Immutable 'drain_filter' predicate

client/finality-grandpa/src/authorities.rs

@@ -229,7 +229,7 @@ where
 	where
 		F: Fn(&H, &H) -> Result<bool, E>,
 	{
-		let mut filter = |node_hash: &H, node_num: &N, _: &PendingChange<H, N>| {
+		let filter = |node_hash: &H, node_num: &N, _: &PendingChange<H, N>| {
 			if number >= *node_num &&
 				(is_descendent_of(node_hash, &hash).unwrap_or_default() || *node_hash == hash)
 			{
@@ -245,7 +245,7 @@ where
 		};
 
 		// Remove standard changes.
-		let _ = self.pending_standard_changes.drain_filter(&mut filter);
+		let _ = self.pending_standard_changes.drain_filter(&filter);
 
 		// Remove forced changes.
 		self.pending_forced_changes

utils/fork-tree/src/lib.rs

@@ -670,14 +670,16 @@ where
 	}
 
 	/// Remove from the tree some nodes (and their subtrees) using a `filter` predicate.
+	///
 	/// The `filter` is called over tree nodes and returns a filter action:
 	/// - `Remove` if the node and its subtree should be removed;
 	/// - `KeepNode` if we should maintain the node and keep processing the tree.
 	/// - `KeepTree` if we should maintain the node and its entire subtree.
+	///
 	/// An iterator over all the pruned nodes is returned.
-	pub fn drain_filter<F>(&mut self, mut filter: F) -> impl Iterator<Item = (H, N, V)>
+	pub fn drain_filter<F>(&mut self, filter: F) -> impl Iterator<Item = (H, N, V)>
 	where
-		F: FnMut(&H, &N, &V) -> FilterAction,
+		F: Fn(&H, &N, &V) -> FilterAction,
 	{
 		let mut removed = vec![];
 		let mut retained = Vec::new();