2592: Add std::ops::Index support for infering r=edwin0cheng a=edwin0cheng
see also #2534
Seem like this can't fix#2534 for this case:
```rust
fn foo3(bar: [usize; 2]) {
let baz = bar[1]; // <--- baz is still unknown ?
println!("{}", baz);
}
```
Co-authored-by: Edwin Cheng <edwin0cheng@gmail.com>
When calling a function, argument-position impl Trait is transparent; same for
return-position impl Trait when inside the function. So in these cases, we need
to represent that type not by `Ty::Opaque`, but by a type variable that can be
unified with whatever flows into there.
If we are expecting a `&Foo` and get a `&something`, when checking the
`something`, we are *expecting* a `Foo`, but we shouldn't try to unify whatever
we get with that expectation, because it could actually be a `&Foo`, and `&&Foo`
coerces to `&Foo`. So this fixes quite a few false type mismatches.
