use_debug in Debug implementation

[jsim2010]

In my opinion, clippy::use_debug should allow the debug format "{:?}" within Debug trait implementations. This seems to be backed up by the comment in the test here:

rust-clippy/tests/ui/print.rs

Lines 15 to 20 in da7aebc

	impl Debug for Foo {
	fn fmt(&self, f: &mut Formatter) -> Result {
	// ok, we can use `Debug` formatting in `Debug` implementations
	write!(f, "{:?}", 42.718)
	}
	}

However, the expected output actually expects an error in this case

rust-clippy/tests/ui/print.stderr

Lines 9 to 13 in da7aebc

	error: use of `Debug`-based formatting
	--> $DIR/print.rs:18:19
	|
	LL | write!(f, "{:?}", 42.718)
	| ^^^^^^

I propose that this error be removed from the test and the code adjusted to allow for debug formatting in Debug. If this is not desired, then the comment in tests/ui/print.rs should be removed to avoid confusion.

[JohnTitor]

I think it's a regression caused by #2949.