Re: [PATCH v8] rust: add new macro for common bitflag operations

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

On Thu Jan 15, 2026 at 12:14 PM GMT, Andreas Hindborg wrote:
> Andreas Hindborg <a.hindborg@kernel.org> writes:
>
>> "Filipe Xavier" <felipeaggger@gmail.com> writes:
>>
>>> We have seen a proliferation of mod_whatever::foo::Flags
>>> being defined with essentially the same implementation
>>> for BitAnd, BitOr, contains and etc.
>>>
>>> This macro aims to bring a solution for this,
>>> allowing to generate these methods for user-defined structs.
>>> With some use cases in KMS and upcoming GPU drivers.
>>>
>>> Link: https://rust-for-linux.zulipchat.com/#narrow/channel/288089-General/topic/We.20really.20need.20a.20common.20.60Flags.60.20type
>>> Suggested-by: Daniel Almeida <daniel.almeida@collabora.com>
>>> Suggested-by: Lyude Paul <lyude@redhat.com>
>>> Reviewed-by: Daniel Almeida <daniel.almeida@collabora.com>
>>> Reviewed-by: Lyude Paul <lyude@redhat.com>
>>> Signed-off-by: Filipe Xavier <felipeaggger@gmail.com>
>>
>> Tested-by: Andreas Hindborg <a.hindborg@kernel.org>
>> Reviewed-by: Andreas Hindborg <a.hindborg@kernel.org>
>>
>>
>> I think it would be useful to add:
>>
>> impl Flags {
>>      unsafe from_raw(value: Repr) -> {
>>      ...
>>      }
>> }
>>
>> impl TryFrom<Repr> for Flag {
>>      // Succeed if `value` is a valid `Flag` enum bit pattern.
>> }
>>
>> impl TryFrom<Repr> for Flags {
>>      // Succeed if `value` is the logical OR of valid enum bit patterns.
>> }
>
> Also bitwise operations on the underlying type such as
>
>   impl BitOrAssign<Flag> for u32 {
>   ...
>   }
>
> Not sure if that is possible with the orphan rule, but it would be nice.
> I was trying to
>
>   lim.features |= request::Feature::Rotational;

This is allowed by orphan rules, in a similar way that you can implement
`From<Local> for Foreign`.

However I don't think this is desirable feature? Why can't a conversion be used
if you're using raw reprs?

I think we should discourage the use of raw integers when bitflags can be used,
adding that (or even `TryFrom`) can be a negative incentive.

Best,
Gary

>
> where `lim: bindings::queue_limits`.
>
>
> Best regards,
> Andreas Hindborg