Add trait for signed to unsigned conversion

[sffc]

I would like a trait such as

pub trait ToUnsigned {
    type UnsignedType: Unsigned;

    fn abs_unsigned(&self) -> Self::UnsignedType;
}

// Example for i32
impl ToUnsigned for i32 {
    type UnsignedType = u32;

    fn abs_unsigned(&self) -> Self::UnsignedType {
        self.abs() as Self::UnsignedType
    }
}

// Example for u32
impl ToUnsigned for u32 {
    type UnsignedType = u32;

    fn abs_unsigned(&self) -> Self::UnsignedType {
        *self
    }
}

Ideally, I would like the traits to "bubble" across the UnsignedType. For example, if my function accepts a T: ToUnsigned + Integer, then I would ideally want to be able to call .abs_unsigned() on T and get back something implementing both Unsigned and Integer.

[cuviper]

I'm open to it. There's a similar request in rust-num/num-traits#163.

Ideally, I would like the traits to "bubble" across the UnsignedType. For example, if my function accepts a T: ToUnsigned + Integer, then I would ideally want to be able to call .abs_unsigned() on T and get back something implementing both Unsigned and Integer.

The way you've written the trait will always get you Unsigned, but I don't think it's possible to express the conditional part in an automatic way. You can manually write that like:

where
    T: ToUnsigned + Integer,
    <T as ToUnsigned>::UnsignedType: Integer,

You might try to make a wrapper trait:

pub trait ToUnsignedInteger: ToUnsigned + Integer
where
    <Self as ToUnsigned>::UnsignedType: Integer
{}

But I think that added constraint will have limited use do to rust-lang/rust#20671.

[sffc]

One snag with the trait definitions in my OP: they panic when given std::i32::MIN since it is not possible to call .abs() on that value. The actual trait implementations should handle this case.