Throwing error from Rust code (module) causes "error object is not a string"

[mauricelam]

How do I throw an error in Rust so that it will be printed out when called from lua?

Here's my test case for the behavior:

use mlua::prelude::*;

#[mlua::lua_module]
fn luaerror(_lua: &Lua) -> LuaResult<LuaTable> {
    Err("MyError".into_lua_err())
}

I am tried both 0.9.0-beta.1 and 0.8.8 on Mac using lua 5.1. My lua file is literally just require('luaerror') and the output I get is

$ cargo build && lua5.1 test.lua
lua5.1: (error object is not a string)

[khvzak]

I'm afraid lua 5.1 does not call __tostring metamethod when printing exceptions.
Normally you can print error by catching it and using tostring function.