Function mono_reachable_as_bitset 

pub fn mono_reachable_as_bitset<'a, 'tcx>(
    body: &'a Body<'tcx>,
    tcx: TyCtxt<'tcx>,
    instance: Instance<'tcx>,
) -> DenseBitSet<BasicBlock>

MonoReachable internally accumulates a DenseBitSet of visited blocks. This is just a convenience function to run that traversal then extract its set of reached blocks.