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

[Dave Abrahams]

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.

* 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.

-- 
-Dave