Solving Rust Error E0446: Private Type in Public Interface

Understanding Rust Error E0446

Rust Error E0446 occurs when a private type is used in a public type signature. This can lead to violations of the intended encapsulation and generate issues when other modules try to access the public interface. To better understand this error, let's consider the following erroneous code example:

#![deny(private_in_public)]
struct Bar(u32);

mod foo {
    use crate::Bar;
    pub fn bar() -> Bar { // error: private type in public interface
        Bar(0)
    }
}

fn main() {}

Solving Rust Error E0446

There are two primary methods for resolving Rust Error E0446, which include modifying the visibility of the private type or the module containing it.

Method 1: Restrict Visibility

The first approach involves restricting the public type signature's visibility to a module that also has access to the private type. This can be achieved using pub(crate) or pub(in crate::my_mod::etc). This means only the module specified will be able to access the function. Here's an example of this solution:

struct Bar(u32);

mod foo {
    use crate::Bar;
    pub(crate) fn bar() -> Bar { // only public to crate root
        Bar(0)
    }
}

fn main() {}

Method 2: Make the Private Type Public

Another way to resolve Rust Error E0446 is by making the private type public. This adjustment grants other modules access to both the public interface and the previously private type. Here's an example of this solution:

pub struct Bar(u32); // we set the Bar type public
mod foo {
    use crate::Bar;
    pub fn bar() -> Bar { // ok!
        Bar(0)
    }
}

fn main() {}

Conclusion

Resolving Rust Error E0446 is crucial for maintaining appropriate encapsulation and ensuring that other modules have access to the necessary types and functions. By either restricting function visibility or making the private type public, developers can fix this error and maintain well-organized and functional codebases.