[Omit needless words] Fix lowercasing of initialisms with two-letter initial words.

Fixes rdar://problem/26643039.

Author: DougGregor

lib/Basic/StringExtras.cpp

@@ -857,14 +857,13 @@ camel_case::toLowercaseInitialisms(StringRef string,
   // Lowercase until we hit the an uppercase letter followed by a
   // non-uppercase letter.
   llvm::SmallString<32> scratchStr;
-  scratchStr.push_back(clang::toLowercase(string[0]));
-  for (unsigned i = 1, n = string.size(); i != n; ++i) {
+  for (unsigned i = 0, n = string.size(); i != n; ++i) {
     // If the next character is not uppercase, stop.
     if (i < n - 1 && !clang::isUppercase(string[i+1])) {
       // If the next non-uppercase character was not a letter, we seem
-      // to have a plural, we should still lowercase the character
-      // we're on.
-      if (!clang::isLetter(string[i+1]) ||
+      // to have a plural, or we're at the beginning, we should still
+      // lowercase the character we're on.
+      if (i == 0 || !clang::isLetter(string[i+1]) ||
           isPluralSuffix(camel_case::getFirstWord(string.substr(i+1)))) {
         scratchStr.push_back(clang::toLowercase(string[i]));
         ++i;

test/ClangModules/MixedSource/mixed-nsuinteger.swift

@@ -15,7 +15,7 @@ var pp: AutoreleasingUnsafeMutablePointer<AnyObject?>? = nil
 var userTypedObj = NSUIntTest()
 
 // Check that the NSUInteger comes across as UInt from user Obj C modules. 
-var ur: UInt = userTypedObj.myCustomMethodThatOperates(onnsuIntegers: ui)
+var ur: UInt = userTypedObj.myCustomMethodThatOperates(onNSUIntegers: ui)
 
 userTypedObj.intProp = ui
 userTypedObj.typedefProp = ui

test/IDE/Inputs/custom-modules/OmitNeedlessWords.h

@@ -9,7 +9,8 @@
 
 typedef NS_OPTIONS(NSUInteger, OMWWobbleOptions) {
   OMWWobbleSideToSide = 0x01,
-  OMWWobbleBackAndForth = 0x02
+  OMWWobbleBackAndForth = 0x02,
+  OMWWobbleToXMLHex = 0x04
 };
 
 @interface OmitNeedlessWords : NSObject

test/IDE/print_omit_needless_words.swift

@@ -260,6 +260,11 @@
 // CHECK-APPKIT: func add(_: Rect)
 // CHECK-APPKIT: class func conjureRect(_: Rect)
 
+// CHECK-OMIT-NEEDLESS-WORDS: struct OMWWobbleOptions
+// CHECK-OMIT-NEEDLESS-WORDS:   static var sideToSide: OMWWobbleOptions
+// CHECK-OMIT-NEEDLESS-WORDS:   static var backAndForth: OMWWobbleOptions
+// CHECK-OMIT-NEEDLESS-WORDS:   static var toXMLHex: OMWWobbleOptions
+
 // CHECK-OMIT-NEEDLESS-WORDS: func jump(to: URL)
 // CHECK-OMIT-NEEDLESS-WORDS: func objectIs(compatibleWith: AnyObject) -> Bool
 // CHECK-OMIT-NEEDLESS-WORDS: func insetBy(x: Int, y: Int)

unittests/Basic/StringExtrasTest.cpp

@@ -192,6 +192,14 @@ TEST(ToLowercaseTest, Words) {
   EXPECT_EQ(camel_case::toLowercaseWord("", scratch), "");
 }
 
+TEST(ToLowercaseInitialismsTest, Words) {
+  llvm::SmallString<64> scratch;
+
+  EXPECT_EQ(camel_case::toLowercaseInitialisms("ToXML", scratch), "toXML");
+  EXPECT_EQ(camel_case::toLowercaseInitialisms("URLsInHand", scratch),
+            "urlsInHand");
+}
+
 TEST(ToSentencecaseTest, Words) {
   llvm::SmallString<64> scratch;