Consider using macro for support of unsized values on stable Rust about smallbox

[andylokandy]

AFAICS, there are some problems of the new macro:

1. We have to expose private fields of Stackbox, but it would be unsafe behaviors to change any of it, and these operations do not require an unsafe block.

2. It is not possible to use such macros in a crate with #![deny(unsafe_code)].

[RReverser]

I would say (2) doesn't matter as much. As for the first one - yeah, macro will have to use a method that, as similar macros usually do, can be hidden from docs for general consumers with #[doc(hidden)] but used by the macro itself.

[andylokandy]

#[doc(hidden)] doesn't prevent crate user from invoking undefined behaviors or even segfaults. This problem is much more severe than phf_macro in which struct fields violation only causes control flow error. I prefer to stick with Unsize traits for type system guarantee for now.

[andylokandy]

Stable unsize feature is no longer viable due to the CoerceUnsize commit cda7d6b.

[RReverser]

I still think unsized on stack is possible in a safe way on stable Rust, but if you feel strongly against this idea, it's fair point and I suppose I can implement it in a separate crate.

[andylokandy]

Do you have any idea of how to do that?

I don't know how to resolve #7 in stable rust.

[andylokandy]

Ok, I got it. The new_unsized method and the macro initializer. You might make a hard fork for that, it's good for the user to have more options.

[andylokandy]

I've come up with an elegant solution about the macro story. I'll try it out soon.

[andylokandy]

👏 👏 Stable unsized type is done!!! Thanks for your help!!

[RReverser]

@andylokandy Heh, I've been away for a week, but glad to see this got resolved!