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

["Gary Guo" <gary@garyguo.net>]

On Sat May 9, 2026 at 7:48 AM BST, Onur Özkan wrote:
> On Fri, 08 May 2026 16:29:49 +0100
> Gary Guo <gary@kernel.org> wrote:
>
>> From: Gary Guo <gary@garyguo.net>
>> 
>> 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@garyguo.net>
>> ---
>>  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.

zeroing_trailer is introduced in
https://lore.kernel.org/rust-for-linux/20260427-pin-init-fix-v3-0-496a699674dd@garyguo.net/
which is currently in rust-fixes.

Best,
Gary