Improve upvar analysis for deref of child capture
Two fixes to the heuristic I implemented in #123660. As I noted in the code:
> Luckily, if this function is not correct, then the program is not unsound, since we still borrowck and validate the choices made from this function -- the only side-effect is that the user may receive unnecessary borrowck errors.
This indeed fixes unnecessary borrowck errors.
r? oli-obk
---
The heuristic is only valid if we deref a `&T`, not a `&mut T` or `Box<T>`, so make sure to check the type. This fixes:
```rust
struct Foo { precise: i32 }
fn mut_ref_inside_mut(f: &mut Foo) {
let x: impl AsyncFn() = async move || {
let y = &f.precise;
};
}
```
Since the capture from `f` to `&f.precise` needs to be treated as a lending borrow from the parent coroutine-closure to the child coroutine.
---
The heuristic is also valid if *any* deref projection in the child capture's projections is a `&T`, but we were only looking at the last one. This ensures that this function is considered not to be lending:
```rust
struct Foo { precise: i32 }
fn ref_inside_mut(f: &mut &Foo) {
let x: impl Fn() -> _ = async move || {
let y = &f.precise;
};
}
```
(Specifically, checking that `impl Fn() -> _` is satisfied is exercising that the coroutine is not considered to be lending.)
e9f6e01b3a