That isn't an implementation of addition between Spans and other Spans. It looks like there isn't one in the library right now. `impl<'a> Add<Span> for &'a Zoned` means a borrow of Zoned is on the left hand side, and a Span on the right. So it says that if z is a Zoned (not a Span) and s is a Span, you can do `&z + s` to add a span to a Zoned. There are a bunch of implementations there, DateTime + Span, Date + Span, Time + Span, Offset + Span. All with Span on the right, but none for Span + Span (nor Span + &Span, or &Span + &Span, ...).
This is correct. You can't do a `span1 + span2`. You'd have to use `span1.checked_add(span2)`. The main problem I had with overloading `+` for span addition is that, in order to add two spans with non-uniform units (like years and months), you need a relative datetime. So `+` would effectively have to panic if you did `1.year() + 2.months()`, which seems like a horrific footgun.
It would be plausible to make `+` for spans do _only_ component wise addition, but this would be an extremely subtle distinction between `+` and `Span::checked_add`. To the point where sometimes `+` and `checked_add` would agree on the results and sometimes they wouldn't. I think that would also be bad.
So I started conservative for the time being: no `+` for adding spans together.
wging|1 year ago
burntsushi|1 year ago
It would be plausible to make `+` for spans do _only_ component wise addition, but this would be an extremely subtle distinction between `+` and `Span::checked_add`. To the point where sometimes `+` and `checked_add` would agree on the results and sometimes they wouldn't. I think that would also be bad.
So I started conservative for the time being: no `+` for adding spans together.