regression: array methods unstable feature use

[Mark-Simulacrum]

Several crates are erroring on as_slice/as_mut_slice being added to the array types (unstably).

• https://crater-reports.s3.amazonaws.com/beta-1.48-1/beta-2020-10-09/reg/unicorn_hat_hd-0.2.1/log.txt
• https://crater-reports.s3.amazonaws.com/beta-1.48-1/beta-2020-10-09/reg/rgb-0.8.25/log.txt

I suspect this is because method resolution is now seeing the method on arrays, whereas previously we derefed to &mut [T] and then there was a trait impl on that? There was another similar issue recently, but I forget what the resolution there was.

cc #76118

[camelid]

Relevant wg-prioritization discussion.

[eddyb]

Shouldn't #48552 prevent this from being an issue at all? Do we need to extend it in some way?
cc @nikomatsakis @pnkfelix