Feature request: Expand on probe_lazy to support shared processing for multiple arguments

[VorpalBlade]

Currently, probe_lazy!(provider, name, expensive().a, expensive().b) doesn't let me share the computation of expensive() (you are at the whim of the optimiser here).

It would be good to be able to share this computation.

usdt is a crate for dtrace probes which allows this syntax:

probe_macro!(|| { let tmp = expensive(); (tmp.a, tmp.b) });
// Or any other lambda that returns a tuple of the expected length.
// They define tracepoints separately from using them, so each tracepoint generates a unique macro.

I don't know that this would work with the current design of probe but having something which lets me only invoke expensive() once would be good.

[cuviper]

You may be able to do something like this:

let tmp = std::cell::LazyCell::new(|| expensive());

probe_lazy!(foo, bar, tmp.a);

probe_lazy!(foo, baz, tmp.b);

That expensive call won't happen until the first deref of tmp, which is behind each lazy probe's guard. If both are enabled, it will still only be called once.