Implement trait for trait with static function

Question

trait Trait<T> {
    fn equality() -> bool;
}

impl<T> PartialEq for Trait<T> {
    fn eq(&self, other: &Trait<T>) -> bool {
        self.equality()
    }
}

Results in

main.rs:5:23: 5:31 error: the trait `Trait` cannot be made into an object [E0372]
main.rs:5 impl<T> PartialEq for Trait<T> {

Removing the static method makes it compilable. Methods with &self parameter compile, too.

Answer 1

It comes down to a matter known as object safety, which you can find information of in RFC 255; Huon has a good explanation of object safety in his blog, too.

Basically, making a trait object requires an implied definition of the trait for its own trait object; in this case, that would be impl<'a, T> Trait<T> for Trait<T> + 'a. If it is possible to write meaningful definitions of all the methods, then a trait is object safe. Static methods don’t make sense in this context—what would the body of fn equality() -> bool be, with no Self type around to call the equality method on? It would need to pull a boolean out of thin air, which it respectfully declines to do.

Answer 2

Expanding on Chris' answer, what you probably want is fn equality(&self) -> bool. fn equality() -> bool is a static method, also known as an associated function. It is called as Trait::equality(), not obj.equality(), and can't access the fields and methods of the object it is called on.