Ideas about the rust type system

Reference <: Ownership

// Copyright 2022 Google LLC.
// SPDX-License-Identifier: Apache-2.0

trait ToRef<T> {
    fn to_ref(&self) -> &T;
}
trait ToRefMut<T>: ToRef<T> {
    fn to_mut(&mut self) -> &mut T;
}

impl<T> ToRef<T> for T {
    fn to_ref(&self) -> &T {
        self
    }
}
impl<T> ToRefMut<T> for T {
    fn to_mut(&mut self) -> &mut T {
        self
    }
}

impl<T> ToRef<T> for &mut T {
    fn to_ref(&self) -> &T {
        self
    }
}
impl<T> ToRefMut<T> for &mut T {
    fn to_mut(&mut self) -> &mut T {
        self
    }
}

impl<T> ToRef<T> for &T {
    fn to_ref(self: &Self) -> &T {
        self
    }
}

fn f(a: impl ToRef<u32>) {
    let a = a.to_ref();
    dbg!(a);
}

fn g(mut a: impl ToRefMut<u32>) {
    let a = a.to_mut();
    *a += 1;
    dbg!(a);
}

fn main() {
    let mut a = 3;
    
    f(&a);
    g(&mut a);
    g(a); // This gets a copy
    f(a); // This gets the old `a`.
}