[swift-evolution] Pitch: Restrict Cross-module Struct Initializers

[Jordan Rose]

While working on the non-exhaustive enums proposal I had it pointed out to me that structs actually currently leak implementation details across module boundaries, specifically their full set of stored properties. This only comes up in one place: when making an initializer for the struct in an extension in another module. We really want people to be able to change the stored properties in their structs between releases without it being a source break—that's half the point of computed properties—and it's also important for a struct author to be able to enforce invariants across the struct's properties. So after talking to a few other Apple Swift folks I put together this proposal:

https://github.com/jrose-apple/swift-evolution/blob/restrict-cross-module-struct-initializers/proposals/nnnn-restrict-cross-module-struct-initializers.md <https://github.com/jrose-apple/swift-evolution/blob/restrict-cross-module-struct-initializers/proposals/nnnn-restrict-cross-module-struct-initializers.md>

This one's way smaller than the enum one, and hopefully fairly uncontroversial. Feedback welcome!

Jordan
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