E0277 message is misleading when there's a blanket impl

[aldanor]

trait Foo { fn foo() {} }
trait Bar {}
impl<T: Bar> Foo for T {}

fn main() {
    <i32 as Foo>::foo();
}

Reported error tries to be too smart:

error[E0277]: the trait bound `i32: Bar` is not satisfied
  |
  |     <i32 as Foo>::foo();
  |     ^^^^^^^^^^^^^^^^^ the trait `Bar` is not implemented for `i32`
  |
  = note: required because of the requirements on the impl of `Foo` for `i32`
  = note: required by `Foo::foo`

... but is misleading since the real error is that i32: Foo is not satisfied; i32 may happen to implement Foo and not implement Bar, so the error message is actually just wrong.

[aldanor]

Hypothetical alternative error message:

error[E0277]: the trait bound `i32: Foo` is not satisfied
  |
  |     <i32 as Foo>::foo();
  |     ^^^^^^^^^^^^^^^^^ the trait `Foo` is not implemented for `i32`
  |
  = note: the trait bound could be satisfied by implementing `Bar` for `i32`
  = note: required by `Foo::foo`

[estebank]

Current output:

error[E0277]: the trait bound `i32: Bar` is not satisfied
 --> src/main.rs:6:5
  |
1 | trait Foo { fn foo() {} }
  |             -------- required by `Foo::foo`
...
6 |     <i32 as Foo>::foo();
  |     ^^^^^^^^^^^^^^^^^ the trait `Bar` is not implemented for `i32`
  |
  = note: required because of the requirements on the impl of `Foo` for `i32`

It should be closer to

error[E0277]: the trait bound `i32: Bar` is not satisfied
 --> src/main.rs:6:5
  |
3 | impl<T: Bar> Foo for T {}
  |      ------ required by this bound
...
6 |     <i32 as Foo>::foo();
  |     ^^^^^^^^^^^^^^^^^ the trait `Bar` is not implemented for `i32`
  |
  = note: required because of the requirements on the impl of `Foo` for `i32`

[jplatte]

This should be tagged A-traits, no?

[estebank]

Current output:

error[E0277]: the trait bound `i32: Bar` is not satisfied
 --> src/main.rs:6:5
  |
6 |     <i32 as Foo>::foo();
  |     ^^^^^^^^^^^^^^^^^ the trait `Bar` is not implemented for `i32`
  |
note: required for `i32` to implement `Foo`
 --> src/main.rs:3:14
  |
3 | impl<T: Bar> Foo for T {}
  |              ^^^     ^

[estebank]

Current output:

error[E0277]: the trait bound `i32: Bar` is not satisfied
 --> src/main.rs:6:6
  |
6 |     <i32 as Foo>::foo();
  |      ^^^ the trait `Bar` is not implemented for `i32`
  |
help: this trait has no implementations, consider adding one
 --> src/main.rs:2:1
  |
2 | trait Bar {}
  | ^^^^^^^^^
note: required for `i32` to implement `Foo`
 --> src/main.rs:3:14
  |
3 | impl<T: Bar> Foo for T {}
  |         ---  ^^^     ^
  |         |
  |         unsatisfied trait bound introduced here

[estebank]

I believe the current output is good enough to explain what's going on, pointing at the blanket impl that matches but that introduces an unmet requirement.

[jplatte]

I don't like that the current output still mentions Bar first, as if the user had written <i32 as Bar>. IMHO the output should be something along the lines of

error[E0277]: the trait bound `i32: Foo` is not satisfied
<snippet>
note: there is a generic implementation, but `i32` does not satisfy it
<snippet>

[estebank]

Sure, we could potentially try to do that.