On Tue Sep 1, 2026 at 11:27 AM BST, Danilo Krummrich wrote: > On Mon Aug 31, 2026 at 10:42 PM CEST, Gary Guo wrote: >> I think if you change the signatue of `registration_data_with` slightly: >> >> pub fn registration_data_with<'this, F: ForLt + 'static, R>( >> &'this self, >> f: impl for<'a> FnOnce(Pin<&'this F::Of<'a>>) -> R, >> ^ note this is changed from 'a to 'this >> ) -> Result<R>; >> >> then there will be an implied bound available inside the callback where 'a >> outlives 'this, and thus the function callback is able to perform coercion of >> any T<'a> to T<'this> provided that `T` is covariant over lifetime `'a`. >> >> [ The coercion won't work when doing abstract `F::Of` on the bus abstraction >> side, but for any user it is dealing with concrete types so the compiler sees >> specific types and thus can check variance ] >> >> Then your projection can just be >> >> aux.registration_data_project(|x| &x.field) >> >> I haven't tried it out but I think it should work. > > I gave this a shot and it seems to work out, it's a good simplification. I > think > we don't even need a dedicated project method in this case. We could add an > alias for with() just to clarify the intent, but not sure that's worth.
Right, I don't think it's needed. It's just a typo in the example given that I write "_project" instead of "_with". Unless you have a case where it actually needs `&'a F::Of<'a>` instead of `&'this F::Of<'a>` (which I think is not very likely, changing signatures of existing users should be sufficient), I think we should just modify the signature of `registration_data_with`. Best, Gary
