Re: [PATCH] rust: pin_init: internal: use `loop {}` to produce never value

[Onur Özkan]

On Fri, 08 May 2026 16:29:49 +0100
Gary Guo <gary@xxxxxxxxxx> wrote:

> From: Gary Guo <gary@xxxxxxxxxxx>
> 
> In the `init!`/`pin_init!` macros, we rely on a trick that assigns never
> (`!`) values to all mentioned fields in never-executed code to let the
> compiler check that all fields have been initialized.
> 
> Currently we use `::core::panic!()` to produce this value, but before Rust
> 1.91.0, it creates outlined `panic_cold_explicit` functions which do not
> get removed by the optimizer, thus leaving dead code behind in the binary.
> This has been fixed by [1], which lands in Rust 1.91.0+, higher than the
> kernel minimum version 1.85.0.
> 
> This causes ~200 dead `panic_cold_explicit` instances being included in the
> binary, with ~90 of them from nova-core's usage of pin-init.
> 
> Work around the issue by using `loop {}` which creates the never value
> without macro expansion or function call at all. All instances of
> `panic_cold_explicit` outside libcore are removed by this change in my
> kernel build.
> 
> Link: https://github.com/rust-lang/rust/pull/145304 [1]
> Signed-off-by: Gary Guo <gary@xxxxxxxxxxx>
> ---
>  rust/pin-init/internal/src/init.rs | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/rust/pin-init/internal/src/init.rs b/rust/pin-init/internal/src/init.rs
> index fbc8286263b2..b1cb2e53ee6f 100644
> --- a/rust/pin-init/internal/src/init.rs
> +++ b/rust/pin-init/internal/src/init.rs
> @@ -338,7 +338,7 @@ fn make_field_check(
>              ::core::ptr::write(slot, #path {
>                  #(
>                      #(#field_attrs)*
> -                    #field_name: ::core::panic!(),
> +                    #field_name: loop {},
>                  )*
>                  #zeroing_trailer
>              })
> 
> -- 
> 2.51.2
> 

I don't know where this patch is based on (on current upstream there's no
zeroing_trailer thing), but at least on the upstream side there's another use of
::core::panic in the same module.

-Onur