[Rusky]

> Suppose, instead, we had a mechanism that allowed registering arbitrary panic hooks, and unregistering them when no longer needed, in any order. Then, we could do RAII-style resource handling: you could have a `CursesTerminal` type, which is responsible for cleaning up the terminal, and it cleans up the terminal on `Drop` and on panic. To do the latter, it would register a panic hook, and deregister that hook on `Drop`.

This doesn't get rid of unwinding at all- it's an inefficient reimplementation of it. There's a reason language implementations have switched away from having the main execution path register and unregister destructors and finally blocks, to storing them in a side table and recovering them at the time of the throw.

[JoshTriplett]

The difference is that unwinding unwinds code that isn't necessarily prepared for it, rather than only code that explicitly wants it.

And I would expect to turn it into an efficient solution, in part by doing the "store in a side table" approach for hook registration.

[Rusky]

Giving special treatment to code that "explicitly wants" to handle unwinding means two things:

* You have to know when an API can unwind, and you have to make it an error to unwind when the caller isn't expecting it. If this is done statically, you are getting into effect annotation territory. If this is done dynamically, are essentially just injecting drop bombs into code that doesn't expect unwinding. Either way, you are multiplying complexity for generic code. (Not to mention you have to invent a whole new set of idioms for panic-free code.)

* You still have to be able to clean up the resources held by a caller that does expect unwinding. So all your vocabulary/glue/library code (the stuff that can't just assume panic=abort) still needs these "scoped panic hooks" in all the same places it has any level of panic awareness in Drop today.

So for anyone to actually benefit from this, they have to be writing panic-free code with whatever new static or dynamic tools come with this, and they have to be narrowly scoped and purpose-specific enough that they could essentially already today afford panic=abort. Who is this even for?

[JoshTriplett]

To be very explicit about something: these are all vague design handwaves, and until they become not only concrete but sufficiently clear to handle use cases people have, they're not going to go anywhere. They're vague ideas we're thinking about. Right now, panic unwind isn't going anywhere.