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

[Alice Ryhl <aliceryhl@google.com>]

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.

Alice