Refactor fmt::Display and fmt::Debug impls in ppaux
Also fixes#44887.
There was a problem that unnamed late-bound regions are *always* named `'r` while they are displayed using `std::fmt::Display`.
---
```rust
fn main() {
f(|_: (), _: ()| {});
}
fn f<F>(_: F) where F: Fn(&(), for<'r> fn(&'r ())) {}
```
Before (incorrectly shadows lifetime, `for<'r>` omitted for the second argument):
```
error[E0631]: type mismatch in closure arguments
--> test.rs:2:5
|
2 | f(|_: (), _: ()| {});
| ^ ----------------- found signature of `fn((), ()) -> _`
| |
| expected signature of `for<'r> fn(&'r (), fn(&'r ())) -> _`
|
= note: required by `f`
```
After:
```
error[E0631]: type mismatch in closure arguments
--> test.rs:2:5
|
2 | f(|_: (), _: ()| {});
| ^ ----------------- found signature of `fn((), ()) -> _`
| |
| expected signature of `for<'s> fn(&'s (), for<'r> fn(&'r ())) -> _`
|
= note: required by `f`
```
r? @nikomatsakis
cbf5d39cca