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

[Benno Lossin <benno.lossin@proton.me>]

On 18.02.25 12:14, Andreas Hindborg wrote:
> "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?

I think it's better to depend on `Index` compared to `SliceIndex`.

---
Cheers,
Benno