Tracking issue for existential types (RFC 2071)

[withoutboats]

This is the tracking issue for existential types, a part of rust-lang/rfcs#2071

Blocking stabilization:

• Implement
• Document
• Decide final syntax

[nikomatsakis]

I am closing this as a duplicate of #34511

[Tyler-Hardin]

I don't see a mention of anything like this in the referenced duplicate. It might be good to add it to the todo list there if this is going to be closed as a dup. (I would do it myself, but I barely grasp the RFC, and even then only in the specific context in which it would be useful to me.)

[Ekleog]

@aturon This tracking issue having been closed in favor of #34511, would it be possible to update the checklist up there with tracking information for this feature, so that it's easy to figure out whether this part of rust-lang/rfcs#2071 has been implemented, is still under debate, is pending stabilisation, etc.? :)