Fix whitespace

Co-authored-by: Esteban Kuber <estebank@users.noreply.github.com>

There was one directory that had only a single test and there was also a
test in the top-level directory. This moves both of them to
`feature-gates/`.

…s.rs

Co-authored-by: Esteban Kuber <estebank@users.noreply.github.com>

The use of Result allows for making use of a reconstructed original value on failed
projections.

See rust-lang#77433 for why the new heuristic was chosen.

Fixes rust-lang#77433

The first parse is to collect whether the code contains macros, has
`main`, and uses other crates. In that pass we ignore errors as those
will be reported when the test file is actually built.

For that we need to reset errors in the `Diagnostic` otherwise when
dropping it unhandled errors will be reported as compiler bugs.

Fixes rust-lang#80992

This has been fixed between 1.47.0-nightly (663d2f5 2020-08-22) and 1.47.0-nightly (5180f3d 2020-08-23).

…ottmcm

Always use extend in BinaryHeap::append

This is faster, see rust-lang#77433.

Fixes rust-lang#77433

Introduce {Ref, RefMut}::try_map for optional projections in RefCell

This fills a usability gap of `RefCell` I've personally encountered to perform optional projections, mostly into collections such as `RefCell<Vec<T>>` or `RefCell<HashMap<U, T>>`:

> This kind of API was briefly featured under Open questions in rust-lang#10514 back in 2013 (!)

```rust
let values = RefCell::new(vec![1, 2, 3, 4]);
let b = Ref::opt_map(values.borrow(), |vec| vec.get(2));
```

It primarily avoids this alternative approach to accomplish the same kind of projection which is both rather noisy and panicky:
```rust
let values = RefCell::new(vec![1, 2, 3, 4]);

let b = if values.get(2).is_some() {
    Some(Ref::map(values.borrow(), |vec| vec.get(2).unwrap()))
} else {
    None
};
```

### Open questions

The naming `opt_map` is preliminary. I'm not aware of prior art in std to lean on here, but this name should probably be improved if this functionality is desirable.

Since `opt_map` consumes the guard, and alternative syntax might be more appropriate which instead *tries* to perform the projection, allowing the original borrow to be recovered in case it fails:

```rust
pub fn try_map<U: ?Sized, F>(orig: Ref<'b, T>, f: F) -> Result<Ref<'b, U>, Self>
where
    F: FnOnce(&T) -> Option<&U>;
```

This would be more in line with the `try_map` method [provided by parking lot](https://docs.rs/lock_api/0/lock_api/struct.RwLockWriteGuard.html#method.try_map).

Explain why borrows can't be held across yield point in async blocks

For rust-lang#78938.

…crum

Make `x.py --color always` apply to logging too

Follow-up to rust-lang#78548, rust-lang#79004.

r? `@Mark-Simulacrum`

…acrum

Add a regression test for rust-lang#76281

This has been fixed between 1.47.0-nightly (663d2f5 2020-08-22) and 1.47.0-nightly (5180f3d 2020-08-23). Maybe fixed by rust-lang#73526?

Created `wasm` dir, it currently has only one test but I'll move some wasm-related tests there as a follow-up.

Closes rust-lang#76281

…Simulacrum

Stabilize the poll_map feature

Stabilizes the `poll_map` feature as tracked by rust-lang#63514 (with a completed FCP).

…nization, r=Mark-Simulacrum

Put all feature gate tests under `feature-gates/`

There was one directory that had only a single test and there was also a
test in the top-level directory. This moves both of them to
`feature-gates/`.

…r=jyn514

Remove doctree::Import

Per the title. Part of cleaning up doctree

doctest: Reset errors before dropping the parse session

The first parse is to collect whether the code contains macros, has
`main`, and uses other crates. In that pass we ignore errors as those
will be reported when the test file is actually built.

For that we need to reset errors in the `Diagnostic` otherwise when
dropping it unhandled errors will be reported as compiler bugs.

Fixes rust-lang#80992

…=jyn514

codegen_cranelift: Fix redundant semicolon warn