Re: [PATCH v3 09/13] rust: init: add `Zeroable` trait and `init::zeroed` function

[Alice Ryhl]

On 3/30/23 00:33, y86-dev@xxxxxxxxxxxxxx wrote:
> From: Benno Lossin <y86-dev@xxxxxxxxxxxxxx>
>
> Add the `Zeroable` trait which marks types that can be initialized by
> writing `0x00` to every byte of the type. Also add the `init::zeroed`
> function that creates an initializer for a `Zeroable` type that writes
> `0x00` to every byte.
>
> Signed-off-by: Benno Lossin <y86-dev@xxxxxxxxxxxxxx>
> ---
> +// SAFETY: `null` pointer is valid.
> +unsafe impl<T: ?Sized> Zeroable for *mut T {}
> +unsafe impl<T: ?Sized> Zeroable for *const T {}

Actually, I just realized that this is not ok for unsized types. When T 
is unsized, the raw pointer is a fat pointer with a vtable, and the 
vtable part is not necessarily zeroable.

However, it would be ok to do it for `*const [T]` since the fat part of 
the pointer is just the length in this case, and a length of zero is fine.

See more here:
https://github.com/Lokathor/bytemuck/blob/8391afa876ba2e99dffb0c991cc7fa775287d106/src/zeroable.rs#L56-L65