[String] Bug fix for empty-range getCharacters

Author: milseman

stdlib/public/core/StringUTF16View.swift

@@ -537,6 +537,8 @@ extension String {
   ) {
     _internalInvariant(_guts.isFastUTF8)
 
+    if _slowPath(range.isEmpty) { return }
+
     return _guts.withFastUTF8 { utf8 in
       var writeIdx = 0
       let writeEnd = buffer.count

validation-test/stdlib/StringBreadcrumbs.swift

@@ -38,14 +38,17 @@ let StringBreadcrumbsTests = TestSuite("StringBreadcrumbsTests")
 
 func validateBreadcrumbs(_ str: String) {
   var utf16CodeUnits = Array(str.utf16)
-  var utf16Indices = Array(str.utf16.indices)
   var outputBuffer = Array<UInt16>(repeating: 0, count: utf16CodeUnits.count)
 
-  for i in 0..<(utf16CodeUnits.count-1) {
-    for j in (i+1)..<utf16CodeUnits.count {
+  // Include the endIndex, so we can test end conversions
+  var utf16Indices = Array(str.utf16.indices) + [str.utf16.endIndex]
+
+  for i in 0...utf16CodeUnits.count {
+    for j in i...utf16CodeUnits.count {
       let range = Range(uncheckedBounds: (i, j))
 
       let indexRange = str._toUTF16Indices(range)
+
       // Range<String.Index> <=> Range<Int>
       expectEqual(utf16Indices[i], indexRange.lowerBound)
       expectEqual(utf16Indices[j], indexRange.upperBound)
@@ -69,6 +72,13 @@ func validateBreadcrumbs(_ str: String) {
   }
 }
 
+StringBreadcrumbsTests.test("uniform strings") {
+  validateBreadcrumbs(smallASCII)
+  validateBreadcrumbs(largeASCII)
+  validateBreadcrumbs(smallUnicode)
+  validateBreadcrumbs(largeUnicode)
+}
+
 StringBreadcrumbsTests.test("largeString") {
   validateBreadcrumbs(largeString)
 }