Add missing Copy/Clone impls for Iterators

[cramertj]

There's a bit of an open question here that probably requires some libs/lang team discussion: do we want to implement Copy for Iterator types, or are the ergonomic and performance benefits outweighed by the potential for confusion? @nikomatsakis previously proposed introducing a Copy-lint that would warn against types that usually wouldn't be Copyd. Personally, I think we should implement Copy for nearly any type possible to allow users the flexibility to make their own iterator-containing types Copy where appropriate.

[rust-highfive]

r? @dtolnay

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[eddyb]

The Copy iterator discussion has a messy past, please don't combine it with fixing the calendar example (which should be trivial wrt Copy closures)

[cramertj]

@eddyb Sure-- I'll split it out. Sorry, I think there's some history here that I'm not aware of.

[Mark-Simulacrum]

I'm inclined to say that it's unlikely we'll accept Copy iterators at this point -- from past discussion(s) the current consensus seems to be that it's too error prone to have them since accidentally copying is too easy. That may have since changed, though.

[eddyb]

@Mark-Simulacrum I thought there was a consensus that if we add lints we can get away with it.

[kennytm]

Could we impl Copy for Range if that's the case? 🤩

[Mark-Simulacrum]

Not sure. Seems okay, and probably doesn't affect too many crates in practice. I'm okay with this, but do think we should get consensus before going ahead with it from at least some of the libs team.

[eddyb]

@kennytm Well, yes, but again, it's predicated on someone actually writing those lints.
They might require operating on MIR (which recently gained lint information, so we could do it now), but I don't recall whether we know when we'd emit warnings (cc @nikomatsakis).

[nikomatsakis]

I think we should add some sort of lint. It's not that hard. Then let's aggressively add Copy.

I had previously opened #45683 for tracking that. I can put up some kind of simple, first-step proposal.

[cramertj]

Closing to keep the PR set clean until we've got the appropriate lint in place.