rust trait implementation

foo.rs

trait HasName {
    fn get_name(&self) -> ~str;
}

trait HasGreeting {
    fn get_greeting(&self) -> ~str;
}

impl <T: HasName> HasGreeting for T {
    fn get_greeting(&self) -> ~str {
        "welcome: " + self.get_name()
    }
}

struct Foo {
    priv name: ~str
}

impl HasName for Foo {
    fn get_name(&self) -> ~str {
        self.name.clone()
    }
}

fn main() {
    let foo1 = Foo{name: ~"Shahid"};
    println(foo1.get_greeting());
    let foo2 = ~Foo{name: ~"John"};
    println((*foo2).get_greeting());

    // the following line doesn't compile
    // println(foo2.get_greeting());
    //
    // foo.rs:29:12: 29:32 error: failed to find an implementation of trait HasName for ~Foo
    // foo.rs:29     println(foo2.get_greeting());
    //                       ^~~~~~~~~~~~~~~~~~~~
}