Understanding Rust Error E0624: Private Items Used Outside of Their Scope

What is Rust Error E0624?

Rust Error E0624 occurs when you attempt to use a private item outside of its defined scope. This error is raised to protect encapsulation and ensure the proper usage of private items within a module or function.

An example of Rust Error E0624

Consider the following erroneous code snippet that demonstrates Rust Error E0624:

#![allow(unused)]
fn main() {
mod inner {
    pub struct Foo;

    impl Foo {
        fn method(&self) {}
    }
}

let foo = inner::Foo;
foo.method(); // error: method `method` is private
}

In this example, a private `method` is defined within the implementation of the `Foo` struct in the `inner` module. Trying to access this method outside the scope of the `inner` module triggers the E0624 error since the function `method()` is marked as private.

Solving Rust Error E0624

There are two primary approaches to resolving Rust Error E0624:

1. Use the item in its defined scope

You can create a public function within the module that encapsulates the logic of the private function, as shown below:

#![allow(unused)]
fn main() {
mod inner {
    pub struct Foo;

    impl Foo {
        fn method(&self) {}
    }

    pub fn call_method(foo: &Foo) {
        foo.method();
    }
}

let foo = inner::Foo;
inner::call_method(&foo);
}

In this modified example, we've added a public function `call_method` within the `inner` module that invokes the private `method`. Users outside the module can then access this private method by calling the public `call_method` function.

2. Make the item public

Another simple solution is to make the private item public, allowing users to access the function directly:

#![allow(unused)]
fn main() {
mod inner {
    pub struct Foo;

    impl Foo {
        pub fn method(&self) {}
    }
}

let foo = inner::Foo;
foo.method();
}

By changing the private `method` function to a public function in the `Foo` struct, this example no longer triggers Rust Error E0624 when invoking `method()`. Keep in mind that making a function public exposes it to external users, which might not always be appropriate depending on the desired level of encapsulation.