Add as_slice() to slice::IterMut and vec::Drain

[cuviper]

In indexmap-rs/indexmap#88, we found that there was no easy way to implement
Debug for our IterMut and Drain iterators. Those are built on
slice::IterMut and vec::Drain, which implement Debug themselves,
but have no other way to access their data. With a new as_slice()
method, we can read the data and customize its presentation.

[rust-highfive]

r? @dtolnay

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[cuviper]

FWIW, these could both support as_mut_slice() too, but I have no immediate use for that. I'm willing to add that for completeness if desired.

[dtolnay]

Thanks! This looks good to me. Please open a tracking issue and add the number.

[cuviper]

OK, I've hopefully addressed @Centril's feedback, and added tracking issue #58957.

[dtolnay]

@bors r+

[bors]

📌 Commit e478cad has been approved by dtolnay

[dtolnay]

FYI @rust-lang/libs: two unstable methods:

• https://doc.rust-lang.org/nightly/std/slice/struct.IterMut.html#method.as_slice
• https://doc.rust-lang.org/nightly/std/vec/struct.Drain.html#method.as_slice

impl<'a, T> slice::IterMut<'a, T> {
    pub fn as_slice(&self) -> &[T];
}

impl<'a, T> vec::Drain<'a, T> {
    pub fn as_slice(&self) -> &[T];
}

Tracking issue: #58957.