Incompatible type error should explain when types come from default type parameters

[varkor]

trait Trait<T = ()> {
    fn foo() -> T;
}

struct S;

impl Trait for S {
    fn foo() -> u32 { 0 }
}

produces:

error[E0053]: method `foo` has an incompatible type for trait
  --> src/lib.rs:28:17
   |
22 |     fn foo() -> T;
   |                 - type in trait
...
28 |     fn foo() -> u32 { 0 }
   |                 ^^^ expected `()`, found `u32`
   |
   = note: expected fn pointer `fn()`
              found fn pointer `fn() -> u32`

The diagnostic doesn't point out why T is (), which comes from the default type parameter. It would be helpful to explain this, as it may not be immediately apparent.

[estebank]

Current output:

error[E0053]: method `foo` has an incompatible type for trait
 --> src/lib.rs:8:17
  |
8 |     fn foo() -> u32 { 0 }
  |                 ^^^
  |                 |
  |                 expected `()`, found `u32`
  |                 help: change the output type to match the trait: `()`
  |
note: type in trait
 --> src/lib.rs:2:17
  |
2 |     fn foo() -> T;
  |                 ^
  = note: expected signature `fn()`
             found signature `fn() -> u32`

We still don't point at line 1.

[estebank]

Minor change with new suggestion:

error[E0053]: method `foo` has an incompatible type for trait
 --> src/lib.rs:8:17
  |
8 |     fn foo() -> u32 { 0 }
  |                 ^^^ expected `()`, found `u32`
  |
note: type in trait
 --> src/lib.rs:2:17
  |
2 |     fn foo() -> T;
  |                 ^
  = note: expected signature `fn() -> ()`
             found signature `fn() -> u32`
help: change the output type to match the trait
  |
8 -     fn foo() -> u32 { 0 }
8 +     fn foo() -> () { 0 }
  |

We must specifically handle this case.