type_eq.rs

#![feature(specialization)]

const fn type_eq<T: ?Sized, U: ?Sized>() -> bool {
    trait Bool {
        const VALUE: bool;
    }
    
    struct True;
    
    impl Bool for True {
        const VALUE: bool = true;
    }
    
    struct False;
    
    impl Bool for False {
        const VALUE: bool = false;
    }
    
    trait TypeEq<T: ?Sized> {
        type Equals: Bool;
    }
    
    impl<T: ?Sized> TypeEq<T> for T {
        type Equals = True;
    }
    
    impl<T: ?Sized, U: ?Sized> TypeEq<T> for U {
        default type Equals = False;
    }
    
    <T as TypeEq<U>>::Equals::VALUE
}

fn main() {
    assert!(!type_eq::<String, str>());
    assert!(!type_eq::<&'static str, str>());
    assert!(type_eq::<String, String>());
}

Implement a function const fn type_eq<T: ?Sized, U: ?Sized>() -> bool that returns true or false at compile time whether two generic type parameters are the same or not. Requires specialization on nightly.