A private type was used in a public type signature.
Erroneous code example:
```compile_fail,E0446
#![deny(private_in_public)]
mod Foo {
struct Bar(u32);
pub fn bar() -> Bar { // error: private type in public interface
Bar(0)
}
}
fn main() {}
```
To solve this error, please ensure that the type is also public. The type
can be made inaccessible if necessary by placing it into a private inner
module, but it still has to be marked with `pub`.
Example:
```
mod Foo {
pub struct Bar(u32); // we set the Bar type public
pub fn bar() -> Bar { // ok!
Bar(0)
}
}
fn main() {}
```