Test with non-fused iterators

library/core/tests/iter.rs

@@ -4,6 +4,43 @@ use core::cell::Cell;
 use core::convert::TryFrom;
 use core::iter::*;
 
+/// An iterator wrapper that panics whenever `next` or `next_back` is called
+/// after `None` has been returned.
+struct Unfuse<I> {
+    iter: I,
+    exhausted: bool,
+}
+
+fn unfuse<I: IntoIterator>(iter: I) -> Unfuse<I::IntoIter> {
+    Unfuse { iter: iter.into_iter(), exhausted: false }
+}
+
+impl<I> Iterator for Unfuse<I>
+where
+    I: Iterator,
+{
+    type Item = I::Item;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        assert!(!self.exhausted);
+        let next = self.iter.next();
+        self.exhausted = next.is_none();
+        next
+    }
+}
+
+impl<I> DoubleEndedIterator for Unfuse<I>
+where
+    I: DoubleEndedIterator,
+{
+    fn next_back(&mut self) -> Option<Self::Item> {
+        assert!(!self.exhausted);
+        let next = self.iter.next_back();
+        self.exhausted = next.is_none();
+        next
+    }
+}
+
 #[test]
 fn test_lt() {
     let empty: [isize; 0] = [];
@@ -148,14 +185,14 @@ fn test_iterator_chain_advance_by() {
         let len = xs.len() + ys.len();
 
         for i in 0..xs.len() {
-            let mut iter = xs.iter().chain(ys);
+            let mut iter = unfuse(xs).chain(unfuse(ys));
             iter.advance_by(i).unwrap();
             assert_eq!(iter.next(), Some(&xs[i]));
             assert_eq!(iter.advance_by(100), Err(len - i - 1));
         }
 
         for i in 0..ys.len() {
-            let mut iter = xs.iter().chain(ys);
+            let mut iter = unfuse(xs).chain(unfuse(ys));
             iter.advance_by(xs.len() + i).unwrap();
             assert_eq!(iter.next(), Some(&ys[i]));
             assert_eq!(iter.advance_by(100), Err(ys.len() - i - 1));
@@ -181,14 +218,14 @@ fn test_iterator_chain_advance_back_by() {
         let len = xs.len() + ys.len();
 
         for i in 0..ys.len() {
-            let mut iter = xs.iter().chain(ys);
+            let mut iter = unfuse(xs).chain(unfuse(ys));
             iter.advance_back_by(i).unwrap();
             assert_eq!(iter.next_back(), Some(&ys[ys.len() - i - 1]));
             assert_eq!(iter.advance_back_by(100), Err(len - i - 1));
         }
 
         for i in 0..xs.len() {
-            let mut iter = xs.iter().chain(ys);
+            let mut iter = unfuse(xs).chain(unfuse(ys));
             iter.advance_back_by(ys.len() + i).unwrap();
             assert_eq!(iter.next_back(), Some(&xs[xs.len() - i - 1]));
             assert_eq!(iter.advance_back_by(100), Err(xs.len() - i - 1));