Unhelpful error message when using bad receiver type

[RalfJung]

Consider the following piece of code:

use std::pin::Pin;

trait Test {
    fn test(self: Pin<&mut Self>);
}

fn foo(t: impl Test) {
    t.test()
}

The error here is that test can only be called on Pin<&mut Self>, so t.test does not work. But the error message is less than helpful:

error[E0599]: no method named `test` found for type `impl Test` in the current scope
 --> src/lib.rs:8:7
  |
8 |     t.test()
  |       ^^^^
  |
  = help: items from traits can only be used if the trait is implemented and in scope
  = note: the following trait defines an item `test`, perhaps you need to implement it:
          candidate #1: `Test`

It suggests I import a trait that is already in scope. This leads down the totally wrong path -- not showing the "help" and "note" at all would be better than this. Ideally it could point out the receiver type mismatch.

[memoryruins]

Re-posting from #63966 since it was closed in favor of this thread.

---

playground 1.39.0-nightly 2019-08-23 9eae1fc0ea9b00341b8f

mod First {
    trait Foo { fn m(self: Box<Self>); }
    fn foo<T: Foo>(a: T) {
        a.m();
    }
}
mod Second {
    trait Bar { fn m(self: std::sync::Arc<Self>); }
    fn bar(b: impl Bar) {
        b.m();
    }
}

help: the following traits define an item `m`, perhaps you need to restrict type parameter `T` with one of them:
  |
3 |     fn foo<T: First::Foo + Foo>(a: T) {
  |            ^^^^^^^^^^^^^^^
3 |     fn foo<T: Second::Bar + Foo>(a: T) {
  |            ^^^^^^^^^^^^^^^^

The above suggests a trait from another module with a different name

help: the following traits define an item `m`, perhaps you need to restrict type parameter `impl Bar` with one of them:
   |
9  |     fn bar(b: impl Bar: First::Foo + {
   |               ^^^^^^^^^^^^^^^^^^^^^^
9  |     fn bar(b: impl Bar: Second::Bar + {
   |               ^^^^^^^^^^^^^^^^^^^^^^^

and the next suggestion isn't syntactically correct.

[estebank]

Current output:

error[E0599]: no method named `m` found for type `T` in the current scope
 --> src/lib.rs:4:11
  |
4 |         a.m();
  |           ^ method not found in `T`
  |
  = help: items from traits can only be used if the type parameter is bounded by the trait
help: the following traits define an item `m`, perhaps you need to restrict type parameter `T` with one of them:
  |
3 |     fn foo<T: First::Foo + Foo>(a: T) {
  |            ^^^^^^^^^^^^^^^
3 |     fn foo<T: Second::Bar + Foo>(a: T) {
  |            ^^^^^^^^^^^^^^^^

error[E0599]: no method named `m` found for type `impl Bar` in the current scope
  --> src/lib.rs:10:11
   |
10 |         b.m();
   |           ^ method not found in `impl Bar`
   |
   = help: items from traits can only be used if the type parameter is bounded by the trait
help: the following traits define an item `m`, perhaps you need to restrict type parameter `impl Bar` with one of them:
   |
9  |     fn bar(b: impl Bar + First::Foo) {
   |               ^^^^^^^^^^^^^^^^^^^^^
9  |     fn bar(b: impl Bar + Second::Bar) {
   |               ^^^^^^^^^^^^^^^^^^^^^^

error[E0599]: no method named `test` found for type `impl Test` in the current scope
 --> src/lib.rs:8:7
  |
8 |     t.test()
  |       ^^^^ method not found in `impl Test`
  |
  = help: items from traits can only be used if the type parameter is bounded by the trait
help: the following trait defines an item `test`, perhaps you need to restrict type parameter `impl Test` with it:
  |
7 | fn foo(t: impl Test + Test) {
  |           ^^^^^^^^^^^^^^^^

[Nemo157]

@rustbot modify labels to +AsyncAwait-OnDeck

This error message comes up a lot when users attempt to manually implement Future or the associated traits like futures::io::{AsyncRead, AsyncWrite} by delegating to a field without pinning it.