Function update_res 

fn update_res(
    cx: &LateContext<'_>,
    parent_certainty: Certainty,
    path_segment: &PathSegment<'_>,
    resolves_to_type: bool,
) -> Option<Res>

For at least some QPath::TypeRelative, the path segment’s res can be Res::Err. update_res tries to fix the resolution when parent_certainty is Certain(Some(..)).