Merge pull request #11201 from milseman/4_0_very_persistent

[4.0] [stdlib] Fix bug in Substring's _persistentContent

Author: milseman

stdlib/public/core/String.swift

@@ -150,13 +150,6 @@ extension StringProtocol {
     }
     return String(String.CharacterView(self))
   }
-
-  internal var _persistentString : String {
-    if _fastPath(self is _SwiftStringView) {
-      return (self as! _SwiftStringView)._persistentContent
-    }
-    return String(String.CharacterView(self))
-  }
 }
 
 extension String : _SwiftStringView {

stdlib/public/core/Substring.swift.gyb

@@ -266,10 +266,16 @@ extension Substring : _SwiftStringView {
   var _persistentContent : String {
     let wholeCore = _slice._base._core
     let native = wholeCore.nativeBuffer
-    if _fastPath(native != nil), let n = native {
+    if _fastPath(native != nil) {
+      let wholeString = String(wholeCore)
+      let n = native._unsafelyUnwrappedUnchecked
       if _fastPath(
-        n.start == wholeCore._baseAddress && n.usedCount == wholeCore.count) {
-        return String(wholeCore)
+        n.start == wholeCore._baseAddress
+        && n.usedCount == wholeCore.count
+        && _slice.startIndex == wholeString.startIndex
+        && _slice.endIndex == wholeString.endIndex
+      ) {
+        return wholeString
       }
     }
     var r = String()

test/stdlib/subString.swift

@@ -182,4 +182,11 @@ SubstringTests.test("UTF8View") {
   expectEqual("", String(u.dropLast(10))!)
 }
 
+SubstringTests.test("Persistent Content") {
+  var str = "abc"
+  str += "def"
+  expectEqual("bcdefg", str.dropFirst(1) + "g")
+  expectEqual("bcdefg", (str.dropFirst(1) + "g") as String)
+}
+
 runAllTests()