[stdlib] Failable String inits from UTF8/16 views

String.UTF8View and String.UTF16View may be slices that don't both start
and end on Unicode scalar boundaries, in which case conversion to a
String must fail.

Swift SVN r24669

stdlib/core/StringUTF16.swift

@@ -165,9 +165,18 @@ extension String {
   }
 
   /// Construct the `String` corresponding to the given sequence of
-  /// UTF16 code units.
-  public init(_ utf16: UTF16View) {
-    self.init(utf16._core[utf16._offset..<utf16._offset+utf16._length])
+  /// UTF-16 code units.  If `utf16` contains unpaired surrogates, the
+  /// result is `nil`.
+  public init?(_ utf16: UTF16View) {
+    let wholeString = String(utf16._core)
+    
+    if let start = utf16.startIndex.samePositionIn(wholeString) {
+      if let end = utf16.endIndex.samePositionIn(wholeString) {
+        self = wholeString[start..<end]
+        return
+      }
+    }
+    return nil
   }
   
   /// The index type for subscripting a `String`\ 's `utf16` view.
@@ -242,7 +251,7 @@ extension String.UTF16View.Index {
   /// `utf8Index`. If no such position exists, the result is `nil`.
   ///
   /// Requires: `utf8Index` is an element of
-  /// `indices(String(utf16).utf8)`.
+  /// `indices(String(utf16)!.utf8)`.
   public init?(
     _ utf8Index: String.UTF8Index, within utf16: String.UTF16View
   ) {
@@ -263,7 +272,7 @@ extension String.UTF16View.Index {
   /// `unicodeScalarIndex`.
   ///
   /// Requires: `unicodeScalarIndex` is an element of
-  /// `indices(String(utf16).unicodeScalars)`.
+  /// `indices(String(utf16)!.unicodeScalars)`.
   public init(
     _ unicodeScalarIndex: String.UnicodeScalarIndex,
     within utf16: String.UTF16View) {
@@ -274,7 +283,7 @@ extension String.UTF16View.Index {
   /// `characterIndex`.
   ///
   /// Requires: `characterIndex` is an element of
-  /// `indices(String(utf16))`.
+  /// `indices(String(utf16)!)`.
   public init(_ characterIndex: String.Index, within utf16: String.UTF16View) {
     _offset = characterIndex._utf16Index
   }
@@ -283,7 +292,7 @@ extension String.UTF16View.Index {
   /// to `self`, or if no such position exists, `nil`.
   ///
   /// Requires: `self` is an element of
-  /// `indices(String(utf8).utf16)`.
+  /// `indices(String(utf8)!.utf16)`.
   public func samePositionIn(
     utf8: String.UTF8View
   ) -> String.UTF8View.Index? {

stdlib/core/StringUTF8.swift

@@ -273,9 +273,18 @@ extension String {
   }
 
   /// Construct the `String` corresponding to the given sequence of
-  /// UTF8 code units.
-  public init(_ utf8: UTF8View) {
-    self.init(utf8._core)
+  /// UTF-8 code units.  If `utf8` contains unpaired surrogates, the
+  /// result is `nil`.
+  public init?(_ utf8: UTF8View) {
+    let wholeString = String(utf8._core)
+    
+    if let start = utf8.startIndex.samePositionIn(wholeString) {
+      if let end = utf8.endIndex.samePositionIn(wholeString) {
+        self = wholeString[start..<end]
+        return
+      }
+    }
+    return nil
   }
   
   /// The index type for subscripting a `String`\ 's `.utf8` view.
@@ -327,7 +336,7 @@ extension String.UTF8View.Index {
   /// `utf16Index`. If no such position exists, the result is `nil`.
   ///
   /// Requires: `utf8Index` is an element of
-  /// `indices(String(utf16).utf8)`.
+  /// `indices(String(utf16)!.utf8)`.
   public init?(_ utf16Index: String.UTF16Index, within utf8: String.UTF8View) {
     let utf16 = String.UTF16View(utf8._core)
     
