One of the topics discussed in the final breakouts of last week's TC39 meeting was something like "self-hosted implementation of cross-realm internal slots". I facetiously joked about using the global symbol registry (which is shared across realms), but it was pointed out that such a mechanism would not be private.
I've been thinking about it since, and I believe that the current surface area of the language is sufficient for this to be possible without introducing something like private symbols—specifically by using built-in functions whose identity (regardless of originating realm) is verified with Function.prototype.toString. There's a demonstration of using this technique to build custom brand-sharing classes of the sort that could trust each other across realms, and I'd love to get some feedback on it: https://jsbin.com/vujehanosu . If this actually is as robust as it seems to be, then it can also be a foundation for cross-realm access to custom analogs of internal slots.
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