Simplify if let.

src/func/call.rs

@@ -577,11 +577,12 @@ impl Engine {
             }
 
             let mut empty_scope;
-            let scope = if let Some(scope) = _scope {
-                scope
-            } else {
-                empty_scope = Scope::new();
-                &mut empty_scope
+            let scope = match _scope {
+                Some(scope) => scope,
+                None => {
+                    empty_scope = Scope::new();
+                    &mut empty_scope
+                }
             };
 
             let result = if _is_method_call {