Trap &mut String parameters.

tests/string.rs

@@ -64,6 +64,25 @@ fn test_string_dynamic() -> Result<(), Box<EvalAltResult>> {
     Ok(())
 }
 
+#[test]
+fn test_string_mut() -> Result<(), Box<EvalAltResult>> {
+    let mut engine = Engine::new();
+
+    engine.register_fn("foo", |x: INT, s: &str| s.len() as INT + x);
+    engine.register_fn("bar", |x: INT, s: String| s.len() as INT + x);
+    engine.register_fn("baz", |s: &mut String| s.len());
+
+    assert_eq!(engine.eval::<INT>(r#"foo(1, "hello")"#)?, 6);
+    assert_eq!(engine.eval::<INT>(r#"bar(1, "hello")"#)?, 6);
+    assert!(
+        matches!(*engine.eval::<INT>(r#"baz("hello")"#).expect_err("should error"),
+            EvalAltResult::ErrorFunctionNotFound(f, _) if f == "baz (&str | ImmutableString | String)"
+        )
+    );
+
+    Ok(())
+}
+
 #[cfg(not(feature = "no_object"))]
 #[test]
 fn test_string_substring() -> Result<(), Box<EvalAltResult>> {