error[E0369]: binary operation `==` cannot be applied to type `fn() {f}`
  --> $DIR/fn-compare-mismatch.rs:4:15
   |
LL |     let x = f == g;
   |             - ^^ - fn() {g}
   |             |
   |             fn() {f}
   |
help: use parentheses to call these
   |
LL |     let x = f() == g();
   |              ++     ++

error[E0308]: mismatched types
  --> $DIR/fn-compare-mismatch.rs:4:18
   |
LL |     let x = f == g;
   |                  ^ expected fn item, found a different fn item
   |
   = note: expected fn item `fn() {f}`
              found fn item `fn() {g}`

error: aborting due to 2 previous errors

Some errors have detailed explanations: E0308, E0369.
For more information about an error, try `rustc --explain E0308`.