fix Const generics are handled incorrectly

[rchaser53]

fix: #3554

[topecongiro]

Thank you for the PR! The fix looks ok, though it seems a bit hacky. The actual problem is that we are somehow passing an invalid span to recover_missing_comment_in_span. I suspect it is because of the opening brace in const generics, which messes up the BytePos created below:

rustfmt/src/visitor.rs

Lines 363 to 365 in 531b2d9

	let where_span_end = snippet
	.find_uncommented("{")
	.map(|x| BytePos(x as u32) + source!(self, item.span).lo());

I think that the where_span_end should be calculated from the span between the self type of impl and its closing brace. E.g., like the code snippet below:

                ast::ItemKind::Impl(_, _, _, _, _, ref self_ty, _) => {
                    let misssing_span = mk_sp(self_ty.span.hi(), item.span.hi());
                    let where_span_end = self.snippet_provider.opt_span_before(misssing_span, "{");

[rchaser53]

Good idea. I fixed it.

[scampi]

Rand is failing because of the rename of format_doc_comments.