[stdlib] Allow Unicode Encoding to fail

This is needed for encodings such as Latin-1

Author: Dave Abrahams

stdlib/public/core/UTF16.swift

@@ -37,7 +37,7 @@ extension _Unicode.UTF16 : UnicodeEncoding {
     return UnicodeScalar(_unchecked: value)
   }
 
-  public static func encode(_ source: UnicodeScalar) -> EncodedScalar {
+  public static func encode(_ source: UnicodeScalar) -> EncodedScalar? {
     let x = source.value
     if _fastPath(x < (1 << 16)) {
       return EncodedScalar(_storage: x, _bitCount: 16)

stdlib/public/core/UTF32.swift

@@ -34,7 +34,7 @@ extension _Unicode.UTF32 : UnicodeEncoding {
   }
 
   @inline(__always)
-  public static func encode(_ source: UnicodeScalar) -> EncodedScalar {
+  public static func encode(_ source: UnicodeScalar) -> EncodedScalar? {
     return EncodedScalar(source.value)
   }
 

stdlib/public/core/UTF8.swift

@@ -58,7 +58,7 @@ extension _Unicode.UTF8 : UnicodeEncoding {
 
   @inline(__always)
   @_inlineable
-  public static func encode(_ source: UnicodeScalar) -> EncodedScalar {
+  public static func encode(_ source: UnicodeScalar) -> EncodedScalar? {
     var c = source.value
     if _fastPath(c < (1&<<7)) {
       return EncodedScalar(_storage: c, _bitCount: 8)

stdlib/public/core/Unicode.swift

@@ -265,7 +265,7 @@ extension _Unicode.UTF8 : UnicodeCodec {
     _ input: UnicodeScalar,
     into processCodeUnit: (CodeUnit) -> Void
   ) {
-    var s = encode(input)._storage
+    var s = encode(input)!._storage
     processCodeUnit(UInt8(extendingOrTruncating: s))
     s &>>= 8
     if _fastPath(s == 0) { return }
@@ -414,7 +414,7 @@ extension _Unicode.UTF16 : UnicodeCodec {
     _ input: UnicodeScalar,
     into processCodeUnit: (CodeUnit) -> Void
   ) {
-    var s = encode(input)._storage
+    var s = encode(input)!._storage
     processCodeUnit(UInt16(extendingOrTruncating: s))
     s &>>= 16
     if _fastPath(s == 0) { return }

stdlib/public/core/UnicodeEncoding.swift

@@ -24,8 +24,9 @@ public protocol _UnicodeEncoding {
   /// Converts from encoded to encoding-independent representation
   static func decode(_ content: EncodedScalar) -> UnicodeScalar
 
-  /// Converts from encoding-independent to encoded representation
-  static func encode(_ content: UnicodeScalar) -> EncodedScalar
+  /// Converts from encoding-independent to encoded representation, returning
+  /// `nil` if the scalar can't be represented in this encoding.
+  static func encode(_ content: UnicodeScalar) -> EncodedScalar?
 
   associatedtype ForwardParser : UnicodeParser
   associatedtype ReverseParser : UnicodeParser

test/Prototypes/UnicodeDecoders.swift

@@ -221,12 +221,12 @@ func checkDecodeUTF<Codec : UnicodeCodec & UnicodeEncoding>(
     decoded.append(scalar)
     expectEqual(
       UnicodeScalar(scalar),
-      Codec.decode(Codec.encode(UnicodeScalar(scalar)!)))
+      Codec.decode(Codec.encode(UnicodeScalar(scalar)!)!))
   }
 
   func output1(_ scalar: UnicodeScalar) {
     decoded.append(scalar.value)
-    expectEqual(scalar, Codec.decode(Codec.encode(scalar)))
+    expectEqual(scalar, Codec.decode(Codec.encode(scalar)!))
   }
 
   var result = assertionSuccess()