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

Thu Jul 3 22:41:23 UTC 2025

On Thu, Jul 3, 2025 at 4:36 PM Benno Lossin <lossin at kernel.org> wrote:
>
> On Thu Jul 3, 2025 at 8:55 PM CEST, Tamir Duberstein wrote:
> > On Thu, Jul 3, 2025 at 11:08 AM Benno Lossin <lossin at kernel.org> wrote:
> >> On Thu Jul 3, 2025 at 3:55 PM CEST, Tamir Duberstein wrote:
> >> > On Thu, Jul 3, 2025 at 5:32 AM Benno Lossin <lossin at kernel.org> wrote:
> >> >> On Tue Jul 1, 2025 at 6:49 PM CEST, Tamir Duberstein wrote:
> >> >> > Introduce a `fmt!` macro which wraps all arguments in
> >> >> > `kernel::fmt::Adapter` and a `kernel::fmt::Display` trait. This enables
> >> >> > formatting of foreign types (like `core::ffi::CStr`) that do not
> >> >> > implement `core::fmt::Display` due to concerns around lossy conversions which
> >> >> > do not apply in the kernel.
> >> >> >
> >> >> > Replace all direct calls to `format_args!` with `fmt!`.
> >> >> >
> >> >> > Replace all implementations of `core::fmt::Display` with implementations
> >> >> > of `kernel::fmt::Display`.
> >> >> >
> >> >> > Suggested-by: Alice Ryhl <aliceryhl at google.com>
> >> >> > Link: https://rust-for-linux.zulipchat.com/#narrow/channel/288089-General/topic/Custom.20formatting/with/516476467
> >> >> > Acked-by: Greg Kroah-Hartman <gregkh at linuxfoundation.org>
> >> >> > Reviewed-by: Alice Ryhl <aliceryhl at google.com>
> >> >> > Signed-off-by: Tamir Duberstein <tamird at gmail.com>
> >> >> > ---
> >> >> >  drivers/block/rnull.rs       |  2 +-
> >> >> >  drivers/gpu/nova-core/gpu.rs |  4 +-
> >> >> >  rust/kernel/block/mq.rs      |  2 +-
> >> >> >  rust/kernel/device.rs        |  2 +-
> >> >> >  rust/kernel/fmt.rs           | 89 +++++++++++++++++++++++++++++++++++++++
> >> >> >  rust/kernel/kunit.rs         |  6 +--
> >> >> >  rust/kernel/lib.rs           |  1 +
> >> >> >  rust/kernel/prelude.rs       |  3 +-
> >> >> >  rust/kernel/print.rs         |  4 +-
> >> >> >  rust/kernel/seq_file.rs      |  2 +-
> >> >> >  rust/kernel/str.rs           | 22 ++++------
> >> >> >  rust/macros/fmt.rs           | 99 ++++++++++++++++++++++++++++++++++++++++++++
> >> >> >  rust/macros/lib.rs           | 19 +++++++++
> >> >> >  rust/macros/quote.rs         |  7 ++++
> >> >> >  scripts/rustdoc_test_gen.rs  |  2 +-
> >> >> >  15 files changed, 236 insertions(+), 28 deletions(-)
> >> >>
> >> >> This would be a lot easier to review if he proc-macro and the call
> >> >> replacement were different patches.
> >> >>
> >> >> Also the `kernel/fmt.rs` file should be a different commit.
> >> >
> >> > Can you help me understand why? The changes you ask to be separated
> >> > would all be in different files, so why would separate commits make it
> >> > easier to review?
> >>
> >> It takes less time to go through the entire patch and give a RB. I can
> >> take smaller time chunks and don't have to get back into the entire
> >> context of the patch when I don't have 30-60min available.
> >
> > Ah, I see what you mean. Yeah, the requirement to RB the entire patch
> > does mean there's a benefit to smaller patches.
> >
> >> In this patch the biggest problem is the rename & addition of new
> >> things, maybe just adding 200 lines in those files could be okay to go
> >> together, see below for more.
> >
> > After implementing your suggestion of re-exporting things from
> > `kernel::fmt` the diffstat is
> >
> > 26 files changed, 253 insertions(+), 51 deletions(-)
> >
> > so I guess I could do all the additions in one patch, but then
> > *everything* else has to go in a single patch together because the
> > formatting macros either want core::fmt::Display or
> > kernel::fmt::Display; they can't work in a halfway state.
>
> 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 prefer to keep things in one commit because the changes are highly
> >> > interdependent. The proc macro doesn't make sense without
> >> > kernel/fmt.rs and kernel/fmt.rs is useless without the proc macro.
> >>
> >> I think that `Adapter`, the custom `Display` and their impl blocks
> >> don't need to be in the same commit as the proc-macro. They are related,
> >> but maybe someone is not well-versed in proc-macros and thus doesn't
> >> want to review that part.
> >
> > Sure, I guess I will split them. But as noted above: changing the
> > formatting macros and all the types' trait implementations has to be a
> > "flag day" change.
>
> See above.
>
> >> >> > +impl_fmt_adapter_forward!(Debug, LowerHex, UpperHex, Octal, Binary, Pointer, LowerExp, UpperExp);
> >> >> > +
> >> >> > +/// A copy of [`fmt::Display`] that allows us to implement it for foreign types.
> >> >> > +///
> >> >> > +/// Types should implement this trait rather than [`fmt::Display`]. Together with the [`Adapter`]
> >> >> > +/// type and [`fmt!`] macro, it allows for formatting foreign types (e.g. types from core) which do
> >> >> > +/// not implement [`fmt::Display`] directly.
> >> >> > +///
> >> >> > +/// [`fmt!`]: crate::prelude::fmt!
> >> >> > +pub trait Display {
> >> >> > +    /// Same as [`fmt::Display::fmt`].
> >> >> > +    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result;
> >> >> > +}
> >> >> > +
> >> >> > +impl<T: ?Sized + Display> Display for &T {
> >> >> > +    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
> >> >> > +        Display::fmt(*self, f)
> >> >> > +    }
> >> >> > +}
> >> >> > +
> >> >> > +impl<T: ?Sized + Display> fmt::Display for Adapter<&T> {
> >> >> > +    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
> >> >> > +        let Self(t) = self;
> >> >> > +        Display::fmt(t, f)
> >> >>
> >> >> Why not `Display::fmt(&self.0, f)`?
> >> >
> >> > I like destructuring because it shows me that there's only one field.
> >> > With `self.0` I don't see that.
> >>
> >> And what is the benefit here?
> >
> > In general the benefit is that the method does not ignore some portion
> > of `Self`. A method that uses `self.0` would not provoke a compiler
> > error in case another field is added, while this form would.
>
> Yeah, but why would that change happen here? And even if it got another
> field, why would that invalidate the impl of `fn fmt`?

I don't know, but I would rather force a person to make that decision
when they add another field rather than assume that such an addition
wouldn't require changes here.