Wrong fix suggested for missing as_mut in the presence of RefCell<Option<>>

[khuey]

Given the following code: https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=d77c477c52217e25856306f659a26e42

use std::cell::RefCell;

fn foo(cell: &RefCell<Option<Vec<()>>>) {
    cell.borrow_mut().unwrap().pop().unwrap();
}

The current output is:

Compiling playground v0.0.1 (/playground)
error[[E0507]](https://doc.rust-lang.org/nightly/error-index.html#E0507): cannot move out of dereference of `RefMut<'_, Option<Vec<()>>>`
 [--> src/lib.rs:4:5
](https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=d77c477c52217e25856306f659a26e42#)  |
4 |     cell.borrow_mut().unwrap().pop().unwrap();
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^ move occurs because value has type `Option<Vec<()>>`, which does not implement the `Copy` trait
  |
help: consider borrowing the `Option`'s content
  |
4 |     cell.borrow_mut().unwrap().as_ref().pop().unwrap();
  |                               +++++++++

For more information about this error, try `rustc --explain E0507`.
error: could not compile `playground` due to previous error

There are two separate issues here:

1. The suggested as_ref is in the wrong place and should be after the borrow_mut(), not after the unwrap().
2. The suggested as_ref really should be an as_mut

[George-lewis]

I think I might be able to take this one if that's alright. Was investigating it a bit yesterday, will post a pr shortly

[khuey]

The first half of this is fixed, as_ref is now suggested in the correct spot. But it still should be as_mut that's suggested.

[khuey]

This is getting better, still seems like the compiler could figure out that as_mut is needed though.

Compiling playground v0.0.1 (/playground)
error[[E0507]](https://doc.rust-lang.org/nightly/error-index.html#E0507): cannot move out of dereference of `RefMut<'_, Option<Vec<()>>>`
 --> src/lib.rs:4:5
  |
4 |     cell.borrow_mut().unwrap().pop().unwrap();
  |     ^^^^^^^^^^^^^^^^^^--------
  |     |                 |
  |     |                 value moved due to this method call
  |     help: consider calling `.as_ref()` or `.as_mut()` to borrow the type's contents
  |     move occurs because value has type `Option<Vec<()>>`, which does not implement the `Copy` trait
  |
note: this function takes ownership of the receiver `self`, which moves value

For more information about this error, try `rustc --explain E0507`.
error: could not compile `playground` due to previous error