Define a macro to create self-referencing type and use it for font face

[EHfive]

This makes unsafe part be more concentrated and allows reuse. The macro would be used in later patch.

[hecrj]

Have we considered using ouroboros instead of rolling our own unsafe?

[EHfive]

Have we considered using ouroboros instead of rolling our own unsafe?

There was, but dropped for compilation time reason. 95e3624 Though it's not a big issue for me..

The produced type is basically equivalent to expansion of ouroboros macro so I think it's fine.

And this macro should still be faster than ouroboros as we didn't use proc-macro here.

[Imberflur]

Imo if this will be using custom unsafe, then it is easier to verify the soundness of a particular implementation rather than a generic macro. And I don't see any other motivation for a macro in this case other than encapsulating the unsafe code (edit: Sorry, I missed the mention of the macro being used in a later patch. I still think the correctness of the macro is more difficult to verify, maybe it isn't too bad...)

Also, the current implementation has a soundness issue, but it is not too surprising since this was just fixed in ouroboros someguynamedjosh/ouroboros#88. I can make a PR for this later today.

[Imberflur]

wrapping this up in a module is necessary to prevent accessing the fields incorrectly so it should probably be part of the macro

[EHfive]

@Imberflur After roughly look at the issue you mentioned, and given the motivation of not using ouroboros is to speed up compilation. I think we should just use the self_cell which is proc-macro-free thus being faster to compile. And to avoid implementing indigenous unsafe code here which has a lesser chance to be audited.

[Imberflur]

This field needs to be wrapped in maybe_dangling::MaybeDangling or core::mem::MaybeUninit to prevent the compiler from potentially assuming any references it contains will be valid till the end of a function when $SelfRef<T> is passed as a function parameter (see the issue I linked above). Any such references will only be valid until this value is dropped.

[Imberflur]

@Imberflur After roughly look at the issue you mentioned, and given the motivation of not using ouroboros is to speed up compilation. I think we should just use the self_cell which is proc-macro-free thus being faster to compile. And to avoid implementing indigenous unsafe code here which has a lesser chance to be audited.

That definitely seems like the best approach. We could have removed the extra layer of indirection from the AliasableBox here (and the equivalent in self_cell), since an Arc already wraps the inner data but I think it is much more valuable to rely on unsafe code that will have a higher chance of being well audited.