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

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

[Gary Guo]

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

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@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.

-Onur

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

[Gary Guo]

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