Auto merge of #34155 - ollie27:unzip, r=alexcrichton

Remove unzip() SizeHint hack

This was using an invalid iterator so is likely to end with buggy
behaviour.

It also doesn't even benefit many type in std including Vec so removing it
shouldn't cause any problems.

Fixes: #33468

src/libcore/iter/iterator.rs

@@ -11,7 +11,6 @@
 use clone::Clone;
 use cmp::{Ord, PartialOrd, PartialEq, Ordering};
 use default::Default;
-use marker;
 use num::{Zero, One};
 use ops::{Add, FnMut, Mul};
 use option::Option::{self, Some, None};
@@ -1748,23 +1747,9 @@ pub trait Iterator {
         FromB: Default + Extend<B>,
         Self: Sized + Iterator<Item=(A, B)>,
     {
-        struct SizeHint<A>(usize, Option<usize>, marker::PhantomData<A>);
-        impl<A> Iterator for SizeHint<A> {
-            type Item = A;
-
-            fn next(&mut self) -> Option<A> { None }
-            fn size_hint(&self) -> (usize, Option<usize>) {
-                (self.0, self.1)
-            }
-        }
-
-        let (lo, hi) = self.size_hint();
         let mut ts: FromA = Default::default();
         let mut us: FromB = Default::default();
 
-        ts.extend(SizeHint(lo, hi, marker::PhantomData));
-        us.extend(SizeHint(lo, hi, marker::PhantomData));
-
         for (t, u) in self {
             ts.extend(Some(t));
             us.extend(Some(u));