Function remove_successors_from_switch 

pub(crate) fn remove_successors_from_switch<'tcx>(
    tcx: TyCtxt<'tcx>,
    bb: BasicBlock,
    body: &Body<'tcx>,
    patch: &mut MirPatch<'tcx>,
    is_unreachable_block: impl Fn(BasicBlock) -> bool,
) -> bool

Return whether the current terminator is fully unreachable.