…fs::read_dir

Fixes an issue where we did not nest tables correctly when resolving
associated types in formal argument/return type positions

Performance shouldn't be impacted (see [1] for a perf run) and this
should allow us to catch more bugs, e.g. [2] and [3].

[1]: rust-lang#64262
[2]: rust-lang#64250
[3]: rust-lang#57298

...since the code is literally the same and does the same thing.

This is not perfectly correct as `#[attr] (5)` will still not lint, but
it does seem good enough, in particular as the parentheses in that case
are not unambiguously incorrect.

Document platform-specific behavior of the iterator returned by std::fs::read_dir

… r=varkor

save-analysis: Nest typeck tables when processing functions/methods

Fixes an issue where we did not nest tables correctly when resolving
associated types in formal argument/return type positions.

This was the minimized reproduction case that I tested the fix on:
```rust
pub trait Trait {
    type Assoc;
}

pub struct A;

pub fn func() {
    fn _inner1<U: Trait>(_: U::Assoc) {}
    fn _inner2<U: Trait>() -> U::Assoc { unimplemented!() }

    impl A {
        fn _inner1<U: Trait>(self, _: U::Assoc) {}
        fn _inner2<U: Trait>(self) -> U::Assoc { unimplemented!() }
    }
}
```
using `debug_assertions`-enabled rustc and by additionally passing `-Zsave-analysis`.

Unfortunately the original assertion fired is a *debug* one and from what I can tell we don't run the tests with these on, so I'm not adding a test here. If I missed it and there is a way to run tests with these on, I'd love to add a test case for this.

Closes rust-lang#63663
Closes rust-lang#50328
Closes rust-lang#43982

… r=varkor

Don't mark expression with attributes as not needing parentheses

This is not perfectly correct as `#[attr] (5)` will still not lint, but it does seem good enough, in particular as the parentheses in that case are not unambiguously incorrect; I might personally prefer to see them for clarity.

Fixes rust-lang#43279.