error[E0214]: parenthesized type parameters may only be used with a `Fn` trait --> $DIR/issue-66286.rs:8:22 | LL | pub extern fn foo(_: Vec(u32)) -> u32 { | ^^^^^^^^ only `Fn` traits may use parentheses | help: use angle brackets instead | LL | pub extern fn foo(_: Vec) -> u32 { | ~ ~ error: aborting due to 1 previous error For more information about this error, try `rustc --explain E0214`.