Function obligations 

pub fn obligations<'tcx>(
    infcx: &InferCtxt<'tcx>,
    param_env: ParamEnv<'tcx>,
    body_id: LocalDefId,
    recursion_depth: usize,
    term: Term<'tcx>,
    span: Span,
) -> Option<PredicateObligations<'tcx>>

Returns the set of obligations needed to make term well-formed. If term contains unresolved inference variables, this may include further WF obligations. However, if term IS an unresolved inference variable, returns None, because we are not able to make any progress at all. This is to prevent cycles where we say “?0 is WF if ?0 is WF”.