You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: mod.rs
    path of file: ./repos/mlua/src/util
    
    the code of the file until where you have to start completion: use std::any::{Any, TypeId};
use std::borrow::Cow;
use std::ffi::CStr;
use std::fmt::Write;
use std::mem::MaybeUninit;
use std::os::raw::{c_char, c_int, c_void};
use std::panic::{catch_unwind, resume_unwind, AssertUnwindSafe};
use std::sync::Arc;
use std::{ptr, slice, str};

use once_
    
pub unsafe fn init_userdata_metatable_newindex(state: *mut ffi::lua_State) -> Result<()> {
    let newindex_key = &USERDATA_METATABLE_NEWINDEX as *const u8 as *const _;
    if ffi::lua_rawgetp(state, ffi::LUA_REGISTRYINDEX, newindex_key) == ffi::LUA_TFUNCTION {
        return Ok(());
    }
    ffi::lua_pop(state, 1);

    // Create and cache `__newindex` generator
    let code = cstr!(
        r#"
            local error, isfunction = ...
            return function (__newindex, field_setters)
                return function (self, key, value)
                    if field_setters ~= nil then
                        local field_setter = field_setters[key]
                        if field_setter ~= nil then
                            field_setter(self, value)
                            return
                        end
                    end

                    if isfunction(__newindex) then
                        __newindex(self, key, value)
                    elseif __newindex == nil then
                        error("attempt to set an unknown field '"..key.."'")
                    else
                        __newindex[key] = value
                    end
                end
            end
    "#
    );
    let code_len = CStr::from_ptr(code).to_bytes().len();
    protect_lua!(state, 0, 1, |state| {
        let ret = ffi::luaL_loadbuffer(state, code, code_len, cstr!("__mlua_newindex"));
        if ret != ffi::LUA_OK {
            ffi::lua_error(state);
        }
        ffi::lua_pushcfunction(state, lua_error_impl);
        ffi::lua_pushcfunction(state, lua_isfunction_impl);
        ffi::lua_call(state, 2, 1);

        #[cfg(feature = "luau-jit")]
        if ffi::luau_codegen_supported() != 0 {
            ffi::luau_codegen_compile(state, -1);
        }

        // Store in the registry
        ffi::lua_pushvalue(state, -1);
        ffi::lua_rawsetp(state, ffi::LUA_REGISTRYINDEX, newindex_key);
    })
}