Suggest correct `println` call

[estebank]

When trying to write println!(x);, suggest println!("{}", x);.

https://users.rust-lang.org/t/wtf-is-this-problem/18045

[csmoe]

Since the x may impl Display or Debug, so {:?}/{}, which one should be suggested firstly?

[ollie27]

#30143 might complicate this because the current error message for println!(x); comes from concat! not format_args! like with print!(x);.

[estebank]

@csmoe ideally we would check if x is debug or display and suggest accordingly, but we already have help text when we get that wrong, so we could (temporarily) give a best guess suggestion and let the compiler guide users with the follow up errors.

@ollie27 you're right.