Shared via Rust Playground

playground.rs

struct Unsigned {
   v: u32
}

trait Nat {
   fn set(&self, Unsigned) -> (); // accepts any unsigned value
   fn get(&self) -> Unsigned;     // returns any unsigned value
}

trait Pos {
   fn set(&self, Unsigned) -> (); // does not accept the 0 value input
   fn get(&self) -> Unsigned;     // must only return positive integers > 0
}

trait NatPos: Nat + Pos {
   fn set(&self, Unsigned) -> (); // accepts any unsigned value
   fn get(&self) -> Unsigned;     // returns any unsigned value
}

impl Nat for Unsigned {
   fn set(&self, v: Unsigned) -> () {
      self.v = v.v;
   }
   fn get(&self) -> Unsigned { return Unsigned{ v : self.v }; }
}

impl Pos for Unsigned {
   fn set(&self, v: Unsigned) -> () {
      if v.v < 1 { panic!() }
      self.v = v.v;
   }
   fn get(&self) -> Unsigned { return Unsigned{ v : self.v }; }
}

impl NatPos for Unsigned {
   fn set(&self, v: Unsigned) -> () {
      self.v = v.v;
   }
   fn get(&self) -> Unsigned { return Unsigned{ v : self.v }; }
}

fn main() {
   let mut x: &NatPos = &Unsigned{ v : 1 };
   let z: &Pos = &x;
   {
      let mut y: &Nat = &x;
      y.set(Unsigned{ v : 0});
   }
   println!("Positive integer: {}", z.get().v); // error! prints "Positive integer: 0"
}