error[E0017]: references in constants may only refer to immutable values
  --> $DIR/const_let_assign3.rs:16:5
   |
LL |     s.foo(3); //~ ERROR references in constants may only refer to immutable values
   |     ^ constants require immutable values

error: aborting due to previous error

For more information about this error, try `rustc --explain E0017`.