useless_format should only warn for String

[Manishearth]

It currently warns for format!("{}", e); where e is anything implementing Show.

cc @mcarton

[oli-obk]

The warning is correct imo, e.to_string() is much more obvious. So only the hint should be adjusted.

[mcarton]

Show hasn’t been a thing for a long time now (in Rust history) :p

I that case I like e.to_string() better, no?

[Manishearth]

Using format in lieu of .to_string() is a common pattern, though.

[mcarton]

They are equivalent anyway as the trait implementation is impl<T> ToString for T where T: Display + ?Sized so ok, I’ll limit it to String and &str.

[llogiq]

I still think .to_string() is more readable. Maybe re-add it as an allowed lint?