Luau (Roblox’s custom Lua implementation) does it right by allowing flagging any table as read only. Trying to modify a read only table raises an error.
Since the branches needed to implement this inside the VM are very coherent, the performance overhead is pretty small. I think it would make sense for Lua to support this as well.
Petri
VM modifications aren't necessary from a semantic perspective. As has been mentioned: You can already implement read-only tables and have been able to for quite a long time. You have also been able to make proxies for a very long time, allowing each script's environment to "inherit from" the base environment without permitting modifications back to it. Doing it this way has pretty low memory overhead (you aren't duplicating the entire environment per script) and the performance isn't THAT much worse in practice.
Implementing it into the VM would be an optimization, not something necessary for functionality. This is why we have a wiki page for useful patches that some people might find valuable. Vanilla Lua is primarily focused on keeping a streamlined, elegant implementation, and new features are typically introduced in response to an obvious need that can't be handled in Lua code.
/s/ Adam
