Ops and driver fns for single-threaded unary union don't need mutability

geo/src/algorithm/bool_ops/mod.rs

@@ -190,12 +190,12 @@ pub trait UnaryUnion {
 // Though the operation is carried out via fold() over the tree iterator, there are two actual nested operations:
 // "fold" operations on leaf nodes build up output MultiPolygons by adding Polygons to them via union and
 // "reduce" operations on parent nodes combine these output MultiPolygons from leaf operations by recursion
-fn bottom_up_fold_reduce<T, S, I, F, R>(ops: &mut Ops<I, F, R>, parent: &ParentNode<T>) -> S
+fn bottom_up_fold_reduce<T, S, I, F, R>(ops: &Ops<I, F, R>, parent: &ParentNode<T>) -> S
 where
     T: RTreeObject,
-    I: FnMut() -> S,
-    F: FnMut(S, &T) -> S,
-    R: FnMut(S, S) -> S,
+    I: Fn() -> S,
+    F: Fn(S, &T) -> S,
+    R: Fn(S, S) -> S,
 {
     parent
         .children()
@@ -272,8 +272,8 @@ where
                 .map(|p| CachedEnvelope::new(ObjectRef::new(p)))
                 .collect(),
         );
-        let mut ops = Ops { init, fold, reduce };
-        bottom_up_fold_reduce(&mut ops, rtree.root())
+        let ops = Ops { init, fold, reduce };
+        bottom_up_fold_reduce(&ops, rtree.root())
     }
 }