error[E0599]: no method named `foo` found for type `&b::B` in the current scope
  --> $DIR/issue-10465.rs:17:15
   |
LL |             b.foo();
   |               ^^^
   |
   = help: items from traits can only be used if the trait is in scope
   = note: the following trait is implemented but not in scope, perhaps add a `use` for it:
           `use a::A;`

error: aborting due to previous error

For more information about this error, try `rustc --explain E0599`.