Re: [PATCH v3 10/16] rust: io: register: make register have a typed base

[Alexandre Courbot]

On Thu Aug 27, 2026 at 11:32 PM JST, Gary Guo wrote:
> On Thu Aug 27, 2026 at 2:42 PM BST, Alexandre Courbot wrote:
>> On Wed Aug 19, 2026 at 8:09 PM JST, Gary Guo wrote:
>>> Previously `register!` defined registers can be used on any untyped I/O
>>> regions. With all users specifying their desired register type now,
>>> propagate the specified type and restrict I/O access only when type
>>> matches.
>>>
>>> Also, add an `io_project!` example which is enabled by this change.
>>>
>>> Signed-off-by: Gary Guo <gary@xxxxxxxxxxx>
>>> ---
>>>  rust/kernel/io.rs          | 13 +++++++++++
>>>  rust/kernel/io/register.rs | 55 ++++++++++++++++++++++++++++++++--------------
>>>  rust/macros/io/register.rs | 34 ++++++++++++++--------------
>>>  3 files changed, 68 insertions(+), 34 deletions(-)
>>>
>>> diff --git a/rust/kernel/io.rs b/rust/kernel/io.rs
>>> index 84dd876b3407..4542187d6b91 100644
>>> --- a/rust/kernel/io.rs
>>> +++ b/rust/kernel/io.rs
>>> @@ -1692,21 +1692,34 @@ pub fn project_loc<U, L>(self, location: L) -> <T::Backend as IoBackend>::View<'
>>>  /// The syntax is of form `io_project!(io, proj)` where `io` is an expression to a type that
>>>  /// implements [`Io`] and `proj` is a [projection specification](kernel::ptr::project!).
>>>  ///
>>> +/// `io_project!` can also project to subview of registers defined with [`register!`] macro.
>>
>> nit: "to a subview".
>>
>> <...>
>>> +/// Helper function for register alias implementation.
>>> +///
>>> +/// This is used to enforce base matching. Only called during const eval.
>>> +#[doc(hidden)]
>>> +#[inline(always)]
>>> +pub const fn alias_offset<Base: ?Sized, Alias: Register<Base = Base>>() -> usize {
>>> +    Alias::OFFSET
>>> +}
>>> +
>>> +/// Helper function for register element alias implementation.
>>> +///
>>> +/// This is used to enforce base matching and provide bounds checking. Only called during const
>>> +/// eval.
>>> +#[doc(hidden)]
>>> +#[inline(always)]
>>> +pub const fn element_alias_offset<Base: ?Sized, Alias: RegisterArray<Base = Base>>(
>>> +    idx: usize,
>>> +) -> usize {
>>> +    build_assert!(idx < Alias::SIZE);
>>> +    Alias::OFFSET + idx * Alias::STRIDE
>>> +}
>>
>> You could convert the `build_assert!` (always good to eschew) into a
>> `const_assert!` if you turn `idx` into a generic parameter:
>>
>>     pub const fn element_alias_offset<
>>         Base: ?Sized,
>>         Alias: RegisterArray<Base = Base>,
>>         const IDX: usize,
>>     >() -> usize {
>>         crate::const_assert!(IDX < Alias::SIZE);
>>         Alias::OFFSET + IDX * Alias::STRIDE
>>     }
>>
>> You get a better error message, the const parameter is used in a
>> very controlled environment well within the expressive power of const
>> generics, and that function is not public interface anyway.
>>
>> If you don't like it (recent discussions make me think you might not
>> :)), then let's remove the `build_assert!` and keep the `static_assert`
>> emitted by the macro until this patch; it's not as elegant as checking
>> the condition into the same block of code that uses it, but again we are
>> in a controlled environment and static_assert > const_assert >
>> build_assert so this may actually be my preferred solution. You can
>> document the invariant in `element_alias_offset`.
>
> The error message should be identical? This function is only called during const
> evaluation, so even if I replace this build_assert to assert it'll be fine and
> never actually cause linker error or runtime panic.

Indeed, I missed that. Even in this case, it's still better to use
something the compiler enforces can only run at const eval.

`build_assert` is something that in an ideal world shouldn't need to
exist, so let's avoid proliferating its use if we can avoid it.

>
> I wonder if I should add
>
>     assert_in_const_eval()
>
> or maybe
>
>     #[const_eval_only]
>     fn ...
>
> which expands to
>
>     #[inline(always)]
>     fn const ... {
>         assert_in_const_eval()
>     }
>
> so that it's clear to reader that this is a const-eval-only function.

If that doesn't delay the series, sure.