Function tuple_field_len_kind 

fn tuple_field_len_kind<'tcx>(
    body: &Body<'tcx>,
    operand: &Operand<'tcx>,
    field: usize,
) -> Option<TupleFieldLen>

Classify the length field of a (.., len, ..) tuple, tracing through copy/move temps and a _tmp = (..) tuple aggregate assignment.