Nouveau - Sep 2025 - [PATCH v4 6/6] rust: bitfield: Use 'as' operator for setter type conversion

On Mon Sep 29, 2025 at 10:59 PM JST, Miguel Ojeda wrote:
> On Sat, Sep 20, 2025 at 8:23?PM Joel Fernandes <joelagnelf at
nvidia.com> wrote:
>>
>> The bitfield macro's setter currently uses the From trait for type
>> conversion, which is overly restrictive and prevents use cases such as
>> narrowing conversions (e.g., u32 storage size to u8 field size) which
>> aren't supported by From.
>
> Being restrictive is a good thing
On that note, I have been wondering whether we should not push the
restriction up to having bounded primitive types with only a set number
of bits valid, e.g. `bound_u8::<2>` is guaranteed to only contain values
in the range `0..=3`.

Getters and setters would use these types depending on the number of
bits of the field, meaning that a caller would have to validate the
value they want to write if it does not implement e.g.
`Into<bound_u8<2>>`.

A bit radical maybe, but correcness ensues. :)

On Mon, Sep 29, 2025 at 11:44:56PM +0900, Alexandre Courbot
wrote:
> On Mon Sep 29, 2025 at 10:59 PM JST, Miguel Ojeda wrote:
> > On Sat, Sep 20, 2025 at 8:23?PM Joel Fernandes <joelagnelf at
nvidia.com> wrote:
> >>
> >> The bitfield macro's setter currently uses the From trait for
type
> >> conversion, which is overly restrictive and prevents use cases
such as
> >> narrowing conversions (e.g., u32 storage size to u8 field size)
which
> >> aren't supported by From.
> >
> > Being restrictive is a good thing
> 
> On that note, I have been wondering whether we should not push the
> restriction up to having bounded primitive types with only a set number
> of bits valid, e.g. `bound_u8::<2>` is guaranteed to only contain
values
> in the range `0..=3`.
> 
> Getters and setters would use these types depending on the number of
> bits of the field, meaning that a caller would have to validate the
> value they want to write if it does not implement e.g.
> `Into<bound_u8<2>>`.
> 
> A bit radical maybe, but correcness ensues. :)
In my v5, I will be rejecting setter inputs that are out of range. Do we have a
usecase where we want the inputs to exceed the bit width range? If not, let
us keep the API simple. I should probably post v5 today so we have a full
discussion on the same and get alignment from everyone.

Thanks