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#![no_std]
//! # Equality constraints for types
//!
//! `is_type` gives the trait `Is<X>` which acts like an equality constraint.
//!
//! There's also functions to move between these two equivalent types,
//! as Rust will still not realise these types are
//!
//! The constraint `X : Is<Y>` requires that `X` == `Y`.
//!
//! The idea was stolen from
//! [this comment on equality constraints in where clauses] which
//! contains an example use case.
//!
//! [this comment on equality constraints in where clauses]:
//! https://github.com/rust-lang/rust/issues/20041#issuecomment-414551783
//!
//! Note that `into_val` and `from_val` are basically `into` and `from`,
//! but for this trait to work universally we need a universal instance,
//! which one can't define on `Into` or `From`.
pub trait Is {
    type Type: ?Sized;

    fn into_val(self) -> Self::Type;
    fn into_ref(&self) -> &Self::Type;
    fn into_mut_ref(&mut self) -> &mut Self::Type;

    fn from_val(x: Self::Type) -> Self;
    fn from_ref(x: &Self::Type) -> &Self;
    fn from_mut_ref(x: &mut Self::Type) -> &mut Self;
}

impl<T> Is for T {
    type Type = T;

    fn into_val(self) -> Self::Type {
        self
    }

    fn into_ref(&self) -> &Self::Type {
        self
    }

    fn into_mut_ref(&mut self) -> &mut Self::Type {
        self
    }

    fn from_val(x: Self::Type) -> Self {
        x
    }

    fn from_ref(x: &Self::Type) -> &Self {
        x
    }

    fn from_mut_ref(x: &mut Self::Type) -> &mut Self {
        x
    }
}