Put operands on its own line when each fits in a single line

[topecongiro]

Currently rustfmt puts as many operands as possible in a single line, then after exceeding max width, it puts other operands on its own line. e.g.

        if self.items.len() == 1 && self.one_line_width != 0 && !list_items[0].has_comment()
             && !list_items[0].inner_as_ref().contains('\n')
             && ::lists::total_item_width(&list_items[0]) <= self.one_line_width

Instead, we should put each operand on its own line when it fits in a single line:

        if self.items.len() == 1
             && self.one_line_width != 0
             && !list_items[0].has_comment()
             && !list_items[0].inner_as_ref().contains('\n')
             && ::lists::total_item_width(&list_items[0]) <= self.one_line_width

Closes #2530.

[nrc]

Hmm, so this is a spec change. The style guide is a bit ambiguous, nothing is specified in the text, but the example implies multiple clauses per line. There was no discussion of this in the issue.

I think this is probably better though and we should amend the style guide.

Thanks for the PR!