Is there a way other than traits to add methods to a type I don’t own?

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As of Rust 1.27, no, there is no other way. It’s not possible to define inherent methods on a type defined in another crate.

You can define your own trait with the methods you need, then implement that trait for an external type. This pattern is known as extension traits. The name of extension traits, by convention, ends with Ext, to indicate that this trait is not meant to be used as a generic bound or as a trait object. There are a few examples in the standard library.

trait DoubleExt {
    fn double(&self) -> Self;
}

impl DoubleExt for i32 {
    fn double(&self) -> Self {
        *self * 2
    }
}

fn main() {
    let a = 42;
    println!("{}", 42.double());
}

Other libraries can also export extension traits (example: byteorder). However, as for any other trait, you need to bring the trait’s methods in scope with use SomethingExt;.

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