[Foundation] UUID: LosslessStringConvertible

[benrimmington]

UUID already conforms to CustomStringConvertible,
but in documentation its description property
is only a "textual description of the UUID."

LosslessStringConvertible conformance will make:

• description and uuidString equivalent;
• "\(UUID())" interpolation more useful;
• String(UUID()) creation possible.

[benrimmington]

@parkera Please let me know if these changes can be accepted into the Foundation overlay (and also swift-corelibs-foundation).

[parkera]

Can you expand a bit on what the benefits of adopting the protocol are?

Also, I think we may not be able to do this until Swift provides an ABI-compatible way to adopt an existing protocol on an existing type.

[benrimmington]

Can you expand a bit on what the benefits of adopting the protocol are?

There may be existing code that relies on "\(uuid)" interpolation:

• either accidentally if "\(uuid.uuidString)" was intended;

• or on purpose if someone looked at the implementation and unit test.

If you want to guarantee that description and uuidString are equivalent, updated documentation would be one option, but this isn't always reliable (for example, NSDatas description no longer matches its documentation). The new conformance would be a stronger guarantee.

Also, I think we may not be able to do this until Swift provides an ABI-compatible way to adopt an existing protocol on an existing type.

OK, I didn't realize that. I also forgot to add an @available placeholder to the new initializer. Do you want to close this PR for now?

[parkera]

We can close it, and I've filed a radar for us to track the idea internally as well (rdar://problem/57847799)