Fix construction of UseGroup containing crate roots

Author: dtolnay

src/item.rs

@@ -1411,14 +1411,16 @@ pub(crate) mod parsing {
  &content,
  allow_crate_root_in_path && !this_tree_starts_with_crate_root,
  )? {
- Some(tree) => items.push_value(tree),
- None => has_any_crate_root_in_path = true,
+ Some(tree) if !has_any_crate_root_in_path => items.push_value(tree),
+ _ => has_any_crate_root_in_path = true,
  }
  if content.is_empty() {
  break;
  }
  let comma: Token![,] = content.parse()?;
- items.push_punct(comma);
+ if !has_any_crate_root_in_path {
+ items.push_punct(comma);
+ }
  }
  if has_any_crate_root_in_path {
  Ok(None)