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This is the first of two proposals covering this new feature:
It propose an
Escapable
type marker to indicate that a value of this type can escape the local scope -- by being stored in a global or returned, for example.This new marker would be implicitly satisfied by all current Swift types to avoid breaking existing code and to reflect the expectation that most types can in fact be freely copied and assigned without limit.
We then add a new
~Escapable
type requirement that can be used to selectively remove this capability.We explain the motivation for such types and the basic capabilities and restrictions on their use.
The companion proposal "Lifetime Dependency Constraints for ~Escapable Values" proposes a set of annotations that selectively modify the constraints of
~Escapable
types so they can be used to provide accurate compile-time tracking of values that are inherently subsidiary to some other value (for example, iterators over collections or slices that must not outlive the container to which they refer).These are part of the BufferView roadmap and are prerequisites for that type.