error[E0019]: constant function contains unimplemented expression type
  --> $DIR/const_let_assign3.rs:10:9
   |
LL |         self.state = x;
   |         ^^^^^^^^^^^^^^

error[E0017]: references in constants may only refer to immutable values
  --> $DIR/const_let_assign3.rs:17:5
   |
LL |     s.foo(3); //~ ERROR references in constants may only refer to immutable values
   |     ^ constants require immutable values

error: aborting due to 2 previous errors

Some errors occurred: E0017, E0019.
For more information about an error, try `rustc --explain E0017`.