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

[Benno Lossin]

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.

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.

> 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.

>> > diff --git a/rust/kernel/fmt.rs b/rust/kernel/fmt.rs
>> > new file mode 100644
>> > index 000000000000..348d16987de6
>> > --- /dev/null
>> > +++ b/rust/kernel/fmt.rs
>> > @@ -0,0 +1,89 @@
>> > +// SPDX-License-Identifier: GPL-2.0
>> > +
>> > +//! Formatting utilities.
>> > +
>> > +use core::fmt;
>>
>> I think we should pub export all types that we are still using from
>> `core::fmt`. For example `Result`, `Formatter`, `Debug` etc.
>>
>> That way I can still use the same pattern of importing `fmt` and then
>> writing
>>
>>     impl fmt::Display for MyType {
>>         fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {}
>>     }
>
> Great idea, done for the next spin. It would be nice to be able to
> lint against references to `core::fmt` outside of kernel/fmt.rs.

I think there was something in clippy that can do that globally and we
could allow that in this file?

>> > +
>> > +/// Internal adapter used to route allow implementations of formatting traits for foreign types.
>> > +///
>> > +/// It is inserted automatically by the [`fmt!`] macro and is not meant to be used directly.
>> > +///
>> > +/// [`fmt!`]: crate::prelude::fmt!
>> > +#[doc(hidden)]
>> > +pub struct Adapter<T>(pub T);
>> > +
>> > +macro_rules! impl_fmt_adapter_forward {
>> > +    ($($trait:ident),* $(,)?) => {
>> > +        $(
>> > +            impl<T: fmt::$trait> fmt::$trait for Adapter<T> {
>> > +                fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
>> > +                    let Self(t) = self;
>> > +                    fmt::$trait::fmt(t, f)
>> > +                }
>> > +            }
>> > +        )*
>> > +    };
>> > +}
>> > +
>> > +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?

>> > +
>> > +    let mut args = TokenStream::from_iter(first_opt);
>> > +    {
>> > +        let mut flush = |args: &mut TokenStream, current: &mut TokenStream| {
>>
>> You don't need to pass `args` as a closure argument, since you always
>> call it with `&mut args`.
>
> This doesn't work because of the borrow checker. If I wrote what you
> suggest, then `args` is mutably borrowed by the closure, which
> prohibits the mutable borrow needed for the .extend() call here:

Ahh right... Well then it's fine.

---
Cheers,
Benno