The current liveness code does a simple liveness computation (actually a few such things) and tells you when e.g. assignments are dead and that sort of thing. It does this on the HIR. It would be better to do this on the MIR — in fact, the NLL computation is already computing liveness across all of MIR, so we ought to be able to piggy back on those results I imagine.

It may be a good idea to wait though until the MIR borrowck stuff "settles down" a bit before attempting this.