Universal Function Call Syntax

Sometimes, functions can have the same names. Consider this code:

# #![allow(unused_variables)]
#fn main() {
trait Foo {
    fn f(&self);
}

trait Bar {
    fn f(&self);
}

struct Baz;

impl Foo for Baz {
    fn f(&self) { println!("Baz’s impl of Foo"); }
}

impl Bar for Baz {
    fn f(&self) { println!("Baz’s impl of Bar"); }
}

let b = Baz;
#}

If we were to try to call b.f(), we’d get an error:

error: multiple applicable methods in scope [E0034]
b.f();
  ^~~
note: candidate #1 is defined in an impl of the trait `main::Foo` for the type
`main::Baz`
    fn f(&self) { println!("Baz’s impl of Foo"); }
    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
note: candidate #2 is defined in an impl of the trait `main::Bar` for the type
`main::Baz`
    fn f(&self) { println!("Baz’s impl of Bar"); }
    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

We need a way to disambiguate which method we need. This feature is called ‘universal function call syntax’, and it looks like this:

# #![allow(unused_variables)]
#fn main() {
# trait Foo {
#     fn f(&self);
# }
# trait Bar {
#     fn f(&self);
# }
# struct Baz;
# impl Foo for Baz {
#     fn f(&self) { println!("Baz’s impl of Foo"); }
# }
# impl Bar for Baz {
#     fn f(&self) { println!("Baz’s impl of Bar"); }
# }
# let b = Baz;
Foo::f(&b);
Bar::f(&b);
#}

Let’s break it down.

Foo::
Bar::

These halves of the invocation are the types of the two traits: Foo and Bar. This is what ends up actually doing the disambiguation between the two: Rust calls the one from the trait name you use.

f(&b)

When we call a method like b.f() using method syntax, Rust will automatically borrow b if f() takes &self. In this case, Rust will not, and so we need to pass an explicit &b.

Angle-bracket Form

The form of UFCS we just talked about:

Trait::method(args);

Is a short-hand. There’s an expanded form of this that’s needed in some situations:

<Type as Trait>::method(args);

The <>:: syntax is a means of providing a type hint. The type goes inside the <>s. In this case, the type is Type as Trait, indicating that we want Trait’s version of method to be called here. The as Trait part is optional if it’s not ambiguous. Same with the angle brackets, hence the shorter form.

Here’s an example of using the longer form.

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

struct Bar;

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

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

fn main() {
    assert_eq!(10, <Bar as Foo>::foo());
    assert_eq!(20, Bar::foo());
}

Using the angle bracket syntax lets you call the trait method instead of the inherent one.