linux-kernel - Re: [PATCH v5 04/23] rust: add new `num` module with `PowerOfTwo` type

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Message-Id: <DANPCSU5A3AU.180DZZ9UXU4TY@nvidia.com>
Date: Mon, 16 Jun 2025 14:19:04 +0900
From: "Alexandre Courbot" <acourbot@...dia.com>
To: "Boqun Feng" <boqun.feng@...il.com>
Cc: "Miguel Ojeda" <ojeda@...nel.org>, "Alex Gaynor"
 <alex.gaynor@...il.com>, "Gary Guo" <gary@...yguo.net>,
 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>,
 "David Airlie" <airlied@...il.com>, "Simona Vetter" <simona@...ll.ch>,
 "Maarten Lankhorst" <maarten.lankhorst@...ux.intel.com>, "Maxime Ripard"
 <mripard@...nel.org>, "Thomas Zimmermann" <tzimmermann@...e.de>, "Benno
 Lossin" <lossin@...nel.org>, "John Hubbard" <jhubbard@...dia.com>, "Ben
 Skeggs" <bskeggs@...dia.com>, "Joel Fernandes" <joelagnelf@...dia.com>,
 "Timur Tabi" <ttabi@...dia.com>, "Alistair Popple" <apopple@...dia.com>,
 <linux-kernel@...r.kernel.org>, <rust-for-linux@...r.kernel.org>,
 <nouveau@...ts.freedesktop.org>, <dri-devel@...ts.freedesktop.org>
Subject: Re: [PATCH v5 04/23] rust: add new `num` module with `PowerOfTwo`
 type

On Sun Jun 15, 2025 at 2:31 AM JST, Boqun Feng wrote:
> On Thu, Jun 12, 2025 at 11:01:32PM +0900, Alexandre Courbot wrote:
> [...]
>> +/// An unsigned integer which is guaranteed to be a power of 2.
>> +#[derive(Debug, Clone, Copy)]
>> +#[repr(transparent)]
>> +pub struct PowerOfTwo<T>(T);
>> +
> [...]
>> +impl<T> Deref for PowerOfTwo<T> {
>
> Why do we need `impl Deref` (and the `impl Borrow` below)? A similar
> concept `NonZero` in std doesn't impl them as well.

I wanted to be exhaustive but you're right, we don't really need these
implementations (especially if `NonZero` doesn't provide them either).

>
>> +    type Target = T;
>> +
>> +    fn deref(&self) -> &Self::Target {
>> +        &self.0
>> +    }
>> +}
>> +
>> +impl<T> PartialEq for PowerOfTwo<T>
>
> Any reason you want to impl these manually instead of deriving? For
> `NonZero`, the std wants to impl these traits only for
> `ZeroablePrimitive` types, but we don't have a similar trait here.

Deriving works perfectly well! :) Thanks for pointing this out.