error[E0277]: `::Item` doesn't implement `std::fmt::Display` --> $DIR/issue-97760.rs:4:20 | LL | println!("{x}"); | ^ `::Item` cannot be formatted with the default formatter | = help: the trait `std::fmt::Display` is not implemented for `::Item` = note: in format strings you may be able to use `{:?}` (or {:#?} for pretty-print) instead = note: this error originates in the macro `$crate::format_args_nl` which comes from the expansion of the macro `println` (in Nightly builds, run with -Z macro-backtrace for more info) help: introduce a type parameter with a trait bound instead of using `impl Trait` | LL ~ pub fn print_values(values: &I) LL ~ where ::Item: std::fmt::Display { | error: aborting due to previous error For more information about this error, try `rustc --explain E0277`.