/// Helper type of a temporary returned by `.for_item(...)`.
/// This is necessary because we can't write the following bound:
///
-/// ```rust
+/// ```ignore (illustrative)
/// F: for<'b, 'tcx> where 'tcx FnOnce(FnCtxt<'b, 'tcx>)
/// ```
struct CheckWfFcxBuilder<'tcx> {