Trait Identity

Source

pub trait Identity {
    type Type: ?Sized;

    const TYPE_EQ: TypeEq<Self, Self::Type>;
}

Expand description

Emulation of T == U bounds.

This trait emulates T == U bounds with T: Identity<Type = U>.

§Projection

Because this trait uses TypeEq for casting between Self and Self::Type, you can transform the arguments of that TypeEq to cast any composition of those types, e.g: cast between Vec<Self> and Vec<Self::Type>

§Example

§Type Parameter Alias

(this example requires Rust 1.61.0, because it uses trait bounds in a const fn)

use typewit::{Identity, TypeEq};
 
assert_eq!(foo(3), [3, 3]);
 
assert_eq!(foo::<&str, 2, _>("hello"), ["hello", "hello"]);


const fn foo<T, const N: usize, R>(val: T) -> R
where
    // emulates a `[T; N] == R` bound
    [T; N]: Identity<Type = R>,
    T: Copy,
{
    Identity::TYPE_EQ // returns a `TypeEq<[T; N], R>`
        .to_right([val; N]) // casts `[T; N]` to `R`
}

§Projection

Demonstrating that any projection of Self and Self::Type can be casted to each other.

use typewit::{Identity, TypeEq, type_fn};
 
assert_eq!(make_vec::<u8>(), vec![3, 5, 8]);
 
fn make_vec<T>() -> Vec<T> 
where
    T: Identity<Type = u8>
{
    let te: TypeEq<Vec<T>, Vec<u8>> = T::TYPE_EQ.project::<VecFn>();
     
    te.to_left(vec![3, 5, 8]) // casts `Vec<u8>` to `Vec<T>`
}
 
type_fn!{
    // A type-level function (TypeFn implementor) from `T` to `Vec<T>`
    struct VecFn;
    impl<T> T => Vec<T>
}