A private trait was used on a public type parameter bound.

Erroneous code examples:

```compile_fail,E0445
#![deny(private_in_public)]

trait Foo {
    fn dummy(&self) { }
}

pub trait Bar : Foo {} // error: private trait in public interface
pub struct Bar2<T: Foo>(pub T); // same error
pub fn foo<T: Foo> (t: T) {} // same error

fn main() {}
```

To solve this error, please ensure that the trait is also public. The trait
can be made inaccessible if necessary by placing it into a private inner
module, but it still has to be marked with `pub`. Example:

```
pub trait Foo { // we set the Foo trait public
    fn dummy(&self) { }
}

pub trait Bar : Foo {} // ok!
pub struct Bar2<T: Foo>(pub T); // ok!
pub fn foo<T: Foo> (t: T) {} // ok!

fn main() {}
```
