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Date:   Fri, 31 Mar 2023 00:40:34 +0200
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Subject: Re: [PATCH v3 09/13] rust: init: add `Zeroable` trait and
 `init::zeroed` function
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To:     y86-dev@protonmail.com, Miguel Ojeda <ojeda@kernel.org>,
        Alex Gaynor <alex.gaynor@gmail.com>,
        Wedson Almeida Filho <wedsonaf@gmail.com>,
        Boqun Feng <boqun.feng@gmail.com>, Gary Guo <gary@garyguo.net>,
        =?UTF-8?Q?Bj=c3=b6rn_Roy_Baron?= <bjorn3_gh@protonmail.com>
Cc:     rust-for-linux@vger.kernel.org, linux-kernel@vger.kernel.org,
        patches@lists.linux.dev
References: <20230329223239.138757-10-y86-dev@protonmail.com>
From:   Alice Ryhl <alice@ryhl.io>
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On 3/30/23 00:33, y86-dev@protonmail.com wrote:
> From: Benno Lossin <y86-dev@protonmail.com>
> 
> 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@protonmail.com>
> ---
> +// 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