rustc suggests invalid syntax for missing method on `impl Trait` parameter

[euclio]

Given:

trait Foo {}

trait Bar {
    fn hello(&mut self) {}
}

fn test(foo: &mut impl Foo) {
    foo.hello();
}

rustc outputs:

error[E0599]: no method named `hello` found for type `&mut impl Foo` in the current scope
 --> src/lib.rs:8:9
  |
8 |     foo.hello();
  |         ^^^^^
  |
  = help: items from traits can only be used if the type parameter is bounded by the trait
help: the following trait defines an item `hello`, perhaps you need to restrict type parameter `impl Foo` with it:
  |
7 | fn test(foo: &mut impl Foo: Bar + {
  |                   ^^^^^^^^^^^^^^^