[stdlib] make makeContiguousUTF8 stricter

Author: glessard

stdlib/public/core/StringCreate.swift

@@ -312,9 +312,17 @@ extension String {
   @inline(never) // slow-path
   internal static func _copying(_ str: Substring) -> String {
     if _fastPath(str._wholeGuts.isFastUTF8) {
-      return unsafe str._wholeGuts.withFastUTF8(range: str._offsetRange) {
+      var new = unsafe str._wholeGuts.withFastUTF8(range: str._offsetRange) {
         unsafe String._uncheckedFromUTF8($0)
       }
+#if os(watchOS) && _pointerBitWidth(_32)
+      if str._wholeGuts.isSmall,
+         str._wholeGuts.count > _SmallString.contiguousCapacity() {
+        new.reserveCapacity(_SmallString.capacity + 1)
+        return new
+      }
+#endif
+      return new
     }
     return unsafe Array(str.utf8).withUnsafeBufferPointer {
       unsafe String._uncheckedFromUTF8($0)

stdlib/public/core/StringProtocol.swift

@@ -185,7 +185,17 @@ extension String {
   /// Contiguous strings also benefit from fast-paths and better optimizations.
   ///
   @_alwaysEmitIntoClient
-  public var isContiguousUTF8: Bool { return _guts.isFastUTF8 }
+  public var isContiguousUTF8: Bool {
+    if _guts.isFastUTF8 {
+#if os(watchOS) && _pointerBitWidth(_32)
+      if _guts.isSmall && _guts.count > _SmallString.contiguousCapacity() {
+        return false
+      }
+#endif
+      return true
+    }
+    return false
+  }
 
   /// If this string is not contiguous, make it so. If this mutates the string,
   /// it will invalidate any pre-existing indices.