Fuse FlattenCompat's inner iterator

Author: timvermeulen

src/libcore/iter/adapters/flatten.rs

@@ -1,7 +1,7 @@
 use crate::fmt;
 use crate::ops::Try;
 
-use super::super::{DoubleEndedIterator, FusedIterator, Iterator};
+use super::super::{DoubleEndedIterator, Fuse, FusedIterator, Iterator};
 use super::Map;
 
 /// An iterator that maps each element to an iterator, and yields the elements
@@ -239,14 +239,17 @@ where
 /// this type.
 #[derive(Clone, Debug)]
 struct FlattenCompat<I, U> {
-    iter: I,
+    iter: Fuse<I>,
     frontiter: Option<U>,
     backiter: Option<U>,
 }
-impl<I, U> FlattenCompat<I, U> {
+impl<I, U> FlattenCompat<I, U>
+where
+    I: Iterator,
+{
     /// Adapts an iterator by flattening it, for use in `flatten()` and `flat_map()`.
     fn new(iter: I) -> FlattenCompat<I, U> {
-        FlattenCompat { iter, frontiter: None, backiter: None }
+        FlattenCompat { iter: iter.fuse(), frontiter: None, backiter: None }
     }
 }