Function dep_callee_resolved_def_id 

pub(crate) fn dep_callee_resolved_def_id<'tcx>(
    tcx: TyCtxt<'tcx>,
    caller: DefId,
    func: &Operand<'tcx>,
) -> Option<DefId>

Like dep_callee_def_id, but resolves trait-method callees to their concrete impl so cross-crate Deref/DerefMut bodies — whose trait-method DefId has no available MIR — can still be inlined.