Fix lexer to properly handle single quote strings

Lexer now suggests escaping unescaped double quotes and unescaping escaped single quotes in single quote strings..

Author: Jumhyn

lib/Parse/Lexer.cpp

@@ -1164,11 +1164,31 @@ void Lexer::lexStringLiteral() {
       if (*TokStart == '\'') {
         // Complain about single-quote string and suggest replacement with
         // double-quoted equivalent.
-        // FIXME: Fixit should replace ['"'] with ["\""] (radar 22709931)
         StringRef orig(TokStart, CurPtr - TokStart);
         llvm::SmallString<32> replacement;
         replacement += '"';
-        replacement += orig.slice(1, orig.size() - 1);
+        std::string str = orig.slice(1, orig.size() - 1).str();
+        std::string quot = "\"";
+        size_t pos = 0;
+        while (pos != str.length()) {
+          if (str.at(pos) == '\\') {
+            if (str.at(pos + 1) == '\'') {
+                // Un-escape escaped single quotes.
+                str.replace(pos, 2, "'");
+                ++pos;
+            } else {
+                // Skip over escaped characters.
+                pos += 2;
+            }
+          } else if (str.at(pos) == '"') {
+            str.replace(pos, 1, "\\\"");
+            // Advance past the newly added ["\""].
+            pos += 2;
+          } else {
+            ++pos;
+          }
+        }
+        replacement += StringRef(str);
         replacement += '"';
         diagnose(TokStart, diag::lex_single_quote_string)
           .fixItReplaceChars(getSourceLoc(TokStart), getSourceLoc(CurPtr),

test/expr/expressions.swift

@@ -448,13 +448,11 @@ func testSingleQuoteStringLiterals() {
   'abc" // expected-error{{unterminated string literal}}
   "a'c"
 
-  // FIXME: <rdar://problem/22709931> QoI: Single-quote => double-quote string literal fixit should escape quote chars
-  // FIXME: The suggested replacement should un-escape the single quote
-  // character.
-  'ab\'c' // expected-error{{single-quoted string literal found, use '"'}}{{3-10="ab\\'c"}}
+  'ab\'c' // expected-error{{single-quoted string literal found, use '"'}}{{3-10="ab'c"}}
 
-  // FIXME: The suggested replacement should escape the double-quote character.
-  'ab"c' // expected-error{{single-quoted string literal found, use '"'}}{{3-9="ab"c"}}
+  'ab"c' // expected-error{{single-quoted string literal found, use '"'}}{{3-9="ab\\"c"}}
+  'ab\"c' // expected-error{{single-quoted string literal found, use '"'}}{{3-10="ab\\"c"}}
+  'ab\\"c' // expected-error{{single-quoted string literal found, use '"'}}{{3-11="ab\\\\\\"c"}}
 }
 
 // <rdar://problem/17128913>