Numeric cast (as operator) in generics

as_num.rs

trait FromU32 {
    fn from_u32(v: u32) -> Self;
}

macro_rules! impl_from_u32 {
    ($($ty:ty)*) => {
        $(
            impl FromU32 for $ty {
                #[inline]
                fn from_u32(v: u32) -> $ty {
                    v as $ty
                }
            }
        )*
    }
}

impl_from_u32!(u8 u16 u32 u64 usize);

impl FromU32 for bool {
    #[inline]
    fn from_u32(v: u32) -> Self {
        v != 0
    }
}

fn f<T: FromU32>() -> Option<T>
{
    let x: u32 = 42;
    Some(FromU32::from_u32(x))
}

fn main() {
    println!("{:?}", f::<bool>());
    println!("{:?}", f::<u8>());
    println!("{:?}", f::<u16>());
    println!("{:?}", f::<u32>());
    println!("{:?}", f::<u64>());
    println!("{:?}", f::<usize>());
}