Understanding Rust Error E0562: Abstract Return Types

An Overview of Rust Error E0562

Rust Error E0562 occurs when the abstract return types, written as impl Trait for a specific trait, are used outside of function and inherent method return types. To fix this error, ensure that the impl Trait only appears in the appropriate return-type positions.

Erroneous Code Example:

fn main() {
    let count_to_ten: impl Iterator<Item=usize> = 0..10;
    // error: `impl Trait` not allowed outside of function and inherent method
    //        return types
    for i in count_to_ten {
        println!("{}", i);
    }
}

Explanation and Solution

In the code example above, the error occurs because the abstract return type `impl Iterator<Item=usize>` is used in a variable binding, which is not allowed. To resolve the E0562 error, move the impl Trait to the return-type position within a new function.

Corrected Code Example:

fn count_to_n(n: usize) -> impl Iterator<Item=usize> {
    0..n
}

fn main() {
    for i in count_to_n(10) {  // ok!
        println!("{}", i);
    }
}

Learn More

For more information and guidelines on using abstract return types in Rust, you can refer to RFC 1522. Adhering to the proper usage of impl Trait can prevent Rust Error E0562 and help ensure that your code compiles and runs without issues.