Add constants.

tests/chars.rs

@@ -11,12 +11,12 @@ fn test_chars() -> Result<(), EvalAltResult> {
     {
         assert_eq!(engine.eval::<char>(r#"let x="hello"; x[2]"#)?, 'l');
         assert_eq!(
-            engine.eval::<String>(r#"let x="hello"; x[2]='$'; x"#)?,
+            engine.eval::<String>(r#"let y="hello"; y[2]='$'; y"#)?,
             "he$lo".to_string()
         );
     }
 
-    assert!(engine.eval::<char>("'\\uhello'").is_err());
+    assert!(engine.eval::<char>(r"'\uhello'").is_err());
     assert!(engine.eval::<char>("''").is_err());
 
     Ok(())

tests/constants.rs

@@ -0,0 +1,16 @@
+use rhai::{Engine, EvalAltResult};
+
+#[test]
+fn test_constant() -> Result<(), EvalAltResult> {
+    let mut engine = Engine::new();
+
+    assert_eq!(engine.eval::<i64>("const x = 123; x")?, 123);
+
+    match engine.eval::<i64>("const x = 123; x = 42; x") {
+        Err(EvalAltResult::ErrorAssignmentToConstant(var, _)) if var == "x" => (),
+        Err(err) => return Err(err),
+        Ok(_) => panic!("expecting compilation error"),
+    }
+
+    Ok(())
+}