This PR allows using methods from traits that are visible but are defined in an inaccessible module (fixes#18241). For example,
```rust
mod foo {
pub use foo::bar::Tr;
mod bar { // This module is inaccessible from `g`
pub trait Tr { fn f(&self) {} }
}
}
fn g<T: foo::Tr>(t: T) {
t.f(); // Currently, this is a privacy error even though `foo::Tr` is visible
}
```
After this PR, it will continue to be a privacy error to use a method from a trait that is not visible. This can happen when a public trait inherits from a private trait (in violation of the `public_in_private` lint) -- see @petrochenkov's example in #28504.
r? @nikomatsakis
45f0ce71c1