Merge pull request #4217 from apple/stdlib-string-fixme-abi

stdlib: add a couple of ABI FIXMEs to String implementation

Author: gribozavr

stdlib/public/core/StringCharacterView.swift

@@ -236,6 +236,9 @@ extension String.CharacterView : BidirectionalCollection {
     )
   }
 
+  // FIXME(ABI): don't make this function inlineable.  Grapheme cluster
+  // segmentation uses a completely different algorithm in Unicode 9.0.
+  //
   /// Returns the length of the first extended grapheme cluster in UTF-16
   /// code units.
   @inline(never)
@@ -275,7 +278,10 @@ extension String.CharacterView : BidirectionalCollection {
 
     return start._position - startIndexUTF16
   }
-  
+
+  // FIXME(ABI): don't make this function inlineable.  Grapheme cluster
+  // segmentation uses a completely different algorithm in Unicode 9.0.
+  //
   /// Returns the length of the previous extended grapheme cluster in UTF-16
   /// code units.
   @inline(never)

stdlib/public/core/StringUnicodeScalarView.swift

@@ -577,6 +577,8 @@ extension String.UnicodeScalarIndex {
 }
 
 extension String.UnicodeScalarView {
+  // FIXME(ABI): don't make this function inlineable.  Grapheme cluster
+  // segmentation uses a completely different algorithm in Unicode 9.0.
   internal func _isOnGraphemeClusterBoundary(_ i: Index) -> Bool {
     if i == startIndex || i == endIndex {
       return true

stdlib/public/core/UnicodeTrie.swift.gyb

@@ -206,6 +206,9 @@ struct _UnicodeGraphemeClusterBreakPropertyTrie {
   }
 }
 
+// FIXME(ABI): don't mark this type versioned, or any of its APIs inlineable.
+// Grapheme cluster segmentation uses a completely different algorithm in
+// Unicode 9.0.
 internal struct _UnicodeExtendedGraphemeClusterSegmenter {
   let _noBoundaryRulesMatrix: UnsafePointer<UInt16>