Disallow overriding keywords.

tests/var_scope.rs

@@ -19,6 +19,32 @@ fn test_var_scope() -> Result<(), Box<EvalAltResult>> {
     Ok(())
 }
 
+#[test]
+fn test_var_is_def() -> Result<(), Box<EvalAltResult>> {
+    let engine = Engine::new();
+
+    assert!(engine.eval::<bool>(
+        r#"
+            let x = 42;
+            is_def_var("x")
+    "#
+    )?);
+    assert!(!engine.eval::<bool>(
+        r#"
+            let x = 42;
+            is_def_var("y")
+    "#
+    )?);
+    assert!(engine.eval::<bool>(
+        r#"
+            const x = 42;
+            is_def_var("x")
+    "#
+    )?);
+
+    Ok(())
+}
+
 #[test]
 fn test_scope_eval() -> Result<(), Box<EvalAltResult>> {
     let engine = Engine::new();