles:
>
> 1. We merge the two `fmt.rs` files & each subsystem merges an
>    implementation of `kernel::fmt::Display` for their types, but keeps
>    the `core::fmt::Display` impl around.
> 2. After all subsystems have merged the previous step, we change the
>    implementations of `print_macro!` to use `fmt!` instead of
>    `format_args!`.
> 3. We remove all occurrences of `core::fmt` (& replace them with
>    `kernel::fmt`), removing the `core::fmt::Display` impls.

That would probably work. We will probably see regressions because we
can't just replace `core::fmt` imports with `kernel::fmt`, so new code
may appear that uses the former.

I think this discussion would be productive on the next spin. The
changes in other subsystems are now almost entirely changing of import
paths -- perhaps that would be sufficiently uncontroversial for folks
to give their Acked-bys.