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Some game developers would significantly benefit from a slightly stripped-down[1] Lua C API as a built-in Lua module, as it would give the ability to create perfectly-isolated sub-states from Lua.

Because you'd be able to register (host) Lua functions as (guest) C functions, they'd appear as opaque C functions to the sub-states, which means the debug.* API could be trivially (and safely!) made available to them. For example:

GUEST | call os.time()
HOST | call guestostime()
HOST | <operations to implement the guest's os.time()>
HOST | return to guest
GUEST | get result created by host
GUEST | call debug.* functions against os.time, but get no useful results other than it being a C function.

It is technically possible to implement this in pure-Lua as it currently stands, at a hefty amount of overhead, both in terms of code/maintenance overhead, and CPU time. (it basically involves reimplementing the debug.* API by checking if the given function is a valid target for the debug API, and carefully wrapping host functions so they look like C functions to the guest, and so on.)

[1] - We don't want to expose luaL_newstate, we want lua_newstate to use the current state's allocator and stuff, and things like that.

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