[stdlib] Spot fix for https://bugs.swift.org/browse/SR-5401

The simplest way to keep the string buffer alive is simply to always grab its
iterator, even if we're not going to use it.  Thanks @milseman for that idea and
@jckarter for the diagnosis help!

stdlib/public/core/StringUnicodeScalarView.swift

@@ -193,6 +193,7 @@ extension String {
     /// collection.
     public struct Iterator : IteratorProtocol {
       init(_ _base: _StringCore) {
+        self._iterator = _base.makeIterator()
         if _base.hasContiguousStorage {
           self._baseSet = true
           if _base.isASCII {
@@ -211,7 +212,6 @@ extension String {
         } else {
           self._ascii = false
           self._baseSet = false
-          self._iterator = _base.makeIterator()
         }
       }
 
@@ -236,7 +236,7 @@ extension String {
             result = _decoder.decode(&(self._base!))
           }
         } else {
-          result = _decoder.decode(&(self._iterator!))
+          result = _decoder.decode(&(self._iterator))
         }
         switch result {
         case .scalarValue(let us):
@@ -252,7 +252,7 @@ extension String {
       internal let _ascii: Bool
       internal var _asciiBase: UnsafeBufferPointerIterator<UInt8>!
       internal var _base: UnsafeBufferPointerIterator<UInt16>!
-      internal var _iterator: IndexingIterator<_StringCore>!
+      internal var _iterator: IndexingIterator<_StringCore>
     }
 
     /// Returns an iterator over the Unicode scalars that make up this view.

test/stdlib/StringAPI.swift

@@ -366,6 +366,22 @@ StringTests.test("[String].joined() -> String") {
   _ = s == "" // should compile without error
 }
 
+StringTests.test("UnicodeScalarView.Iterator.Lifetime") {
+  // Tests that String.UnicodeScalarView.Iterator is maintaining the lifetime of
+  // an underlying String buffer. https://bugs.swift.org/browse/SR-5401
+  //
+  // WARNING: it is very easy to write this test so it produces false negatives
+  // (i.e. passes even when the code is broken).  The array, for example, seems
+  // to be a requirement.  So perturb this test with care!
+  let sources = ["𝓣his 𝓘s 𝓜uch 𝓛onger 𝓣han 𝓐ny 𝓢mall 𝓢tring 𝓑uffer"]
+  for s in sources {
+    // Append something to s so that it creates a dynamically-allocated buffer.
+    let i = (s + "X").unicodeScalars.makeIterator()
+    expectEqualSequence(s.unicodeScalars, IteratorSequence(i).dropLast(),
+      "Actual Contents: \(Array(IteratorSequence(i)))")
+  }
+}
+
 var CStringTests = TestSuite("CStringTests")
 
 func getNullUTF8() -> UnsafeMutablePointer<UInt8>? {