Rollup merge of #63691 - timvermeulen:chain-size-hint, r=scottmcm

Fix bug in iter::Chain::size_hint

`Chain::size_hint` currently ignores `self.state`, which means that the size hints of the underlying iterators are always combined regardless of the iteration state. This, of course, should only happen when the state is `ChainState::Both`.

src/libcore/iter/adapters/chain.rs

@@ -173,17 +173,23 @@ impl<A, B> Iterator for Chain<A, B> where
 
     #[inline]
     fn size_hint(&self) -> (usize, Option<usize>) {
-        let (a_lower, a_upper) = self.a.size_hint();
-        let (b_lower, b_upper) = self.b.size_hint();
+        match self.state {
+            ChainState::Both => {
+                let (a_lower, a_upper) = self.a.size_hint();
+                let (b_lower, b_upper) = self.b.size_hint();
 
-        let lower = a_lower.saturating_add(b_lower);
+                let lower = a_lower.saturating_add(b_lower);
 
-        let upper = match (a_upper, b_upper) {
-            (Some(x), Some(y)) => x.checked_add(y),
-            _ => None
-        };
+                let upper = match (a_upper, b_upper) {
+                    (Some(x), Some(y)) => x.checked_add(y),
+                    _ => None
+                };
 
-        (lower, upper)
+                (lower, upper)
+            }
+            ChainState::Front => self.a.size_hint(),
+            ChainState::Back => self.b.size_hint(),
+        }
     }
 }
 

src/libcore/tests/iter.rs

@@ -152,6 +152,54 @@ fn test_iterator_chain_find() {
     assert_eq!(iter.next(), None);
 }
 
+#[test]
+fn test_iterator_chain_size_hint() {
+    struct Iter {
+        is_empty: bool,
+    }
+
+    impl Iterator for Iter {
+        type Item = ();
+
+        // alternates between `None` and `Some(())`
+        fn next(&mut self) -> Option<Self::Item> {
+            if self.is_empty {
+                self.is_empty = false;
+                None
+            } else {
+                self.is_empty = true;
+                Some(())
+            }
+        }
+
+        fn size_hint(&self) -> (usize, Option<usize>) {
+            if self.is_empty {
+                (0, Some(0))
+            } else {
+                (1, Some(1))
+            }
+        }
+    }
+
+    impl DoubleEndedIterator for Iter {
+        fn next_back(&mut self) -> Option<Self::Item> {
+            self.next()
+        }
+    }
+
+    // this chains an iterator of length 0 with an iterator of length 1,
+    // so after calling `.next()` once, the iterator is empty and the
+    // state is `ChainState::Back`. `.size_hint()` should now disregard
+    // the size hint of the left iterator
+    let mut iter = Iter { is_empty: true }.chain(once(()));
+    assert_eq!(iter.next(), Some(()));
+    assert_eq!(iter.size_hint(), (0, Some(0)));
+
+    let mut iter = once(()).chain(Iter { is_empty: true });
+    assert_eq!(iter.next_back(), Some(()));
+    assert_eq!(iter.size_hint(), (0, Some(0)));
+}
+
 #[test]
 fn test_zip_nth() {
     let xs = [0, 1, 2, 4, 5];