TryFrom for Range<U> to Range<V>?

help
1

Hi,

I have some functions which make more sense to return Range<u64> over Range<usize>, mostly because I'm parsing data which I know is going to be the u64 type, and returning usize means that 32-bit software can't parse 64-bit values correctly, which is... wrong, especially since u64 works on 32-bit platforms.

Currently, I'm using a function similar to:

pub fn as_range_usize<T: TryInto<usize>>(range: Range<T>) -> Result<Range<usize>, T::Error> {
    Ok(Range {
        start: range.start.try_into()?,
        end: range.end.try_into()?,
    })
}

However, it would be nice to be able to just do let range: Range<usize> = a_u64_range.try_into()?. Unfortunately, this isn't possible, due to orphan rules (I think).

I could implement a newtype wrapper of Range, but this is just annoying. Is there:
a) Something I'm missing to do this in a good way, or,
b) Any reason why the traits

impl<T> TryFrom<Range<T>> for Range<usize> where usize: TryFrom<T> { /* impl */ }
// or even the more general
impl<U, V> TryFrom<Range<U>> for Range<V> where V: TryFrom<U> { /* impl */ }

are not implemented on Range?

2

Overlapping implementations.

  1. There's a blanket T: From<T>
  2. That results in a T: Into<T> for any T
  3. That results in T: TryFrom<T> for any T
    a. (That results in T: TryInto<T> for any T)

Your suggested implementation in combination with 3 (with T = usize) means that you're trying to implement TryFrom<Range<usize>> for Range<usize> which conflicts with 3 (with T = Range<usize>).

The implementations could exist in std,[1] but they'd be on a laundry-list of specific types, like they are for NonZero<T>.

You could use a custom trait (or traits) instead, so you can use method call syntax.

  1. core technically ↩︎

3

Thanks, that's helpful. Defining a trait would probably be the way to go.

Ok. Slightly rephrasing my question, is there any reason why this wouldn't be the case? I looked at open issues and couldn't find anything about such a thing.

4

Not that I'm aware of. You could seek library team feedback via IRLO or ACP or Zulip.

5

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