strange borrowing suggestion

[est31]

Rustc suggests borrowing of a function's return type when there is a trait not implemented error for the function's parameters.

Not only does the borrow not help, a dereferencing is required instead, but it is suggested for the return type, which is not where the problem is at all.

Consider the following program:

#[derive(Clone, Copy)]
struct Hello;

trait Tr: Clone + Copy {}
impl Tr for Hello {}

fn foo<T: Tr>(_v: T, _w: T) {}

fn main() {
    let hellos = [Hello; 3];
    for hi in hellos.iter() {
        foo(hi, hi);
    }
}

On latest nightly 1.85.0-nightly (2024-12-25 7c002ff9a70cb84fd1a9) (same as on stable and beta):

error[E0277]: the trait bound `&Hello: Tr` is not satisfied
  --> src/main.rs:14:9
   |
14 |         foo(hi, hi);
   |         ^^^ the trait `Tr` is not implemented for `&Hello`
   |
note: required by a bound in `foo`
  --> src/main.rs:7:11
   |
7  | fn foo<T: Tr>(_v: T, _w: T) {}
   |           ^^ required by this bound in `foo`
help: consider borrowing here
   |
14 |         &foo(hi, hi);
   |         +

error[E0277]: the trait bound `&Hello: Tr` is not satisfied
  --> src/main.rs:15:13
   |
15 |         bar(hi);
   |         --- ^^ the trait `Tr` is not implemented for `&Hello`
   |         |
   |         required by a bound introduced by this call
   |
   = help: the trait `Tr` is implemented for `Hello`
note: required by a bound in `bar`
  --> src/main.rs:9:11
   |
9  | fn bar<T: Tr>(_v: T) {}
   |           ^^ required by this bound in `bar`

The example shows that if there is one parameter, a borrow is not suggested. It happens only if there is two parameters.

Related (but not the same): #132041