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

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

On Wed Apr 23, 2025 at 2:03 AM JST, Danilo Krummrich wrote:
>> Well, that turned out to be an interesting rabbit hole.
>> 
>> Leveraging the existing traits seems a bit difficult:
>> 
>> - `ExactSizeIterator` cannot be implemented for adapters that increase the
>>   length of their iterators, because if one of them is already `usize::MAX` long
>>   then the size wouldn't be exact anymore. [1]
>> 
>> - And `TrustedLen` cannot be implemented for adapters that make an iterator
>>   shorter, because if the iterator returns more than `usize::MAX` items (i.e.
>>   has an upper bound set to `None`) then the adapter can't predict the actual
>>   length. [2]
>
> Why is this a problem for the above implementation of Vec::extend()?
>
> I just looked it up and it seems that std [1] does the same thing. Do I miss
> anything?
>
> [1] https://github.com/rust-lang/rust/blob/master/library/alloc/src/vec/spec_extend.rs#L25

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.