error[E0214]: parenthesized type parameters may only be used with a `Fn` trait --> $DIR/unboxed-closure-sugar-used-on-struct.rs:7:15 | LL | fn foo(b: Box<Bar()>) { | ^^^^^ only `Fn` traits may use parentheses error[E0107]: struct takes 1 generic argument but 0 generic arguments were supplied --> $DIR/unboxed-closure-sugar-used-on-struct.rs:7:15 | LL | fn foo(b: Box<Bar()>) { | ^^^ expected 1 generic argument | note: struct defined here, with 1 generic parameter: `A` --> $DIR/unboxed-closure-sugar-used-on-struct.rs:3:8 | LL | struct Bar<A> { | ^^^ - help: add missing generic argument | LL | fn foo(b: Box<Bar(A)>) { | + error: aborting due to 2 previous errors Some errors have detailed explanations: E0107, E0214. For more information about an error, try `rustc --explain E0107`.