borrowed_box should not be emitted for mutable boxed trait object

Hello.
The borrowed_box suggestion can be a false positive for mutable boxed trait object, because it seems there's no way to apply the suggestion (it only works for immutable boxed trait object).

For example, the following code triggers this warning:

fn get_mut<'a>(map: &'a mut HashMap<String, Box<Display>>, key: &str) -> Option<&'a mut Box<Display>> {
    map.get_mut(key)
}

But if I try to fix it by:

fn get_mut<'a>(map: &'a mut HashMap<String, Box<Display>>, key: &str) -> Option<&'a mut Display> {
    map.get_mut(key)
        .map(AsMut::as_mut)
}

I get a compilation error:

error[E0308]: mismatched types
  --> src/main.rs:12:5
   |
12 | /     map.get_mut(key)
13 | |         .map(AsMut::as_mut)
   | |___________________________^ lifetime mismatch
   |
   = note: expected type `std::option::Option<&'a mut std::fmt::Display + 'a>`
              found type `std::option::Option<&mut std::fmt::Display + 'static>`
note: the lifetime 'a as defined on the function body at 11:1...
  --> src/main.rs:11:1
   |
11 | / fn get_mut<'a>(map: &'a mut HashMap<String, Box<Display>>, key: &str) -> Option<&'a mut Display> {
12 | |     map.get_mut(key)
13 | |         .map(AsMut::as_mut)
14 | | }
   | |_^
   = note: ...does not necessarily outlive the static lifetime

(To be honest, I don't understand why I get this error for returning a mutable reference, but not for an immutable reference.)

Thanks to remove the warning in this case.

Start at the borrowed_box lint and reproduce the mutable boxed trait-object example from the issue. Trace why the suggestion is emitted, then verify that this case no longer warns while the applicable immutable case retains its existing behavior.