[swift-evolution] [Late Pitch] Deprecations, Moves, and Renames

[Ben Rimmington]

> On 9 Aug 2016, at 20:09, Dave Abrahams wrote:
> 
> Hi Everybody,
> 
> With another round of apologies for taking late action, we propose to
> make some deprecations, moves, and renames.  The background for these
> moves is as follows:
> 
> We've always known that when Swift reached ABI stability (now slated for
> Swift 4), we would be committed to supporting many of the standard
> library's design decisions for years to come.  What we only realized
> very recently is that, although Swift 3.0 is *not* shipping with a
> stable ABI, the promise that Swift 3.0 code will work with Swift 4.0
> code creates similar implications when it comes to certain protocols,
> today.  Especially where these protocols show up in refinement
> hierarchies, we can't keep Swift 3 code working in the future without
> carrying them forward into future libraries.
> 
> The proposed changes are as follows:
> 
> * Move `CustomPlaygroundQuickLookable` to the PlaygroundSupport module.
>  This component is really only useful for playgrounds, and doesn't
>  belong in the standard library.

I didn't think it was possible to `import PlaygroundSupport` unless the 
current file is within a playground. If so, how can corelibs-foundation 
or third-party modules add `CustomPlaygroundQuickLookable` conformance?

> * Deprecate the Indexable protocols with a message indicating that they
>  will be gone in Swift 4.  These protocols are implementation details
>  of the standard library designed to work around language limitations
>  that we expect to be gone in Swift 4.  There's no reason for anyone to
>  ever touch these; users should always use a corresponding Collection
>  protocol (e.g. instead of MutableIndexable, use MutableCollection).
> 
> * Deprecate the ExpressibleByStringInterpolation protocol with a
>  message indicating that its design is expected to change.  We know
>  this protocol to be mis-designed
>  (https://bugs.swift.org/browse/SR-1260) and limited
>  (https://bugs.swift.org/browse/SR-2303), but there's no time to fix it
>  for Swift 3.  If we knew what the new design should look like, we
>  might be able to calculate that the current API is supportable in a
>  forward-compatible way (as we do for Comparable).  Unfortunately, we
>  do not.
> 
> * Rename Streamable to TextOutputStreamable and add a deprecated
>  Streamable typealias for it.  Now that OutputStream been renamed to
>  TextOutputStream, we should also move Streamable out of the way.
> 
> Deprecation is being proposed instead of underscoring or renaming
> because it allows existing code to keep working (with warnings).  At
> this late stage, it would be bad to actually break anything.

If the SE-0104 (protocol-oriented integers) proposal has been deferred, 
should any protocols (e.g. SignedNumber) be deprecated?

-- Ben