Stand-in lifetime names for printing HRTs doesn’t avoid name collisions.

[steffahn]

E.g.

use std::cell::Cell;

fn f<'a>(f: fn(Cell<(&'a i32, &i32)>)) -> for<'l> fn(Cell<(&'l i32, &'l i32)>) {
    f
}

chooses the lifetime name “'r” for the fn-pointer type of the argument f and prints

error[E0308]: mismatched types
 --> src/lib.rs:4:5
  |
3 | fn f<'a>(f: fn(Cell<(&'a i32, &i32)>)) -> for<'l> fn(Cell<(&'l i32, &'l i32)>) {
  |                                           ------------------------------------ expected `for<'l> fn(Cell<(&'l i32, &'l i32)>)` because of return type
4 |     f
  |     ^ one type is more general than the other
  |
  = note: expected fn pointer `for<'l> fn(Cell<(&'l i32, &'l i32)>)`
             found fn pointer `for<'r> fn(Cell<(&'a i32, &'r i32)>)`

For more information about this error, try `rustc --explain E0308`.

---

However, rewriting the code into

use std::cell::Cell;
type Ty = for<'r> fn(Cell<(&'r i32, &'r i32)>);
fn f<'r>(f: fn(Cell<(&'r i32, &i32)>)) -> Ty {
    f
}

does not currently make the compiler reconsider the choice of “'r” in the lifetime name, resulting in an unhelpful error message

error[E0308]: mismatched types
 --> src/lib.rs:4:5
  |
3 | fn f<'r>(f: fn(Cell<(&'r i32, &i32)>)) -> Ty {
  |                                           -- expected `for<'r> fn(Cell<(&'r i32, &'r i32)>)` because of return type
4 |     f
  |     ^ one type is more general than the other
  |
  = note: expected fn pointer `for<'r> fn(Cell<(&'r i32, &'r i32)>)`
             found fn pointer `for<'r> fn(Cell<(&'r i32, &'r i32)>)`

[b-naber]

@rustbot claim