lifetimes hell (with UserData-s)

[ildar]

Hello,
I'm doing Lua module representing calloop crate.
I found that code is spoiled with lifetime annotations very quickly. Example:

// TimerHandle
#[derive(Clone)]
struct TimerHandle<'a> (rs_timer::TimerHandle<LuaValue<'a>>);
impl LuaUserData for TimerHandle<'_> {}
impl<'a> TimerHandle<'a> {
    fn new(lua: &'static Lua, ls: rs_timer::TimerHandle<LuaValue<'static>>) -> LuaResult<LuaTable<'a>> {
        let res = lua.create_table()?;
        res.set("_self",
            lua.create_userdata(
                TimerHandle(ls)
            )?)?;
        res.set("add_timeout", lua.create_function(TimerHandle::add_timeout)?)?;
        Ok(res)
    }

    pub fn add_timeout(_: &Lua, (luaself, delay_from_now, data): (LuaTable, u64, LuaValue<'static>)) -> LuaResult<()> {
        let ud: LuaAnyUserData = luaself.get("_self")?;
        let ref_self = ud.borrow::<TimerHandle<'static>>()?;
        ref_self.0.add_timeout(std::time::Duration::from_millis(delay_from_now), data);
        Ok(())
    }
}

I may be very wrong but think that it's because AnyUserData is 'static albeit 'lua surely would be enough.

Q: is it very difficult to have an alternative AltAnyUserData with 'lua lifetime?

[khvzak]

I would suggest to try to store LuaValue<'a> as an associated value to a UserData (see set_user_value and get_user_value)

Also you could try to store LuaValue in Lua Registry and refer to it via RegistryKey.
It would significantly simplity the "lifetimes hell".

Also I'm working on alternative implementation that will not require lifetimes for values but it's mostly a concept now.