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

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

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@garyguo.net>
>> ---
>>  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.

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.

Best,
Gary