newproxy is a hidden function in Lua 5.1, it's not mentioned in the manual
but a description and example of its use could be found on Lua User's Wiki Hidden Features Page.
The function was removed past versions 5.1 and grepping the source code of versions 5.2 and above finds no hits of it, since the purpose it served was taken over by metatables on tables.
If we compare metatables part of the manuals of
Lua 5.1
and
Lua 5.2
we can see 5.2 allows setting __len and __gc on a table, while 5.1 does not.
We can test it with a simple program.
local meta = {}
meta.__gc = function(tab) print(tab, "Hello from __gc") end
meta.__len = function(tab) print(tab, "Hello from __len"); return -1 end
local x = setmetatable({}, meta)
local y = #x
$ lua51.exe test.lua
$ lua52.exe test.lua
table: 0000021d84f7a960 Hello from __len
table: 0000021d84f7a960 Hello from __gc
If we adjust the program to use newproxy in Lua 5.1 it will produce a similar result.
local x = newproxy(true)
local meta = getmetatable(x)
meta.__gc = function(tab) print(tab, "Hello from __gc") end
meta.__len = function(tab) print(tab, "Hello from __len"); return -1 end
local y = #x
$ lua51.exe test.lua
userdata: 000002264e1f0ef8 Hello from __len
userdata: 000002264e1f0ef8 Hello from __gc
I personally never used it and didn't even know it works like that in 5.1 until very recently.