list/tuple/sequence: implement Index

[birkenfeld]

See #1667

[davidhewitt]

👍 looks like a great start!

What do you think of supporting full slice indexing? I think this means either implementing SliceIndex or just implementing Index directly for Range<usize>, RangeFull, RangeInclusive<usize> etc.

[birkenfeld]

Yeah, I though about it, but that runs into API consistency again: do we want to panic on invalid indices or not?
In the end, I didn't (and don't) care about it enough to make the change.

[davidhewitt]

That's fair enough. I think for out of bounds slice indexing we should probably panic (to match Rust), given that we already deviated from Python by not supporting negative indexing. That also has the nice dual that again get_slice is the non-panicking companion, even if it doesn't return an error.

I'm happy to implement SliceIndex myself in a follow-up.