error[E0214]: parenthesized parameters may only be used with a trait
  --> $DIR/unboxed-closure-sugar-used-on-struct.rs:7:18
   |
LL | fn foo(b: Box<Bar()>) {
   |                  ^^ only traits may use parentheses

error[E0121]: the type placeholder `_` is not allowed within types on item signatures
  --> $DIR/unboxed-closure-sugar-used-on-struct.rs:7:15
   |
LL | fn foo(b: Box<Bar()>) {
   |               ^^^^^ not allowed in type signatures

error: aborting due to 2 previous errors

Some errors occurred: E0121, E0214.
For more information about an error, try `rustc --explain E0121`.