use num_traits::PrimInt;

/// this only exists to be able to instantiate the value via `Default::default()`.
/// It could, however, be useful for adding other methods too.
trait MyPrimInt: PrimInt + Default {}

macro_rules! impl_my_prim_int {
    ($($ty:ident)*) => ($(impl MyPrimInt for $ty {})*)
}

impl_my_prim_int! { u8 u16 u32 u64 usize i8 i16 i32 i64 isize }

trait MakeOne {
    fn make_one() -> Self;
}

impl MakeOne for String {
    fn make_one() -> Self {
        String::new()
    }
}

impl<T: MyPrimInt> MakeOne for T {
    fn make_one() -> Self {
        Default::default()
    }
}

#[derive(Debug)]
struct TheStruct<T> {
    val: T,
}

impl<T: MakeOne> TheStruct<T> {
    fn new() -> Self {
        TheStruct {
            val: T::make_one()
        }
    }
}

fn main() {
    let str = TheStruct::<String>::new();
    println!("str = {:?}", str);

    let int = TheStruct::<i16>::new();
    println!("int = {:?}", int);
}