`format_args!` doesn't support none Sized types so we should just pass it the references to `left_val` and `right_val`.
The following works:
```rust
assert!([1, 2, 3][..] == vec![1, 2, 3][..])
```
So I would expect this to as well:
```rust
assert_eq!([1, 2, 3][..], vec![1, 2, 3][..])
```
But it fails with "error: the trait `core::marker::Sized` is not implemented for the type `[_]` [E0277]"
I don't know if this change will have any nasty side effects I don't understand.
0bd708461b