8.1 – Changes in the Language

• The concept of environment changed. Only Lua functions have environments. To set the environment of a Lua function, use the variable _ENV or the function load.

  C functions no longer have environments. Use an upvalue with a shared table if you need to keep shared state among several C functions. (You may use luaL_setfuncs to open a C library with all functions sharing a common upvalue.)

  To manipulate the “environment” of a userdata (which is now called user value), use the new functions lua_getuservalue and lua_setuservalue.

• Lua identifiers cannot use locale-dependent letters.

• Doing a step or a full collection in the garbage collector does not restart the collector if it has been stopped.
• Weak tables with weak keys now perform like ephemeron tables.
• The event tail return in debug hooks was removed. Instead, tail calls generate a special new event, tail call, so that the debugger can know that there will not be a corresponding return event.
• Equality between function values has changed. Now, a function definition may not create a new value; it may reuse some previous value if there is no observable difference to the new function.