linux-kernel - Re: [PATCH v6] rust: kernel: add support for bits/genmask macros

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Message-Id: <DAO1WDWJTT04.1P4XT0W2XHPNW@nvidia.com>
Date: Tue, 17 Jun 2025 00:08:52 +0900
From: "Alexandre Courbot" <acourbot@...dia.com>
To: "Daniel Almeida" <daniel.almeida@...labora.com>
Cc: "Miguel Ojeda" <ojeda@...nel.org>, "Alex Gaynor"
 <alex.gaynor@...il.com>, "Boqun Feng" <boqun.feng@...il.com>, "Gary Guo"
 <gary@...yguo.net>, Björn Roy Baron
 <bjorn3_gh@...tonmail.com>, "Benno Lossin" <benno.lossin@...ton.me>,
 "Andreas Hindborg" <a.hindborg@...nel.org>, "Alice Ryhl"
 <aliceryhl@...gle.com>, "Trevor Gross" <tmgross@...ch.edu>, "Danilo
 Krummrich" <dakr@...nel.org>, <linux-kernel@...r.kernel.org>,
 <rust-for-linux@...r.kernel.org>
Subject: Re: [PATCH v6] rust: kernel: add support for bits/genmask macros

On Mon Jun 16, 2025 at 11:14 PM JST, Daniel Almeida wrote:
>>> +macro_rules! impl_bit_fn {
>>> +    (
>>> +        $checked_name:ident, $unbounded_name:ident, $const_name:ident, $ty:ty
>>> +    ) => {
>>> +        /// Computes `1 << n` if `n` is in bounds, i.e.: if `n` is smaller than
>>> +        /// the maximum number of bits supported by the type.
>>> +        ///
>>> +        /// Returns [`None`] otherwise.
>>> +        #[inline]
>>> +        pub fn $checked_name(n: u32) -> Option<$ty> {
>>> +            (1 as $ty) .checked_shl(n)
>>> +        }
>>> +
>>> +        /// Computes `1 << n` if `n` is in bounds, i.e.: if `n` is smaller than
>>> +        /// the maximum number of bits supported by the type.
>>> +        ///
>>> +        /// Returns `0` otherwise.
>>> +        ///
>>> +        /// This is a convenience, as [`Option::unwrap_or`] cannot be used in
>>> +        /// const-context.
>>> +        #[inline]
>>> +        pub fn $unbounded_name(n: u32) -> $ty {
>>> +            match $checked_name(n) {
>>> +                Some(v) => v,
>>> +                None => 0,
>>> +            }
>> 
>> This could more succintly be `$checked_name(n).unwrap_or(0)` (same
>> remark for `$genmask_unbounded` below).
>> 
>
> Wait, I just realized that $unbounded_name is not ‘const fn’, so we don’t need this function at all?
>
> Users can simply do `unwrap_or` on their own.

Agreed, we can probably drop this.

>> 
>> ... or we make the methods generic against `RangeBounds` and allow both
>> `Range` and `RangeInclusive` to be used. But I'm concerned that callers
>> might use `0..1` thinking it is inclusive while it is not.
>> 
>> Thoughts?
>
> I don't think we can do what you suggested here. I assume that we'd have to
> rely on [0] and friends, and these are not const fn, so they can’t be used in
> the const version of genmask.

You are right, this cannot be used here. It's not a big loss, limiting
the API to inclusive ranges as discussed on the other thread might
actually end up being safer than having two options.