[stdlib] String.CharacterView.SubSequence is Substring.CharacterView

This changes String.CharacterView from being its own slice type to
instead use Substring.CharacterView. In the process, this updates
Substring.CharacterView to be RangeReplaceable.

This aligns with the goal of introducing Substring as a separate type
to avoid accidental memory "leaks".

stdlib/public/core/StringCharacterView.swift

@@ -726,6 +726,8 @@ extension String.CharacterView : RangeReplaceableCollection {
 
 // Algorithms
 extension String.CharacterView {
+  public typealias SubSequence = Substring.CharacterView
+
   /// Accesses the characters in the given range.
   ///
   /// The example below uses this subscript to access the characters up to, but
@@ -739,6 +741,12 @@ extension String.CharacterView {
   ///
   /// - Complexity: O(*n*) if the underlying string is bridged from
   ///   Objective-C, where *n* is the length of the string; otherwise, O(1).
+  @available(swift, introduced: 4.0)
+  public subscript(bounds: Range<Index>) -> Substring.CharacterView {
+    return String(self)[bounds].characters
+  }
+
+  @available(swift, obsoleted: 4.0)
   public subscript(bounds: Range<Index>) -> String.CharacterView {
     return String.CharacterView(
       unicodeScalars[bounds]._core,

stdlib/public/core/Substring.swift.gyb

@@ -384,7 +384,7 @@ extension StringProtocol {
 %   ('characters', 'Character')
 % ]:
 %   View = ViewPrefix + 'View'
-%   RangeReplaceable = 'RangeReplaceable' if property == 'unicodeScalars' else ''
+%   RangeReplaceable = 'RangeReplaceable' if property == 'unicodeScalars' or property == 'characters' else ''
 extension Substring {
   public struct ${View} {
     var _slice: ${RangeReplaceable}BidirectionalSlice<String.${View}>
@@ -467,10 +467,10 @@ extension String {
   }
   % end
 }
-% end
 
 // FIXME: The other String views should be RangeReplaceable too.
-extension Substring.UnicodeScalarView : RangeReplaceableCollection {
+% if RangeReplaceable == 'RangeReplaceable':
+extension Substring.${View} : RangeReplaceableCollection {
   public init() { _slice = RangeReplaceableBidirectionalSlice.init() }
 
   public mutating func replaceSubrange<C : Collection>(
@@ -479,6 +479,9 @@ extension Substring.UnicodeScalarView : RangeReplaceableCollection {
     _slice.replaceSubrange(target, with: replacement)
   }
 }
+% end # if RangeReplaceable
+
+% end # for (property, ViewPrefix) in
 
 #if _runtime(_ObjC)
 
@@ -524,6 +527,16 @@ extension Substring : RangeReplaceableCollection {
   }
 }
 
+extension Substring.CharacterView : _SwiftStringView {
+  var _persistentContent : String {
+    return Substring(self)._persistentContent
+  }
+  var _ephemeralContent : String {
+    return Substring(self)._ephemeralContent
+  }
+
+}
+
 extension Substring {
   public func lowercased() -> String {
     return String(self).lowercased()

test/stdlib/ErrorHandling.swift

@@ -107,7 +107,7 @@ ErrorHandlingTests.test("ErrorHandling/index(where:)") {
 
 ErrorHandlingTests.test("ErrorHandling/split") {
   do {
-    let _: [String.CharacterView] = try "foo".characters.split { _ in
+    let _: [Substring] = try "foo".split { _ in
       throw SillyError.JazzHands
       return false
     }

test/stdlib/subString.swift

@@ -121,9 +121,9 @@ SubstringTests.test("CharacterView") {
 
   t.replaceSubrange(t.startIndex...t.startIndex, with: ["C"])
   u.replaceSubrange(u.startIndex...u.startIndex, with: ["E"])
-  expectEqual(String(u), "Efg")
-  expectEqual(String(t), "Cdefg")
-  expectEqual(s, "abcdefg")
+  expectEqual("Efg", String(u))
+  expectEqual("Cdefg", String(t))
+  expectEqual("abcdefg", s)
 }
 
 SubstringTests.test("UnicodeScalars") {

validation-test/stdlib/String.swift

@@ -56,43 +56,47 @@ StringTests.test("sizeof") {
 
 StringTests.test("AssociatedTypes-UTF8View") {
   typealias View = String.UTF8View
+  typealias SubView = Substring.UTF8View
   expectCollectionAssociatedTypes(
     collectionType: View.self,
     iteratorType: View.Iterator.self,
-    subSequenceType: Substring.UTF8View.self,
+    subSequenceType: SubView.self,
     indexType: View.Index.self,
     indexDistanceType: Int.self,
     indicesType: DefaultBidirectionalIndices<View>.self)
 }
 
 StringTests.test("AssociatedTypes-UTF16View") {
   typealias View = String.UTF16View
+  typealias SubView = Substring.UTF16View
   expectCollectionAssociatedTypes(
     collectionType: View.self,
     iteratorType: IndexingIterator<View>.self,
-    subSequenceType: Substring.UTF16View.self,
+    subSequenceType: SubView.self,
     indexType: View.Index.self,
     indexDistanceType: Int.self,
     indicesType: View.Indices.self)
 }
 
 StringTests.test("AssociatedTypes-UnicodeScalarView") {
   typealias View = String.UnicodeScalarView
+  typealias SubView = Substring.UnicodeScalarView
   expectCollectionAssociatedTypes(
     collectionType: View.self,
     iteratorType: View.Iterator.self,
-    subSequenceType: Substring.UnicodeScalarView.self,
+    subSequenceType: SubView.self,
     indexType: View.Index.self,
     indexDistanceType: Int.self,
     indicesType: DefaultBidirectionalIndices<View>.self)
 }
 
 StringTests.test("AssociatedTypes-CharacterView") {
   typealias View = String.CharacterView
+  typealias SubView = Substring.CharacterView
   expectCollectionAssociatedTypes(
     collectionType: View.self,
     iteratorType: IndexingIterator<View>.self,
-    subSequenceType: View.self,
+    subSequenceType: SubView.self,
     indexType: View.Index.self,
     indexDistanceType: Int.self,
     indicesType: DefaultBidirectionalIndices<View>.self)

validation-test/stdlib/StringViews.swift

@@ -482,15 +482,15 @@ tests.test("index-mapping/unicode-scalar-to-character/\(id)") {
     "❄️", err("\u{FE0F}"),
     replacementCharacter, "\u{20}", replacementCharacter, replacementCharacter
   ]
+
+  let mappedChars = winter.unicodeScalars.indices.map {
+    i in mapIndex(i, winter).map {
+      winter[$0]
+    }
+  }
 
   expectEqualSequence(
-    winterUnicodeScalarCharacters,
-
-    winter.unicodeScalars.indices.map {
-      i in mapIndex(i, winter).map {
-        winter[$0]
-      }
-    }, sameValue: ==)
+    winterUnicodeScalarCharacters, mappedChars, sameValue: ==)
 
   expectEqual(winter.endIndex, mapIndex(winter.unicodeScalars.endIndex, winter)!)