Implement RandomAccessIterator where possible

[bluss]

Adaptors like Enumerate, Map, Zip, Take etc need to implement Random access.

I'm working on this.

[huonw]

One has to be careful with Map because it will mean that the mapped function can get evaluated multiple times (if you access a specific element more than once).

[bluss]

That's correct. Since the introduction of double-ended iterator, you can't be sure of the order the map function is invoked either (could be alternating front and back). I don't know how to best handle this.

[bluss]

We could provide a separate adaptor that only takes an extern fn. That Map adaptor will be clonable and random access, and the standard one will not? In the end we don't have any purity guarantees either way.

[huonw]

Not sure that such a restriction is worth it, either no functions or all functions (at least until we get an effects system); since there's no guarantee that extern fn's are idempotent/pure (e.g. fn increment_and_get_network_resource, or something using TLS), and many closure would be (e.g. |&x| x).

[emberian]

Hasn't been fixed entirely, #8120 added a bunch of impls.

[bluss]

"RAI" seems to never have reached usefulness. I'm still not sure if it should even be in rust.

[steveklabnik]

This ticket isn't particularly actionable, if anything is still missing, please open a new one with specifics, thanks.