Bugfix/comment duplication

[GambitingMan]

Fixes #5871

The issue was in the rewrite_paren function.

Since the span on expressions doesn't include attributes and attributes comes before the expression, this caused the attribute to be included in pre_comment since the end of the comment span was subexpr.span.lo().
I changed the logic to use the span of the first attribute, if there are any attributes present. If not, default to the expression span.

[calebcartwright]

Thanks for the PR! You've definitely identified the root cause, as well as the proper underlying technique to address the issue.

However, this is such a common type of occurrence that we've got existing utilities that can and should be reused here instead. Details linked below, but basically using our span() instead of the AST struct's span field will cover the inline logic you originally proposed

rustfmt/src/spanned.rs

Lines 43 to 55 in da7f678

	macro_rules! implement_spanned {
	($this:ty) => {
	impl Spanned for $this {
	fn span(&self) -> Span {
	span_with_attrs!(self)
	}
	}
	};
	}
	// Implement `Spanned` for structs with `attrs` field.
	implement_spanned!(ast::AssocItem);
	implement_spanned!(ast::Expr);

[GambitingMan]

That did the trick 😄

[ytmimi]

Thanks for making the changes @calebcartwright asked for. I think we're good to go here!

[calebcartwright]

Thanks for fixing this and for your first contribution!