Function lua_tolstring 

pub fn lua_tolstring(l: LuaState, ind: c_int, len: *mut SizeT) -> LuaString

Converts the Lua value at the given index to a C string. If len is not None, it also sets *len with the string length. The Lua value must be a string or a number; otherwise, the function returns a std::ptr::null(). If the value is a number, then lua_tolstring also changes the actual value in the stack to a string. (This change confuses lua_next when lua_tolstring is applied to keys during a table traversal.)