Enable dot operations on constant variables.

tests/side_effects.rs

@@ -0,0 +1,39 @@
+use rhai::{Engine, EvalAltResult, RegisterFn, Scope};
+use std::cell::Cell;
+use std::rc::Rc;
+
+#[derive(Debug, Clone)]
+struct CommandWrapper {
+    value: Rc<Cell<i64>>,
+}
+
+impl CommandWrapper {
+    pub fn set_value(&mut self, x: i64) {
+        let val = self.value.get();
+        self.value.set(val + x);
+    }
+}
+
+#[test]
+fn test_side_effects() -> Result<(), EvalAltResult> {
+    let mut engine = Engine::new();
+    let mut scope = Scope::new();
+
+    let payload = Rc::new(Cell::new(12));
+    assert_eq!(payload.get(), 12);
+
+    let command = CommandWrapper {
+        value: payload.clone(),
+    };
+
+    scope.push_constant("Command", command);
+
+    engine.register_type_with_name::<CommandWrapper>("CommandType");
+    engine.register_fn("action", CommandWrapper::set_value);
+
+    engine.eval_with_scope::<()>(&mut scope, "Command.action(30)")?;
+
+    assert_eq!(payload.get(), 42);
+
+    Ok(())
+}