add in-place iteration for Zip

this picks the left hand side as source since it might be more natural to
consume that as IntoIter source

Author: the8472

library/core/src/iter/adapters/zip.rs

@@ -1,7 +1,10 @@
 use crate::cmp;
 use crate::fmt::{self, Debug};
 
-use super::super::{DoubleEndedIterator, ExactSizeIterator, FusedIterator, Iterator, TrustedLen};
+use super::super::{
+    DoubleEndedIterator, ExactSizeIterator, FusedIterator, InPlaceIterable, Iterator, SourceIter,
+    TrustedLen,
+};
 
 /// An iterator that iterates two other iterators simultaneously.
 ///
@@ -327,6 +330,26 @@ where
 {
 }
 
+// Arbitrarily selects the left side of the zip iteration as extractable "source"
+// it would require negative trait bounds to be able to try both
+#[unstable(issue = "0", feature = "inplace_iteration")]
+unsafe impl<S, A, B> SourceIter for Zip<A, B>
+    where
+        A: SourceIter<Source = S>,
+        B: Iterator,
+        S: Iterator,
+{
+    type Source = S;
+
+    #[inline]
+    fn as_inner(&mut self) -> &mut S {
+        SourceIter::as_inner(&mut self.a)
+    }
+}
+
+#[unstable(issue = "0", feature = "inplace_iteration")]
+unsafe impl<A: InPlaceIterable, B: Iterator> InPlaceIterable for Zip<A, B> {}
+
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<A: Debug, B: Debug> Debug for Zip<A, B> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {