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

[estebank]

Only the first error is now emitted. Suggestion still needs to be fixed.

[est31]

Not sure where the bar came from, I guess I meant something like:

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

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

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

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

Nowadays it gives on 1.96.0-nightly (2026-03-09 2d76d9bc76f27b03b489):

error[E0277]: the trait bound `&Hello: Tr` is not satisfied
  --> src/main.rs:15:9
   |
15 |         foo(hi, hi);
   |         ^^^ the trait `Tr` is not implemented for `&Hello`
   |
note: required by a bound in `foo`
  --> src/main.rs:9:11
   |
 9 | fn foo<T: Tr>(_v: T, _w: T) {}
   |           ^^ required by this bound in `foo`
help: consider borrowing here
   |
15 |         &foo(hi, hi);
   |         +

error[E0277]: the trait bound `&Hello: Tr` is not satisfied
  --> src/main.rs:16:13
   |
16 |         bar(hi);
   |         --- ^^ the trait `Tr` is not implemented for `&Hello`
   |         |
   |         required by a bound introduced by this call
   |
note: required by a bound in `bar`
  --> src/main.rs:10:11
   |
10 | fn bar<T: Tr>(_v: T) {}
   |           ^^ required by this bound in `bar`
help: consider dereferencing here
   |
16 |         bar(*hi);
   |             +

The bar suggestion nowadays is accurate!

[arferreira]

Gonna take a look on this!
@rustbot claim

[estebank]

I just started looking at this. I found a couple of things to address, but I can give you some additional context on what's left to do. I'll publish a PR shortly.

[arferreira]

I'll step back on this one. Let me know if there's anything left later.
@rustbot release-assignment

[estebank]

I've got enough to address the lack of borrowing for functions and methods, but it'll be great to also have suggestions for dereferencing and cloning when applicable. Would you want me to ping you when I post the PR and do that follow up work?

Sorry for sniping you out of it, I was walking the dogs yesterday and thought of a way to address this 😅

[arferreira]

Absolutely, please ping me! I'll take the deref and clone follow-up. No worries at all, glad you had the insight on the walk ✌️