Clarify only arrays undergo unsized coercion during dispatch

[tmandry]

A small example to illustrate the difference between array and trait unsized coercions: (playground)

fn check_dyn(t: &Type) {
    check_is_unsize::<Type, dyn Trait>(); // Success
    t.method(); //~ERROR
}

fn check_array(t: &[i32; 3]) {
    check_is_unsize::<[i32; 3], [i32]>(); // Success
    t.len(); // Success
    // ^ Note that len is only defined on [T].
}

fn check_is_unsize<From: ?Sized, To: ?Sized>()
where
    From: std::marker::Unsize<To>,
{}

struct Type;

trait Trait {}

impl dyn Trait {
    fn method(&self) {}
}

impl Trait for Type {}

This came up during a discussion about autoref in zulip: #t-lang/custom-refs > Autoref. Thanks to @BennoLossin for the original example.

The rustc dev guide describes this algorithm similarly to the way the reference does today; it mentions unsized coercions in general, but only uses arrays as an example.

[tmandry]

@BennoLossin and @Nadrieril pointed out that we could remove this step entirely with a simple Deref impl for arrays:

impl<T, const N: usize> Deref for [T; N] {
    type Target = [T];
    fn deref(&self) -> &[T] { &*self }
}

Similarly for DerefMut. I wonder why those impls don't exist.

[rustbot]

This PR was rebased onto a different master commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

[traviscross]

This is true. Thanks for the PR.