Auto merge of #13336 - nyurik:ref-option-sig, r=llogiq

Suggest `Option<&T>` instead of `&Option<T>`

closes #13054

```rust
// bad code
fn foo(a: &Option<T>) {}
fn bar(&self) -> &Option<T> {}

// Use instead
fn foo(a: Option<&T>) {}
fn bar(&self) -> Option<&T> {}
```

Handles argument types and return types in functions, methods, and closures with explicit types.  Honors `avoid_breaking_exported_api` parameter.

See this great [YouTube video](https://www.youtube.com/watch?v=6c7pZYP_iIE) with the in-depth explanation.

### Open Questions
These are not blocking, and could be done in separate PRs if needed.
* [ ] Should `&Option<Box<T>>` be suggested as `Option<&T>` -- without the box?  Handled by [clippy::borrowed_box](https://rust-lang.github.io/rust-clippy/master/index.html#/borrowed_box)
* [ ] Should `&Option<String>` be suggested as `Option<&str>` -- using de-refed type?

### Possible Future Improvements
These cases might also be good to handle, probably in a separate PR.

```rust
fn lambdas() {
  let x = |a: &Option<String>| {};
  let x = |a: &Option<String>| -> &Option<String> { todo!() };
}

fn mut_ref_to_ref(a: &mut &Option<u8>) {}
```

changelog: [`ref_option`]: Suggest `Option<&T>` instead of `&Option<T>`

0 files changed, 0 insertions, 0 deletions