unnecessary sort by: avoid dereferencing the suggested closure parameter
This change tries to simplify the solution for problematic cases but is less restrictive than #6006.
* We can't dereference shared references to non-Copy types, so the new suggestion does not do that. Note that this implies that the suggested closure parameter will be a reference.
* We can't take a reference to the closure parameter in the returned key, so we don't lint in those cases. This can happen either because the key borrows from the parameter (e.g. `|a| a.borrows()`), or because we suggest `|a| Reverse(a)`. If we did we would hit this error:
```
error: lifetime may not live long enough
--> /home/ebroto/src/ebroto-clippy/tests/ui/unnecessary_sort_by.fixed:19:25
|
19 | vec.sort_by_key(|b| Reverse(b));
| -- ^^^^^^^^^^ returning this value requires that `'1` must outlive `'2`
| ||
| |return type of closure is Reverse<&'2 isize>
| has type `&'1 isize`
error: aborting due to previous error
```
Note that Clippy does not currently have the (MIR-based) machinery necessary to check that what is borrowed is actually the closure parameter.
changelog: [`unnecessary_sort_by`]: avoid dereferencing the suggested closure parameter
Fixes#6001
c9fdeef643