linux-kernel - Re: [PATCH 2/2] rust: add `const

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Message-Id: <DG88F5B5IAA2.1XXMBH517NDBW@garyguo.net>
Date: Fri, 06 Feb 2026 22:28:40 +0000
From: "Gary Guo" <gary@...yguo.net>
To: "John Hubbard" <jhubbard@...dia.com>, "Gary Guo" <gary@...yguo.net>,
 "Miguel Ojeda" <ojeda@...nel.org>, "Boqun Feng" <boqun@...nel.org>,
 Björn Roy Baron <bjorn3_gh@...tonmail.com>, "Benno Lossin"
 <lossin@...nel.org>, "Andreas Hindborg" <a.hindborg@...nel.org>, "Alice
 Ryhl" <aliceryhl@...gle.com>, "Trevor Gross" <tmgross@...ch.edu>, "Danilo
 Krummrich" <dakr@...nel.org>, "Alexandre Courbot" <acourbot@...dia.com>,
 "Yury Norov" <yury.norov@...il.com>, "Nathan Chancellor"
 <nathan@...nel.org>, "Nicolas Schier" <nsc@...nel.org>
Cc: <linux-kernel@...r.kernel.org>, <rust-for-linux@...r.kernel.org>,
 <linux-kbuild@...r.kernel.org>
Subject: Re: [PATCH 2/2] rust: add `const_assert!` macro

On Fri Feb 6, 2026 at 10:21 PM GMT, John Hubbard wrote:
> On 2/6/26 9:12 AM, Gary Guo wrote:
>> From: Gary Guo <gary@...yguo.net>
>> 
>> The macro is a more powerful version of `static_assert!` for use inside
>> function contexts. This is powered by inline consts, so enable the feature
>> for old compiler versions that does not have it stably.
>
> This is nice. A nit below about documentation.
>
>> 
>> The `build_assert!` doc is refined to recommend it where possible.
>
> Another piece of the puzzle...see below to continue the treasure
> hunt.
>
>> 
>> While it is possible already to write `const { assert!(...) }`, this
>> provides a short hand that is more uniform with other assertions. It also
>> formats nicer with rustfmt where it will not be formatted into multiple
>> lines.
>> 
>> Two users that would route via the Rust tree are converted.
>> 
>> Signed-off-by: Gary Guo <gary@...yguo.net>
>> ---
>>  rust/kernel/build_assert.rs | 55 +++++++++++++++++++++++++++++++++----
>>  rust/kernel/num/bounded.rs  | 24 ++++++----------
>>  rust/kernel/prelude.rs      |  2 +-
>>  rust/kernel/ptr.rs          | 18 ++++++------
>>  scripts/Makefile.build      |  3 +-
>>  5 files changed, 71 insertions(+), 31 deletions(-)
>> 
>> diff --git a/rust/kernel/build_assert.rs b/rust/kernel/build_assert.rs
>> index d464494d430a..e40f0227e1ef 100644
>> --- a/rust/kernel/build_assert.rs
>> +++ b/rust/kernel/build_assert.rs
>> @@ -41,6 +41,45 @@ macro_rules! static_assert {
>>      };
>>  }
>>  
>> +/// Assertion during constant evaluation.
>> +///
>> +/// This is a more powerful version of `static_assert` that can refer to generics inside functions
>> +/// or implementation blocks. However, it also have a limitation where it can only appear in places
>> +/// where statements can appear; for example, you cannot use it as an item in the module.
>> +///
>> +/// [`static_assert!`] should be preferred where possible.
>
> Over what? There are 3 different assertion types. If it is actually
> possible to list a clear order of preference, then let's list all
> 3 in order, rather than having a scattering around of "A is 
> better than B", "C is sometimes worse than A", and then the
> reader has to play treasure hunt to work it out. haha :)

I am trying to avoid listing all assertion macros in all assertions macros,
that's just going to be duplications.

How about remove `#[doc(hidden)]` on `mod build_assert` and enumerate all
assertions in the module documentation and give them rankings there?

Best,
Gary