error[E0308]: mismatched types
  --> $DIR/existential-printing.rs:7:13
   |
LL | fn f() -> impl FnOnce<()> { || () }
   |           --------------- the expected opaque type
LL |
LL | fn main() { () = f(); }
   |             ^^   --- this expression has type `impl FnOnce<()>`
   |             |
   |             expected opaque type, found `()`
   |
   = note: expected opaque type `impl FnOnce<()>`
                found unit type `()`

error: aborting due to 1 previous error

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