Rollup merge of #35611 - jonathandturner:ptr-helper, r=nikomatsakis

Rollup merge of #35611 - jonathandturner:ptr-helper, r=nikomatsakis

Improve &-ptr printing

This PR replaces printing `&-ptr` with a more readable description.  To do so it uses a few heuristics.

If the name of the type is unknown, too long (longer than just saying "reference"), or too complex (a type with explicit lifetime annotations), it will instead opt to print either "reference" or "mutable reference", depending on the mutability of the type.

Before:

```
error[E0308]: mismatched types
  --> src/test/compile-fail/issue-7061.rs:14:46
   |
14 |     fn foo(&'a mut self) -> Box<BarStruct> { self }
   |                                              ^^^^ expected box, found &-ptr
   |
   = note: expected type `Box<BarStruct>`
   = note:    found type `&'a mut BarStruct`

error: aborting due to previous error
```

After:

```
error[E0308]: mismatched types
  --> src/test/compile-fail/issue-7061.rs:14:46
   |
14 |     fn foo(&'a mut self) -> Box<BarStruct> { self }
   |                                              ^^^^ expected box, found mutable reference
   |
   = note: expected type `Box<BarStruct>`
   = note:    found type `&'a mut BarStruct`

error: aborting due to previous error
```