error[E0277]: the `?` operator can only be used in a function that returns `Result` or `Option` (or another type that implements `std::ops::Try`) --> $DIR/try-operator-on-main.rs:9:5 | LL | std::fs::File::open("foo")?; | ^^^^^^^^^^^^^^^^^^^^^^^^^^^ cannot use the `?` operator in a function that returns `()` | = help: the trait `std::ops::Try` is not implemented for `()` = note: required by `std::ops::Try::from_error` error[E0277]: the `?` operator can only be applied to values that implement `std::ops::Try` --> $DIR/try-operator-on-main.rs:12:5 | LL | ()?; | ^^^ the `?` operator cannot be applied to type `()` | = help: the trait `std::ops::Try` is not implemented for `()` = note: required by `std::ops::Try::into_result` error[E0277]: the trait bound `(): std::ops::Try` is not satisfied --> $DIR/try-operator-on-main.rs:15:25 | LL | try_trait_generic::<()>(); | ^^ the trait `std::ops::Try` is not implemented for `()` ... LL | fn try_trait_generic() -> T { | ----------------- --- required by this bound in `try_trait_generic` error[E0277]: the `?` operator can only be applied to values that implement `std::ops::Try` --> $DIR/try-operator-on-main.rs:22:5 | LL | ()?; | ^^^ the `?` operator cannot be applied to type `()` | = help: the trait `std::ops::Try` is not implemented for `()` = note: required by `std::ops::Try::into_result` error: aborting due to 4 previous errors For more information about this error, try `rustc --explain E0277`.