Re: [PATCH v6 2/6] rust: str: implement `Index` for `BStr`

[Andreas Hindborg <a.hindborg@kernel.org>]

"Gary Guo" <gary@garyguo.net> writes:

> On Tue, 11 Feb 2025 21:24:44 +0100
> Andreas Hindborg <a.hindborg@kernel.org> wrote:
>
>> "Gary Guo" <gary@garyguo.net> writes:
>>
>> > On Tue, 11 Feb 2025 16:57:36 +0100
>> > Andreas Hindborg <a.hindborg@kernel.org> wrote:
>> >
>> >> The `Index` implementation on `BStr` was lost when we switched `BStr` from
>> >> a type alias of `[u8]` to a newtype. This patch adds back `Index` by
>> >> implementing `Index` for `BStr` when `Index` would be implemented for
>> >> `[u8]`.
>> >>
>> >> Signed-off-by: Andreas Hindborg <a.hindborg@kernel.org>
>> >> ---
>> >>  rust/kernel/str.rs | 11 +++++++++++
>> >>  1 file changed, 11 insertions(+)
>> >>
>> >> diff --git a/rust/kernel/str.rs b/rust/kernel/str.rs
>> >> index 002dcddf7c768..1eb945bed77d6 100644
>> >> --- a/rust/kernel/str.rs
>> >> +++ b/rust/kernel/str.rs
>> >> @@ -114,6 +114,17 @@ fn eq(&self, other: &Self) -> bool {
>> >>      }
>> >>  }
>> >>
>> >> +impl<Idx> Index<Idx> for BStr
>> >> +where
>> >> +    Idx: core::slice::SliceIndex<[u8], Output = [u8]>,
>> >
>> > I think I'd prefer
>> >
>> > 	[T]: Index<Idx>,
>>
>> Is that equivalent?
>
> Sorry, I meant `[u8]: Index<Idx>`. This makes more semantic sense that
> "what ever can index a byte slice, it can also index BStr". This is
> also how our CStr and the array primitive type implements its Index
> operation.
>
> They should be equivalent as libcore does
>
> 	impl<T, I> Index<I> for [T] where I: SliceIndex<[T]> { ... }
>

What I originally wrote is `Idx` must be usable as an index for `[u8]`,
yielding `[u8]` when indexing.

The new one you suggest, I parse as `[u8]` should be indexable by `Idx`.
This is less info. The compiler will also complain about the missing info:

error[E0308]: mismatched types
   --> /home/aeh/src/linux-rust/module-params/rust/kernel/str.rs:141:26
    |
141 |         BStr::from_bytes(&self.0[index])
    |         ---------------- ^^^^^^^^^^^^^^ expected `&[u8]`, found `&<[u8] as Index<Idx>>::Output`
    |         |
    |         arguments to this function are incorrect
    |
    = note: expected reference `&[u8]`
               found reference `&<[u8] as Index<Idx>>::Output`
    = help: consider constraining the associated type `<[u8] as Index<Idx>>::Output` to `[u8]`

If I constrain the output it's all fine again:

    [u8]: Index<Idx, Output = [u8]>,


But as I said, I don't think it matters which direction we put this?


Best regards,
Andreas Hindborg