error[E0618]: expected function, found `usize`
  --> $DIR/const-as-fn.rs:4:5
   |
LL | const FOO: usize = 0;
   | ---------------- `FOO` defined here
...
LL |     FOO();
   |     ^^^--
   |     |
   |     call expression requires function

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0618`.