Add FromValueDelegate

[ranfdev]

Makes implementing FromValue safe and trivial

[ranfdev]

I needed to remove this to auto-implement ToValue for T: ValueDelegate. Else they were in conflict.

The tests don't complain so... 🤷🏻

[sdroege]

I think this makes it impossible to pass &Object to e.g. Object::set_property() and only Object would work

[ranfdev]

Uh oh, we don't have GATs in rust 1.64, only from 1.65. 😢

That ValueDelegate trait implementation will require a T: Clone bound then

[sdroege]

Uh oh, we don't have GATs in rust 1.64, only from 1.65. cry

For the next release we can update to 1.65 but that won't do for backporting :)

[RealKC]

I don't think this can be backported anyway? It removes a trait impl and I'm not sure the new one covers the cases the old trait impl did

[ranfdev]

So, in brief, implementing this with traits is impossible. To keep the old ToValue implementation for &T, Rust would need support for specialization.

Fortunately, we have macros!
Instead of implementing ToValue/FromValue with a blanket implementation, we could have a #[derive(ValueDelegate)]
to achieve the same thing.

And that would work for every rust version.
The result is equivalent. The macro would just generate a ToValue/FromValue implementation calling From<DelegatedType> and Into<DelegatedType>

[sdroege]

I think that makes sense. It's even less user code than implementing a trait manually ;)

[pbor]

#1000 has the macro approach