error[E0615]: attempted to take value of method `speak` on type `cat`
  --> $DIR/assign-to-method.rs:30:8
   |
LL |   nyan.speak = || println!("meow"); //~ ERROR attempted to take value of method
   |        ^^^^^
   |
   = help: maybe a `()` to call it is missing?

error: aborting due to previous error

For more information about this error, try `rustc --explain E0615`.