/// When we pass through a set of binders (e.g., when looking into
/// a `fn` type), we push a new bound region scope onto here. This
/// will contain the instantiated region for each region in those
/// When we pass through a set of binders (e.g., when looking into
/// a `fn` type), we push a new bound region scope onto here. This
/// will contain the instantiated region for each region in those