[stdlib] Don't allow String() + Character("x")

Symmetry with what we did for Arrays says that Strings shoudl only
concatenate to Strings using "+".  We have append() for adding single
characters.

Swift SVN r20997

stdlib/core/String.swift

@@ -214,7 +214,7 @@ public func <(lhs: String, rhs: String) -> Bool {
 // Support for copy-on-write
 extension String {
 
-  mutating func _append(rhs: String) {
+  mutating func extend(rhs: String) {
     _core.append(rhs._core)
   }
 
@@ -273,19 +273,10 @@ public func +(var lhs: String, rhs: String) -> String {
   if (lhs.isEmpty) {
     return rhs
   }
-  lhs._append(rhs)
+  lhs._core.extend(rhs._core)
   return lhs
 }
 
-public func +(var lhs: String, rhs: Character) -> String {
-  lhs._append(String(rhs))
-  return lhs
-}
-public func +(lhs: Character, rhs: String) -> String {
-  var result = String(lhs)
-  result._append(rhs)
-  return result
-}
 public func +(lhs: Character, rhs: Character) -> String {
   var result = String(lhs)
   result += String(rhs)
@@ -299,14 +290,10 @@ public func += (inout lhs: String, rhs: String) {
     lhs = rhs
   }
   else {
-    lhs._append(rhs)
+    lhs._core.extend(rhs._core)
   }
 }
 
-public func += (inout lhs: String, rhs: Character) {
-  lhs += String(rhs)
-}
-
 // Comparison operators
 // FIXME: Compare Characters, not code units
 extension String : Comparable {
@@ -543,10 +530,19 @@ extension String : ExtensibleCollectionType {
       S : SequenceType
       where S.Generator.Element == Character
   >(seq: S) {
+    reserveCapacity(underestimateCount(seq))
     for c in seq {
-      self += c
+      self.append(c)
     }
   }
+  
+  public init<
+      S : SequenceType
+      where S.Generator.Element == Character
+  >(seq: S) {
+    self = ""
+    self.extend(seq)
+  }
 }
 
 // Algorithms

test/DebugInfo/iteration.swift

@@ -9,7 +9,7 @@ for letter in puzzleInput {
         case "a", "e", "i", "o", "u", " ":
             continue
         default:
-            puzzleOutput += letter
+            puzzleOutput.append(letter)
     }
 }
 println(puzzleOutput)

test/stdlib/Reverse.swift

@@ -31,16 +31,14 @@ let eager: Array = reverse(lazy(2..<8).map { $0 * 3 })
 println(equal(eager, r))
 
 let raboof = reduce(lazy("foobar").reverse(), "") {
-  // FIXME: "$0 + $1" should work just fine here
+  (s: String, c: Character) in s + String(c)
-  (s: String, c: Character) in s + c
 }
 // CHECK-NEXT: "raboof"
 debugPrintln(raboof)
 
 // Prove that the result is at least bidirectional, i.e. reversible
 let foobar = reduce(lazy("foobar").reverse().reverse(), "") {
-  // FIXME: "$0 + $1" should work just fine here
+  (s: String, c: Character) in s + String(c)
-  (s: String, c: Character) in s + c
 }
 // CHECK-NEXT: "foobar"
 debugPrintln(foobar)