Implement FusedIterator and size_hint for DrainFilter

Author: mbrubeck

src/map.rs

@@ -1390,8 +1390,15 @@ where
     fn next(&mut self) -> Option<Self::Item> {
         self.inner.next(&mut self.f)
     }
+
+    #[inline]
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        (0, self.inner.iter.size_hint().1)
+    }
 }
 
+impl<K, V, F> FusedIterator for DrainFilter<'_, K, V, F> where F: FnMut(&K, &mut V) -> bool {}
+
 /// Portions of `DrainFilter` shared with `set::DrainFilter`
 pub(super) struct DrainFilterInner<'a, K, V> {
     pub iter: RawIter<(K, V)>,

src/set.rs

@@ -1462,8 +1462,15 @@ where
         let (k, _) = self.inner.next(&mut |k, _| f(k))?;
         Some(k)
     }
+
+    #[inline]
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        (0, self.inner.iter.size_hint().1)
+    }
 }
 
+impl<K, F> FusedIterator for DrainFilter<'_, K, F> where F: FnMut(&K) -> bool {}
+
 impl<T, S> Clone for Intersection<'_, T, S> {
     #[cfg_attr(feature = "inline-more", inline)]
     fn clone(&self) -> Self {