_StringGuts._extractSlice: Quickly return an empty string when the range is empty

There is little point to allocating String storage with capacity 0.

Author: lorentey

stdlib/public/core/StringGuts.swift

@@ -979,6 +979,7 @@ extension _StringGuts {
   @_inlineable
   public // @testable
   func _extractSlice(_ range: Range<Int>) -> _StringGuts {
+    if range.isEmpty { return _StringGuts() }
     if range == 0..<count { return self }
     switch (isASCII, _isUnmanaged) {
     case (true, true):

validation-test/stdlib/String.swift

@@ -521,6 +521,26 @@ StringTests.test("appendToEmptyString") {
   expectEqual(b2.bufferID, b2ID)
 }
 
+StringTests.test("Swift3Slice/Empty") {
+  let size = 5
+  let s = String(repeating: "x", count: size)
+  for i in 0 ... size {
+    let slice = s[s.index(_nth: i)..<s.index(_nth: i)]
+    // Most Swift 3 substrings are extracted into their own buffer,
+    // but empty substrings get turned into the empty string singleton
+    expectNil(slice.bufferID)
+  }
+}
+
+StringTests.test("Swift3Slice/Full") {
+  let size = 5
+  let s = String(repeating: "x", count: size)
+  let slice = s[s.startIndex..<s.endIndex]
+  // Most Swift 3 substrings are extracted into their own buffer,
+  // but if the substring covers the full original string, it is used instead.
+  expectEqual(slice.bufferID, s.bufferID)
+}
+
 StringTests.test("appendToSubstring") {
   for initialSize in 1..<16 {
     for sliceStart in [0, 2, 8, initialSize] {