[PATCH] rust: bitfield: mark `Debug` impl as `#[inline]`

[PATCH] rust: bitfield: mark `Debug` impl as `#[inline]`

[Gary Guo]

From: Gary Guo <gary@garyguo.net>

A `Debug` impl is for debugging and is normally not used, and therefore
should ideally not be code-generated unless used. However, Rust has no way
of knowing if a dependent crate is going to use the trait impl or not, so
unless it is marked as `#[inline]`, it will be code-generated in the
defining crate (as it is not generic).

Mark the impl generated by bitfield macro `#[inline]`, so they do not stay
in the binary unless used.

This reduces nova-core.o .text by 17% (from 151922 bytes to 125676 bytes).

Signed-off-by: Gary Guo <gary@garyguo.net>
---
 rust/kernel/bitfield.rs | 1 +
 1 file changed, 1 insertion(+)

diff --git a/rust/kernel/bitfield.rs b/rust/kernel/bitfield.rs
index 554a5a2ff0ab..35ede53f2b8e 100644
--- a/rust/kernel/bitfield.rs
+++ b/rust/kernel/bitfield.rs
@@ -535,6 +535,7 @@ const fn [<__with_ $field>](
     // `Debug` implementation.
     (@debug $name:ident { $($field:ident;)* }) => {
         impl ::kernel::fmt::Debug for $name {
+            #[inline]
             fn fmt(&self, f: &mut ::kernel::fmt::Formatter<'_>) -> ::kernel::fmt::Result {
                 f.debug_struct(stringify!($name))
                     .field("<raw>", &::kernel::prelude::fmt!("{:#x}", self.inner))

base-commit: 7f07865bda5f5770d2f7927bc76cb2b4acd46074
-- 
2.54.0

Re: [PATCH] rust: bitfield: mark `Debug` impl as `#[inline]`

[Alice Ryhl]

On Thu, Jun 11, 2026 at 08:05:54PM +0100, Gary Guo wrote:
> From: Gary Guo <gary@garyguo.net>
> 
> A `Debug` impl is for debugging and is normally not used, and therefore
> should ideally not be code-generated unless used. However, Rust has no way
> of knowing if a dependent crate is going to use the trait impl or not, so
> unless it is marked as `#[inline]`, it will be code-generated in the
> defining crate (as it is not generic).
> 
> Mark the impl generated by bitfield macro `#[inline]`, so they do not stay
> in the binary unless used.
> 
> This reduces nova-core.o .text by 17% (from 151922 bytes to 125676 bytes).
> 
> Signed-off-by: Gary Guo <gary@garyguo.net>

Reviewed-by: Alice Ryhl <aliceryhl@google.com>

Re: [PATCH] rust: bitfield: mark `Debug` impl as `#[inline]`

[Alexandre Courbot]

On Fri Jun 12, 2026 at 4:05 AM JST, Gary Guo wrote:
> From: Gary Guo <gary@garyguo.net>
>
> A `Debug` impl is for debugging and is normally not used, and therefore
> should ideally not be code-generated unless used. However, Rust has no way
> of knowing if a dependent crate is going to use the trait impl or not, so
> unless it is marked as `#[inline]`, it will be code-generated in the
> defining crate (as it is not generic).
>
> Mark the impl generated by bitfield macro `#[inline]`, so they do not stay
> in the binary unless used.
>
> This reduces nova-core.o .text by 17% (from 151922 bytes to 125676 bytes).
>
> Signed-off-by: Gary Guo <gary@garyguo.net>

Ouch, well-spotted. Thanks for this.

Acked-by: Alexandre Courbot <acourbot@nvidia.com>

Re: [PATCH] rust: bitfield: mark `Debug` impl as `#[inline]`

[Alexandre Courbot]

On Fri Jun 12, 2026 at 1:40 PM JST, Alexandre Courbot wrote:
> On Fri Jun 12, 2026 at 4:05 AM JST, Gary Guo wrote:
>> From: Gary Guo <gary@garyguo.net>
>>
>> A `Debug` impl is for debugging and is normally not used, and therefore
>> should ideally not be code-generated unless used. However, Rust has no way
>> of knowing if a dependent crate is going to use the trait impl or not, so
>> unless it is marked as `#[inline]`, it will be code-generated in the
>> defining crate (as it is not generic).
>>
>> Mark the impl generated by bitfield macro `#[inline]`, so they do not stay
>> in the binary unless used.
>>
>> This reduces nova-core.o .text by 17% (from 151922 bytes to 125676 bytes).
>>
>> Signed-off-by: Gary Guo <gary@garyguo.net>
>
> Ouch, well-spotted. Thanks for this.
>
> Acked-by: Alexandre Courbot <acourbot@nvidia.com>

Btw, should this be

Fixes: b7b8b4ccdad4 ("rust: extract `bitfield!` macro from `register!`")

?

Re: [PATCH] rust: bitfield: mark `Debug` impl as `#[inline]`

[Miguel Ojeda]

On Thu, Jun 11, 2026 at 9:06 PM Gary Guo <gary@kernel.org> wrote:
>
> This reduces nova-core.o .text by 17% (from 151922 bytes to 125676 bytes).

That's definitely something :)

I wonder if we could lint this for `fmt` bits.

Cheers,
Miguel

Re: [PATCH] rust: bitfield: mark `Debug` impl as `#[inline]`

[Miguel Ojeda]

On Sat, Jun 13, 2026 at 7:43 AM Alexandre Courbot <acourbot@nvidia.com> wrote:
>
> Btw, should this be
>
> Fixes: b7b8b4ccdad4 ("rust: extract `bitfield!` macro from `register!`")
>
> ?

I think it is reasonable either way. One can see it as a "size
improvement" or as a "fix" if it was unintentional to not have the
annotation.

It also depends on the magnitude of the size reduction -- if it were
very low, would we see it as a fix?

Cheers,
Miguel