linux-kernel - Re: [PATCH v3 13/15] rust: pin-init: internal: init: add escape hatch for referencing initialized fields

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Message-Id: <DFOJJTV5PAXE.3E5P6MHNX2AC0@garyguo.net>
Date: Wed, 14 Jan 2026 18:58:40 +0000
From: "Gary Guo" <gary@...yguo.net>
To: "Benno Lossin" <lossin@...nel.org>, "Gary Guo" <gary@...yguo.net>,
 "Miguel Ojeda" <ojeda@...nel.org>, "Boqun Feng" <boqun.feng@...il.com>,
 Björn Roy Baron <bjorn3_gh@...tonmail.com>, "Andreas
 Hindborg" <a.hindborg@...nel.org>, "Alice Ryhl" <aliceryhl@...gle.com>,
 "Trevor Gross" <tmgross@...ch.edu>, "Danilo Krummrich" <dakr@...nel.org>
Cc: "Janne Grunau" <j@...nau.net>, <rust-for-linux@...r.kernel.org>,
 <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 13/15] rust: pin-init: internal: init: add escape
 hatch for referencing initialized fields

On Wed Jan 14, 2026 at 6:18 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.
> 
> Requested-by: Janne Grunau <j@...nau.net>
> Link: https://lore.kernel.org/all/20251206170214.GE1097212@robin.jannau.net [1]
> Tested-by: Andreas Hindborg <a.hindborg@...nel.org>
> Signed-off-by: Benno Lossin <lossin@...nel.org>

Reviewed-by: Gary Guo <gary@...yguo.net>

> ---
> Changes in v3: none
> Changes in v2:
> * silence clippy warning
> ---
>  rust/pin-init/internal/src/init.rs | 76 +++++++++++++++++++++---------
>  1 file changed, 53 insertions(+), 23 deletions(-)