Is it possible to declare a trait that accepts more methods that the ones declared in its body?

Question

Something like:

pub trait MyTrait {
    // No methods defined/declared
}

impl MyTrait for MyType {
    fn method_one_not_declared_neither_defined(&self) {
        // body of trait method impl here
    }

    fn method_two_not_declared_neither_defined(&self) {
        // body of trait method impl here
    }
}

Answer 1

While the trait itself cannot be extended, you can still add behaviour to all types which implement a trait. This can be done using extension traits and blanket implementations.

trait MyTrait {}

struct MyType;

impl MyTrait for MyType {}

trait TraitExt {
    fn foo(&self) {
        println!("Foo");
    }
}

impl<T> TraitExt for T where T: MyTrait {}

fn main() {
    let t = MyType {};
    t.foo();
}

Playground

This is not the same as extending MyTrait however - TraitExt must be in scope for foo to be called.