Re: [PATCH v7 05/10] rust: io: add IoLoc and IoWrite types

[Alexandre Courbot]

On Sat Mar 7, 2026 at 12:10 AM JST, Alexandre Courbot wrote:
> On Fri Mar 6, 2026 at 11:32 PM JST, Alexandre Courbot wrote:
>>> `MyRegArray` here is a type name that is a bitfield and not an array. `foo` returns a
>>> single value and not an array. "at" here is saying that the register is at a
>>> specific location and doesn't really indicate the array nature.
>>>
>>> This is why I insist that I would prefer an explicit location
>>>
>>>     bar.write(REG_ARRAY.at(10), Reg::foo())
>>>
>>> would have no ambiguity whatsoever about user's intent.
>>
>> IIUC `REG_ARRAY` would be a const ZST and `at` a method returning an
>> `AtArray(usize)`? I still have doubts that its generic type could be
>> inferred automatically but it's worth giving it a go.
>>
>> If that works, then I assume fixed register writes would look like
>>
>>     bar.write(FIXED, Reg::foo());
>>
>> Unless we have a specialized `write` variant for them.
>
> So, I tried this. It seemed to work really well - but then I realized
> there is a problem:
>
> The constructor of the `AtArray` needs to check that the index is in
> bounds for the register array. And to do that, it needs... the type of
> the register as a generic parameter. So we would be back to repeating
> the register's type in both arguments, unfortunately. :/

If we add the register type as a generic argument of `AtArray`, we end
up with exactly the already-existing `ArrayRegisterLoc` type, which
performs the bounds-checking and also implements `IoLoc`.

The last remaining question is whether it can be instanciated from a
2-arguments write without repeating the type, and yes it does! So that
part does work quite nicely at least.