Error suggests implementing private trait instead of importing it.

[pierzchalski]

If I implement a private trait on a public struct, then use a trait method elsewhere, the diagnostic message suggests implementing the already implemented private trait instead of making the trait public and importing it. For example, this code:

mod m {
    trait T {
        fn f(&self);
    }
    
    pub struct S;
    
    impl T for S {
        fn f(&self) {}
    }
}

fn main() {
    let s = m::S;
    s.f();
}

Results in this diagnostic:

error[E0599]: no method named `f` found for type `m::S` in the current scope
  --> src/main.rs:15:7
   |
6  |     pub struct S;
   |     ------------- method `f` not found for this
...
15 |     s.f();
   |       ^
   |
   = help: items from traits can only be used if the trait is implemented and in scope
   = note: the following trait defines an item `f`, perhaps you need to implement it:
           candidate #1: `m::T`

The note should suggest making m::T public and useing it at the top level.

[estebank]

Current output:

error[E0599]: no method named `f` found for struct `S` in the current scope
  --> src/main.rs:15:7
   |
6  |     pub struct S;
   |     ------------- method `f` not found for this
...
15 |     s.f();
   |       ^ method not found in `S`
   |
   = help: items from traits can only be used if the trait is implemented and in scope
note: `T` defines an item `f`, perhaps you need to implement it
  --> src/main.rs:2:5
   |
2  |     trait T {
   |     ^^^^^^^

We still don't check for accessibility.

[estebank]

Triage: no change