BorrowPlus.rs

use std::boxed::Box;
use std::rc::Rc;
use std::borrow::Borrow;
use std::mem;

struct BP_REF;
struct BP_BOX;
struct BP_OWNED;
struct BP_RC;

struct Ref<'a, T: 'a>(&'a T);

unsafe trait BorrowPlus<Borrowed: ?Sized, TRef>
    : Borrow<Borrowed> + std::marker::Sized {
    fn borrowPlusToRC(self) -> Rc<Borrowed>
        where Self: BorrowPlus<Borrowed, BP_RC>
    {
        panic!("Impossible")
    }
    fn borrowPlusToOwned(self) -> Self
        where Self: BorrowPlus<Borrowed, BP_OWNED>
    {
        panic!("Impossible")
    }
    fn borrowPlusToBox(self) -> Box<Borrowed>
        where Self: BorrowPlus<Borrowed, BP_BOX>
    {
        panic!("Impossible")
    }
}

unsafe impl<Borrowed> BorrowPlus<Borrowed, BP_RC> for Rc<Borrowed> {
    fn borrowPlusToRC(self) -> Rc<Borrowed> {
        self
    }
}

unsafe impl<'a, Borrowed> BorrowPlus<Borrowed, BP_REF> for &'a Borrowed {}

unsafe impl<Borrowed> BorrowPlus<Borrowed, BP_BOX> for Box<Borrowed> {
    fn borrowPlusToBox(self) -> Box<Borrowed> {
        self
    }
}

unsafe impl<Borrowed> BorrowPlus<Borrowed, BP_OWNED> for Borrowed {
    fn borrowPlusToOwned(self) -> Borrowed {
        self
    }
}

fn asRc<T, BP: BorrowPlus<T, BP_RC>>(x: BP) -> Rc<T> {
    x.borrowPlusToRC()
}

fn asBox<T, BP: BorrowPlus<T, BP_BOX>>(x: BP) -> Box<T> {
    x.borrowPlusToBox()
}



fn main() {
    let x = 3;
    let y = &x;
    // let z = asRc(y); //should not compile

    let x = 3;
    let y = Rc::new(x);
    let z = asRc(y);
    ()
}