Re: [PATCH 1/2] rust: num: casts: replace const type narrowing methods with a macro

["Gary Guo" <gary@garyguo.net>]

On Tue Aug 25, 2026 at 3:26 PM BST, Alexandre Courbot wrote:
> On Tue Aug 25, 2026 at 9:01 PM JST, Gary Guo wrote:
>> On Tue Aug 25, 2026 at 9:25 AM BST, Miguel Ojeda wrote:
>>> What I wouldn't want is a raw `as`, because the point of the saga we
>>> started a long time ago is to introduce better tools that allow us to
>>> get rid of the almighty `as` into weaker (i.e. safer) options, even if
>>> some uses of `as` may be "obviously right".
>>
>> I think that is rather a linting issue, not something that warrants extra code
>> in kernel. We have been requesting some extra clippy features and I think that
>> is the correct way to go, not add a ton of methods and macros. Yes, it wasn't
>> moving on clippy end, but I could add a feature to klint instead?
>>
>> Do you think we still need all these extra function and macros if we
>> can get clippy (or klint) to enforce CAST comments?
>>
>> I can imagine the following rules that would practically solve all the footgun
>> of `as` numerical casts without having to use awkward syntax:
>>
>> * widening casts are allowed
>> * narrowing casts is disallowed unless CAST comment exists, except where its
>>   value is constant and truncation does not happen.
>
> These rules classify casts by width, but the footguns really are about
> which values are actually being converted.
>
> In particular for value narrowing we still end up with CAST comments,
> whose existence a lint can check, but not their correctness (for
> instance, a bindgen-provided constant that changes in a breaking way).
> `const_as!` lets us drop them altogether.

The rule says "except where its value is constant and truncation does not
happen".

So I'd imagine just writing

    bindings::FOO as u32

and *NOT* have CAST comment, and a warning being generated if truncation
happens.

Best,
Gary