error[E0308]: mismatched types
  --> $DIR/occurs-check.rs:5:9
   |
LL |     f = box f;
   |         ^^^^^ cyclic type of infinite size
   |
help: try using a conversion method
   |
LL |     f = (box f).to_string();
   |         +     +++++++++++++

error: aborting due to previous error

For more information about this error, try `rustc --explain E0308`.