traitor

src/main.rs

 //#![feature(const_size_of)]
+#![feature(optin_builtin_traits)]
 use std::mem::{transmute, size_of};
 use std::marker::Sized;
 
@@ -25,8 +26,30 @@ where
 }
 
 
+
+trait OkInState {}
+impl OkInState for .. {}
+
+impl !OkInState for Point {}
+
+// You cannot implement it yourself
+//impl OkInState for Point {}
+
+
+struct Cont<T>
+where
+    T: OkInState,
+{
+    pub data: T,
+}
+
+// this is ok though
+struct X {}
+
 fn main() {
     let mut p = Point { x: 0, y: 1 };
+    let _c = Cont { data: X {} };
+    let _c = Cont { data: p };
     println!(" p {:?}", p);
 
     let mut x = 0;