@@ -354,7 +363,7 @@ extension String.UTF8View.Index {
   /// `unicodeScalarIndex`.
   ///
   /// Requires: `unicodeScalarIndex` is an element of
-  /// `indices(String(utf8).unicodeScalars)`.
+  /// `indices(String(utf8)!.unicodeScalars)`.
   public init(
     _ unicodeScalarIndex: String.UnicodeScalarIndex,
     within utf8: String.UTF8View
@@ -366,7 +375,7 @@ extension String.UTF8View.Index {
   /// `characterIndex`.
   ///
   /// Requires: `characterIndex` is an element of
-  /// `indices(String(utf8))`.
+  /// `indices(String(utf8)!)`.
   public init(_ characterIndex: String.Index, within utf8: String.UTF8View) {
     self.init(utf8._core, _utf16Offset: characterIndex._base._position)
   }
@@ -374,7 +383,7 @@ extension String.UTF8View.Index {
   /// Return the position in `utf16` that corresponds exactly
   /// to `self`, or if no such position exists, `nil`.
   ///
-  /// Requires: `self` is an element of `indices(String(utf16).utf8)`.
+  /// Requires: `self` is an element of `indices(String(utf16)!.utf8)`.
   public func samePositionIn(
     utf16: String.UTF16View
   ) -> String.UTF16View.Index? {

stdlib/core/StringUnicodeScalarView.swift

@@ -415,7 +415,7 @@ extension String.UnicodeScalarIndex {
   /// Return the position in `utf8` that corresponds exactly
   /// to `self`.
   ///
-  /// Requires: `self` is an element of `indices(String(utf8))`.
+  /// Requires: `self` is an element of `indices(String(utf8)!)`.
   public func samePositionIn(utf8: String.UTF8View) -> String.UTF8View.Index {
     return String.UTF8View.Index(self, within: utf8)
   }
@@ -423,7 +423,7 @@ extension String.UnicodeScalarIndex {
   /// Return the position in `utf16` that corresponds exactly
   /// to `self`.
   ///
-  /// Requires: `self` is an element of `indices(String(utf16))`.
+  /// Requires: `self` is an element of `indices(String(utf16)!)`.
   public func samePositionIn(
     utf16: String.UTF16View
   ) -> String.UTF16View.Index {

test/1_stdlib/StringViews.swift

@@ -697,25 +697,35 @@ tests.test("UTF8 indexes") {
 tests.test("UTF16->String") {
   let s = summer + winter + winter + summer
   let v = s.utf16
-  for i in indices(s) {
-    for j in i..<s.endIndex {
-      expectEqual(
-        s[i..<j],
-        String(v[i.samePositionIn(v)..<j.samePositionIn(v)])
-      )
+  for i in indices(v) {
+    for j in i..<v.endIndex {      
+      if let si = i.samePositionIn(s) {
+        if let sj = j.samePositionIn(s) {
+          expectEqual(i, i1)
+          expectEqual(j, j1)
+          expectEqual(s[si..<sj], String(v[i..<j]))
+          continue
+        }
+      }
+      expectEmpty(String(v[i..<j]))
     }
   }
 }
 
 tests.test("UTF8->String") {
   let s = summer + winter + winter + summer
-  let v = s.utf16
-  for i in indices(s) {
-    for j in i..<s.endIndex {
-      expectEqual(
-        s[i..<j],
-        String(v[i.samePositionIn(v)..<j.samePositionIn(v)])
-      )
+  let v = s.utf8
+  for i in indices(v) {
+    for j in i..<v.endIndex {      
+      if let si = i.samePositionIn(s) {
+        if let sj = j.samePositionIn(s) {
+          expectEqual(i, i1)
+          expectEqual(j, j1)
+          expectEqual(s[si..<sj], String(v[i..<j]))
+          continue
+        }
+      }
+      expectEmpty(String(v[i..<j]))
     }
   }
 }