Fix some infinite init recursion

Author: milseman

stdlib/public/core/StringGuts.swift

@@ -70,15 +70,6 @@ extension _StringGuts {
   }
 
   internal init(_ storage: _SharedStringStorage) {
-    // TODO(UTF8 perf): Is it better, or worse, to throw the object away if
-    // immortal? We could avoid having to ARC the object itself when owner is
-    // nil, at the cost of repeatedly creating a new one when this string is
-    // bridged back-and-forth to ObjC. For now, we're choosing to keep the
-    // object allocation, perform some ARC, and require a dependent load for
-    // literals that have bridged back in from ObjC. Long term, we will likely
-    // emit some kind of "immortal" object for literals with efficient bridging
-    // anyways, so this may be a short-term decision.
-
     // TODO(UTF8): We should probably store perf flags in the object
     self.init(_StringObject(storage, isASCII: false))
   }

stdlib/public/core/StringRangeReplaceableCollection.swift

@@ -56,11 +56,7 @@ extension String: RangeReplaceableCollection {
   public init<S : Sequence & LosslessStringConvertible>(_ other: S)
   where S.Element == Character {
     if let str = other as? String {
-      self.init(str)
-      return
-    }
-    if let subStr = other as? Substring {
-      self.init(subStr)
+      self = str
       return
     }
     self = other.description
@@ -87,7 +83,7 @@ extension String: RangeReplaceableCollection {
   public init<S : Sequence>(_ characters: S)
   where S.Iterator.Element == Character {
     if let str = characters as? String {
-      self.init(str)
+      self = str
       return
     }
     if let subStr = characters as? Substring {

stdlib/public/core/Substring.swift

@@ -723,11 +723,11 @@ extension Substring : RangeReplaceableCollection {
   public init<S : Sequence>(_ elements: S)
   where S.Element == Character {
     if let str = elements as? String {
-      self.init(str)
+      self = str[...]
       return
     }
     if let subStr = elements as? Substring {
-      self.init(subStr)
+      self = subStr
       return
     }
     self = String(elements)[...]