Uses the same UserData trait, and should at least in theory support everything
that 'static UserData does, except that any functions added that rely on
AnyUserData are pretty much useless.
Probably pretty slow and I'm not sure how to make it dramatically faster, which
is a shame because generally when you need non'-static userdata you might be
creating it kind of a lot (if it was long-lived, it would probably be 'static).
Haven't added tests yet, will do that next.
Also rename to/from/pack/unpack to pack/unpack/pack_multi/unpack_multi, I don't
know if this makes their usage clearer, and it IS a bit confusing that I'm
changing the meaning of the words 'pack' and 'unpack'
This required a lot of little adjustments where we used std's `String`
before. In downstream code, this shouldn't be necessary, as you can just
do `use rlua::String as LuaString` to disambiguate.
It, ahem "should not" be possible to exhaust lua stack space in normal usage,
and causing stack errors to be Err is slightly obnoxious. I have been wanting
to make this change for a while, and removing the callback API from tables makes
this sensible *I think*.
I can think of a couple of ways that this is not technically true, but I think
that they are acceptable, or should be handled differently.
One, you can make arbitrarily sized LuaVariadic values. I think this is maybe a
bug already, because there is an argument limit in Lua which is lower than the
stack limit. I'm not sure what happens there, but if it is a stack based panic,
(or any panic?) it is a bug.
Two, I believe that if you recurse over and over between lua -> rust -> lua ->
rust etc, and call rlua API functions, you might get a stack panic. I think for
trusted lua code, this is morally equivalent to a regular stack overflow in
plain rust, which is already.. well it's not a panic but it's some kind of safe
crash I'm not sure, so I think this is acceptable. For *untrusted* lua code,
this could theoretically be a problem if the API provided a callback that would
call back into lua, then some lua script could force a stack based panic. There
are so many concerns with untrusted lua code, and this library is NOT safe
enough yet for untrusted code (it doesn't even provide an option to limit lua to
the safe API subset yet!), so this is not currently an issue. When the library
provides support for "safe lua", it should come with big warnings anyway, and
being able to force a stack panic is pretty minor in comparison.
I think if there are other ways to cause unbounded stack usage, that it is a
bug, or there can be an error just for that situation, like argument count
limits.
This commit also fixes several stupid bugs with tests, stack checking, and
panics.
