Re: [PATCH v3] rust: alloc: implement `extend` for `Vec`

["Alexandre Courbot" <acourbot@nvidia.com>]

On Wed Apr 23, 2025 at 5:51 PM JST, Alice Ryhl wrote:
> On Wed, Apr 23, 2025 at 10:02:58AM +0900, Alexandre Courbot wrote:
>> The problem I see is that if you try and do something like:
>> 
>>   vec.extend((0..10).into_iter().skip(2));
>> 
>> with the standard library, then the use of `skip` will remove the
>> `TrustedLen` implementation from the resulting iterator and
>> `extend_desugared` will be called instead of `extend_trusted`, which
>> could add some unwanted (and unexpected) overhead.
>> 
>> If we want an implementation of `extend` as simple as "confidently
>> increase the length of the vector and copy the new items into it, once",
>> then we need a trait that can be implemented on both shrinking and
>> extending adapters. Anything else and we might trick the caller into a
>> code path less efficient than expected (i.e. my original version, which
>> generates more core even for the obvious cases that are `extend_with`
>> and `extend_from_slice`). Or if we rely on `TrustedLen` solely in the
>> kernel, then `extend` could not be called at all with this particular
>> iterator.
>> 
>> There is also the fact that `TrustedLen` is behind a nightly feature,
>> which I guess is another obstacle for using it.
>
> The stdlib alloc crate relies on specialization to speed up methods
> related to iterators. We can't use specialization, so losing these
> optimizations is simply a cost of not using the upstream alloc library
> that we have to accept.

Yeah I was surprised to see

  impl<T, I, A: Allocator> SpecExtend<T, I> for Vec<T, A>
  where
      I: Iterator<Item = T>

and

  impl<T, I, A: Allocator> SpecExtend<T, I> for Vec<T, A>
  where
      I: TrustedLen<Item = T>

in the standard library, which clearly looks like an overlap. Didn't
know it was relying on a non-standard feature.

That's going to limit what we can do in the kernel, but nonetheless if
we can support only the cases that can be optimized I think we would
have our bases covered.