Add Extend impls for tuples of arity 1 through 12

[shahn]

No description provided.

[rustbot]

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[shahn]

This was discussed in #85835 (comment) and I want to get that effort started. It's been forever since I contributed to rustc, so please excuse if I did something wrong. If this is deemed OK, I would be happy to follow this up with the same treatmeant for FromIterator and maybe `unzip, too - but those probably deserve their own PR?

[scottmcm]

r? libs-api

[dtolnay]

@rust-lang/libs-api:
@rfcbot fcp merge

Among other things, this feature makes unzip work for larger (and smaller) tuples.
Among other things, this feature makes collect_into (which is still unstable) work for larger tuples.

  fn main() {
      let iter = std::iter::repeat_n(('a', 'b', 'c'), 3);

-     let mut collection = (Vec::new(), (Vec::new(), Vec::new()));
-     iter.map(|(a, b, c)| (a, (b, c))).collect_into(&mut collection);
-     let (va, (vb, vc)) = collection;
+     let mut collections = (Vec::new(), Vec::new(), Vec::new());
+     iter.collect_into(&mut collections);
+     let (va, vb, vc) = collections;
      println!("{:?}\n{:?}\n{:?}", va, vb, vc);
}

[rfcbot]

Team member @dtolnay has proposed to merge this. The next step is review by the rest of the tagged team members:

• @Amanieu
• @BurntSushi
• @dtolnay
• @joshtriplett
• @m-ou-se

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[cuviper]

Among other things, this feature makes unzip work for larger (and smaller) tuples.

How so? This doesn't change Iterator::unzip AFAICS, which explicitly operates on pairs -- but either of those pairs could be a nested tuple of arity 1..=12 after this change.

[dtolnay]

You are right. I've updated the motivating use case to use collect_into instead of unzip. Or please let me know if someone knows a better one.

[shahn]

I noticed I made a silly mistake, I added tuple impls for not just 12-tuples but 13-tuples accidentally. I am also adding some tests for these impls, but I wonder, is there a good example to verify 13-tuples do not have the Extend impl?

I also want to add the FromIterator<(EA, ...)> for (A, ...) impls. Should these go into a new PR? That would enable this:

    let a = vec![(1, 2, 3), (4, 5, 6), (7, 8, 9)];
    let b: (Vec<_>, Vec<_>, Vec<_>) = a.into_iter().collect();