Re: [PATCH v3 17/25] rust: alloc: implement `collect` for `IntoIter`

[Alice Ryhl <aliceryhl@google.com>]

On Thu, Aug 1, 2024 at 2:08 AM Danilo Krummrich <dakr@kernel.org> wrote:
>
> Currently, we can't implement `FromIterator`. There are a couple of
> issues with this trait in the kernel, namely:
>
>   - Rust's specialization feature is unstable. This prevents us to
>     optimze for the special case where `I::IntoIter` equals `Vec`'s
>     `IntoIter` type.
>   - We also can't use `I::IntoIter`'s type ID either to work around this,
>     since `FromIterator` doesn't require this type to be `'static`.
>   - `FromIterator::from_iter` does return `Self` instead of
>     `Result<Self, AllocError>`, hence we can't properly handle allocation
>     failures.
>   - Neither `Iterator::collect` nor `FromIterator::from_iter` can handle
>     additional allocation flags.
>
> Instead, provide `IntoIter::collect`, such that we can at least convert
> `IntoIter` into a `Vec` again.
>
> Signed-off-by: Danilo Krummrich <dakr@kernel.org>

I'm not convinced a collect implementation specific to IntoIter is necessary?

> +
> +        // SAFETY: `buf` points to the start of the backing buffer and `len` is guaranteed to be
> +        // smaller than `cap`. Depending on `alloc` this operation may shrink the buffer or leaves
> +        // it as it is.
> +        ptr = match unsafe { A::realloc(Some(buf.cast()), layout, flags) } {

Why would you shrink it? You can just keep the capacity.

Alice