Fix incorrect lower bounds

src/libcore/char.rs

@@ -905,7 +905,12 @@ macro_rules! continuation_byte {
 
     #[inline]
     fn size_hint(&self) -> (usize, Option<usize>) {
-        self.0.size_hint()
+        let len = self.iter.len();
+
+        // A code point is at most 4 bytes long.
+        let min_code_points = len / 4;
+
+        (min_code_points, Some(len))
     }
 }
 

src/libcore/iter/traits.rs

@@ -903,7 +903,12 @@ fn next(&mut self) -> Option<Self::Item> {
     }
 
     fn size_hint(&self) -> (usize, Option<usize>) {
-        self.iter.size_hint()
+        if self.error.is_some() {
+            (0, Some(0))
+        } else {
+            let (_, upper) = self.iter.size_hint();
+            (0, upper)
+        }
     }
 }