Changes like #52767 and existence of the useless_format clippy lint show that people end up writing format!("{}", a_string) way more often than they should (i.e., >0 times).

Since format is a proc macro/builtin in the compiler, and since this specific case should not have any side effects other than producing a String, I want to propose adding this special case:

When expanding a call of the format macro (and before using format_args!), check if the formatting string is literally "{}", and, if the only other parameter x is either of type &str, String, or Cow<str>, expand the macro to ToString::to_string(x).

This is quite a conservative and concrete refinement. It could easily be adjusted to support more string-like types.

And additional special case to consider is format!("foo") (with only a literal formatting string). It should expand to String::from("foo").