error[E0277]: `()` doesn't implement `std::fmt::Display` --> $DIR/doesnt-satisfy.rs:9:17 | LL | fn bar() -> () {} | ^^ `()` cannot be formatted with the default formatter | = help: the trait `std::fmt::Display` is not implemented for `()` = note: in format strings you may be able to use `{:?}` (or {:#?} for pretty-print) instead note: required by a bound in `Foo::{opaque#0}` --> $DIR/doesnt-satisfy.rs:5:22 | LL | fn bar() -> impl std::fmt::Display; | ^^^^^^^^^^^^^^^^^ required by this bound in `Foo::{opaque#0}` error: aborting due to previous error For more information about this error, try `rustc --explain E0277`.