Error message for type mismatch between impl Traits needs better explanation

[kornelski]

The message "expected type impl Trait (opaque type), found type impl Trait (opaque type)" sounds like a paradox: the compiler has found exactly what it expected, and yet it doesn't match.

The explanatory text of E0308 is the same as for all other banal type errors, so it's not addressing this problem specifically.

From the questions I'm seeing in the user forum I have an impression that users think impl Trait is like dyn Trait and try to use it as an abstraction for mixing different types.

fn f1() -> impl Copy {
    "1"
}
fn f2() -> impl Copy {
    2
}

fn main() {
    let mut x = f1();
    x = f2();
}

10 | x = f2();
| ^^^^ expected opaque type, found a different opaque type
|
= note: expected type impl std::marker::Copy (opaque type)
found type impl std::marker::Copy (opaque type)

---

Suggestions:

• Make it a separate error, so that it'll get its own rustc --explain document.

• If possible, instead of "(opaque type)" print more internal type information, e.g. like for closures:

  = note: expected type `impl [std::marker::Copy@src/main.rs:2:10]`
          found type `impl [std::marker::Copy@src/main.rs:5:10]`
  
  = note: each `impl Trait`, even for the same trait, is a different type.
  = help: consider using `Box<dyn Trait>` or separate generic parameters.
  

[estebank]

Current output:

error[E0308]: mismatched types
  --> src/main.rs:10:9
   |
10 |     x = f2();
   |         ^^^^ expected opaque type, found a different opaque type
   |
   = note: expected type `impl std::marker::Copy` (opaque type at <src/main.rs:1:12>)
              found type `impl std::marker::Copy` (opaque type at <src/main.rs:4:12>)
   = note: distinct uses of `impl Trait` result in different opaque types

The suggestion to use Box<dyn Trait> could be interesting as text, but giving a structured suggestion would be quite difficult.

[kornelski]

Addition of the line where the type comes from is a significant improvement. Shall I close the issue?

[estebank]

Let's keep it open focused on getting some span labels pointing at those places and nothing else.