Callbacks should not be able to capture their arguments and hold onto them,
because the `&Lua` used in previous calls will not remain valid across calls.
One could imagine an API where the specific `&Lua` is simply stored inside the
`Scope` itself, but this is harder to do, and would (badly) encourage storing
references inside Lua userdata.
Ideally, the only way it should be possible to store Lua handles inside Lua
itself is through usafety or the `rental` crate or other self-borrowing
techniques to make references into 'static types. If at all possible this
roadblock should stay, because reference types inside userdata are almost always
going to lead to a a memory leak, and if you accept the risks you should just
use `RegistryKey` with its manual removal.
Vastly simpler and less magical than using a fixed size magical section of the
active stack, and seems to be no slower. The only real downside is that
it *seems* extremely extremely hacky (and to be fair, it is).
This simplifies the Scope lifetimes, and should make it a compile error for
scope created handles to exit the scope. This should be strictly better, as you
would never WANT to do this, but I hope that I have not caused a subtle lifetime
problem that would prevent passing those created handles back into Lua. I've
tested every situation I can think of, and it doesn't appear to be an issue, but
I admit that I don't fully understand everything involved and I could be missing
something.
The reason that I needed to do this is that if you can let a scope handle escape
the scope, you have a LuaRef with an unused registry id, and that can lead to
UB. Since not letting the scope references escape is a strict improvement
ANYWAY (if I haven't caused a lifetime issue), this is the easiest fix.
This is technically a breaking change but I think in most cases if you notice it
you would be invoking UB, or you had a function that accepted a Scope or
something. I don't know if it's worth a version bump?
kyren