Re: [PATCH] rust: pin-init: add references to previously initialized fields

[Benno Lossin]

On Wed Dec 3, 2025 at 11:05 PM CET, Janne Grunau wrote:
> On Fri, Sep 05, 2025 at 04:00:46PM +0200, Benno Lossin wrote:
>> After initializing a field in an initializer macro, create a variable
>> holding a reference that points at that field. The type is either
>> `Pin<&mut T>` or `&mut T` depending on the field's structural pinning
>> kind.
>
> just as a heads up: creating references broke part of the agx firmware
> init structs which uses a `#[repr(C, packed)]` struct as field in
> another struct. This fails with
>
> | error[E0793]: reference to packed field is unaligned
> |    --> ../drivers/gpu/drm/asahi/initdata.rs:722:28
> |     |
> | 722 |                       sub <- try_init!(raw::GlobalsSub::ver {
> |     |  ____________________________^
> | 723 | |                         unk_54: cfg.global_unk_54,
> | 724 | |                         unk_56: 40,
> | 725 | |                         unk_58: 0xffff,
> | ...   |
> | 731 | |                         ..Zeroable::init_zeroed()
> | 732 | |                     }),
> |     | |______________________^
> |     |
> |     = note: packed structs are only aligned by one byte, and many modern architectures penalize unaligned field accesses
> |     = note: creating a misaligned reference is undefined behavior (even if that reference is never dereferenced)
> |     = help: copy the field contents to a local variable, or replace the reference with a raw pointer and use `read_unaligned`/`write_unaligned` (loads and stores via `*p` must be properly aligned even when using raw pointers)
> |     = note: this error originates in the macro `$crate::__init_internal` which comes from the expansion of the macro `try_init` (in Nightly builds, run with -Z macro-backtrace for more info)
>
> This was easy enough to work around, I don't see how this embedding of a
> `#[repr(C, packed)]` struct was necessary or at least helpful. The code
> is not expected to be included in the upstream driver so it no worth
> spending effort on this.
>
> I don't think it's likely that anyone else will run into this but I
> thought I mention it at least.
>
> The asahi driver also ran into the discussed variable shadowing issue (a
> variable was used to initialize a field of the same name and was later
> used to initialize another field). This was trivially fixed by renaming
> the variable.

Thanks for the report, I expected the latter kind of error, but was not
aware of the packed struct issue. If anyone needs a proper workaround
from pin-init, let me know.

Cheers,
Benno