unnecessary sort by: avoid dereferencing the suggested closure parameter

[ebroto]

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

[rust-highfive]

r? @phansch

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[phansch]

@bors r+

[bors]

📌 Commit 9365660 has been approved by phansch

[bors]

⌛ Testing commit 9365660 with merge c9fdeef...

[bors]

☀️ Test successful - checks-action_dev_test, checks-action_remark_test, checks-action_test
Approved by: phansch
Pushing c9fdeef to master...