librustc: Allow coercions through arrays.

[luqmana]

Fixes #11205.

[sfackler]

😄

[sfackler]

Needs a rebase (and review)

[sfackler]

ping @pcwalton

[pcwalton]

I'm confused as to how this works. Wouldn't trans have to loop through the array to do this properly?

This seems a little too magical…

[luqmana]

@pcwalton All it's doing is adding an auto adjustment for each element.

If we have something like:

trait Foo {}
impl Foo for int {}

let x: [&Foo, ..2] = [&1i, &2i];

When we typeck [&1i, &2i] we'll have an expected type of [&Foo, ..2] before we'd just fail since the types didn't match up but now instead it'll try and coerce each element to &Foo.