Re: [PATCH 12/12] rust: pin-init: internal: init: add escape hatch for referencing initialized fields

["Benno Lossin" <lossin@kernel.org>]

On Fri Jan 9, 2026 at 2:58 PM CET, Gary Guo wrote:
> On Thu Jan 8, 2026 at 1:50 PM GMT, Benno Lossin wrote:
>> The initializer macro emits mutable references for already initialized
>> fields, which allows modifying or accessing them later in code blocks or
>> when initializing other fields. This behavior results in compiler errors
>> when combining with packed structs, since those do not permit creating
>> references to misaligned fields. For example:
>>
>>     #[repr(C, packed)]
>>     struct Foo {
>>         a: i8,
>>         b: i32,
>>     }
>>
>>     fn main() {
>>         let _ = init!(Foo { a: -42, b: 42 });
>>     }
>>
>> This will lead to an error like this:
>>
>>     error[E0793]: reference to field of packed struct is unaligned
>>       --> tests/ui/compile-fail/init/packed_struct.rs:10:13
>>        |
>>     10 |     let _ = init!(Foo { a: -42, b: 42 });
>>        |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>>        |
>>        = note: this struct is 1-byte aligned, but the type of this field may require higher alignment
>>        = 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 `init` (in Nightly builds, run with -Z macro-backtrace for more info)
>>
>> This was requested by Janne Grunau [1] and will most certainly be used
>> by the kernel when we eventually end up with trying to initialize packed
>> structs.
>>
>> Thus add an initializer attribute `#[disable_initialized_field_access]`
>> that does what the name suggests: do not generate references to already
>> initialized fields.
>>
>> There is space for future work: add yet another attribute which can be
>> applied on fields of initializers that ask for said field to be made
>> accessible. We can add that when the need arises.
>
> An alternative might be checking if the specific field is actually used later
> and only generate references if so. Although, that might be "too much magic"?

Sounds like a good idea, I'll consider for a future series.

Cheers,
Benno