The Index trait is used to specify the functionality of indexing operations like arr[idx].

Example

A trivial implementation of Index. When Foo[Foo] happens, it ends up calling index, and therefore, main prints Indexing!.

struct Foo;

impl Index<Foo, Foo> for Foo {
    fn index(&self, _rhs: &Foo) -> Foo {
        println!("Indexing!");
        *self
    }
}

fn main() {
    Foo[Foo];
}