linux-kernel - Re: [PATCH 1/2] rust: Zeroable: allow struct update syntax outside init macros

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Message-ID: <CAH5fLgjJbzAbf5CO3xjHxcThpqju3j0tNdo+QiGupARVmoThYw@mail.gmail.com>
Date: Thu, 28 Nov 2024 15:40:54 +0100
From: Alice Ryhl <aliceryhl@...gle.com>
To: Paolo Bonzini <pbonzini@...hat.com>
Cc: rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org, 
	boqun.feng@...il.com, ojeda@...nel.org, benno.lossin@...ton.me, 
	axboe@...nel.dk, tmgross@...ch.edu, bjorn3_gh@...tonmail.com, 
	gary@...yguo.net, alex.gaynor@...il.com, a.hindborg@...nel.org
Subject: Re: [PATCH 1/2] rust: Zeroable: allow struct update syntax outside
 init macros

On Thu, Nov 28, 2024 at 3:13 PM Paolo Bonzini <pbonzini@...hat.com> wrote:
>
> The Zeroable trait is a marker trait, even though the various init macros
> use a "fake" struct update syntax.  Sometimes, such a struct update
> syntax can be useful even outside the init macros.  Add an associated
> const that returns an all-zero instance of a Zeroable type.
>
> The exact syntax used by the init macros cannot be reproduced without
> forgoing the ability to use Zeroable::ZERO in const context.  However,
> it might not be a good idea to add a fn zeroed() inside the
> Zeroable trait, to avoid confusion with the init::zeroed() function
> and because Zeroable::ZERO is unrelated to the Init and PinInit
> traits.  In other words, let's treat this difference as a
> feature rather than a bug.
>
> The definition of the ZERO constant requires adding a Sized boundary, but
> this is not a problem either because neither slices nor trait objects
> are zeroable.
>
> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>

Slices are zeroable. I know they don't implement the trait, but they
could implement it, and this could be used to implement e.g.:

pub fn write_zero<T: Zeroed + ?Sized>(value: &mut T) {
    memset(0, ...);
}

Alice