[Pitch] Noncopyable (or "move-only") structs and enums

[Torust (Thomas Roughton)]

@filip-sakel

I don’t think the problem here is the negative copyable constraint ?: Copyable, but rather implicit requirements. I think a better name for your attribute is @implicitConstraint(MyProtocol), which —as you say— could generalize to other feature like reflection.

I think the key distinction to draw is that I’m suggesting @noImplicitCopy be automatically applied to all code that isn’t in an ImplicitlyCopyable scope – or, to put it another way, that code that isn’t in an ImplicitlyCopyable scope doesn’t automatically synthesise conformances to ImplicitlyCopyable for Copyable types that are used within said scope (wherever they may be defined). That’s in addition to suppressing automatic conformance of Copyable for all types defined outside of an ImplicitlyCopyable scope.

Rephrasing without negative constraints: code within an ImplicityCopyable scope automatically generates conformances to ImplicitlyCopyable for all Copyable types regardless of where they’re defined, and additionally synthesises conformances to Copyable for all types defined within that scope. All current Swift code is assumed to be implicitly within an ImplicityCopyable scope.