book: 'Angle-bracket Form' does not explain why expanded form is needed

[ivan]

https://doc.rust-lang.org/book/ufcs.html#angle-bracket-form gives an example that uses <Bar as Clone>::clone(self);, where using Clone::clone(self); works just as well (tested: rustc 1.6.0-nightly (2e07996 2015-10-29)). And therefore I have no idea why and where I'd need the longer form.

I tested

trait Foo {
    fn clone(&self);
}

#[derive(Clone)]
struct Bar;

impl Foo for Bar {
    fn clone(&self) {
        println!("Making a clone of Bar");

        //<Bar as Clone>::clone(self);
        Clone::clone(self);
    }
}

fn main() {
   let b = Bar;
   Foo::clone(&b);
}

[sfackler]

The expanded form is required when the method you want to call doesn't mention Self in any way. For example:

trait Foo {
    fn foo() -> i32;
}

struct Bar;

impl Foo for Bar {
    fn foo() -> i32 {
        10
    }
}

fn main() {
    println!("{}", <Bar as Foo>::foo());
}

Seems like the example in the book should be updated to something like that.

[steveklabnik]

@sfackler Bar::foo() works just fine there.

I agree we need a better example.... but I'm not sure what that is.

[sfackler]

Ah yeah. If you add a fn foo() directly to Bar as well, then you should need to use the full UFCS form to specify you want to call Foo's foo rather than Bar's:

trait Foo {
    fn foo() -> i32;
}

struct Bar;

impl Bar {
    fn foo() -> i32 {
        20
    }
}

impl Foo for Bar {
    fn foo() -> i32 {
        10
    }
}

fn main() {
    println!("{}", <Bar as Foo>::foo());
}