linux-kernel - Re: [PATCH v5] rust: kernel: add support for bits/genmask macros

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Message-ID: <CANiq72=Lu1nwW0-MP-nJ57v8hh56c6V+r=RsnR6g41Em8MvxCg@mail.gmail.com>
Date: Mon, 19 May 2025 19:57:19 +0200
From: Miguel Ojeda <miguel.ojeda.sandonis@...il.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, Fiona Behrens <me@...enk.dev>
Subject: Re: [PATCH v5] rust: kernel: add support for bits/genmask macros

On Tue, May 13, 2025 at 8:52 PM Daniel Almeida
<daniel.almeida@...labora.com> wrote:
>
> I have been staring at this for a little while.
>
> I wonder what is everyone's opinions on an extra set of:
>
> // e.g.: for u32
> const fn const_genmask_u32<const H: u32, const L: u32>() -> u32 {
>   crate::build_assert!(H >= L);
>   ...
> }
>
> ..on top of the current genmask functions we already have?

It seems you want `consteval` from C++ :)

Without having thought about the particular use case, just a quick
note: if you have const generics, then you can use `const { assert!(H
>= L); }` instead.

Nowadays `.unwrap()` on `Option` is `const`, but not `Result`'s. That
would be a way to have a single fallible function that allows users to
decide to unwrap in a const context or use them fallibly for runtime
values. Another is having a custom `const` unwrap for those concrete
`Result`s.

Cheers,
Miguel