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

From: Paolo Bonzini
Date: Thu Nov 28 2024 - 11:44:06 EST


On 11/28/24 15:40, Alice Ryhl wrote:
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@xxxxxxxxxx>

Slices are zeroable. I know they don't implement the trait,

Right, I should have used the uppercase "Zeroable" for clarity.

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, ...);
}

Yeah, that would be I think

pub fn write_zero<T: Zeroable + ?Sized>(value: &mut T) {
unsafe {
ptr::write_bytes((value as *mut T).cast::<u8>(), 0,
std::mem::size_of_val(value))
}
}

? And it works for both sized values and slices. If Zeroable is limited to sized types, I guess you could still do:

pub fn write_zero_slice<T: Zeroable>(value: &mut [T]) {
ptr::write_bytes(value.as_mut_ptr(), 0, value.len())
}

So the question is whether the ZERO constant is worthwhile enough, to justify the limitation of the Sized bound (e.g. having separate write_zero and write_zero_slice in the future).

Thanks,

Paolo