borrow_check:: path_utils:: is_upvar_field_projection
pub(crate) fn is_upvar_field_projection<'tcx>(
) -> Option<Field>
place is a field projection, and the field is being projected from a closure type,
then returns the index of the field being projected. Note that this closure will always
self in the current MIR, because that is the only time we directly access the fields
of a closure type.