Correctly handle multibyte characters for String#gsub(String, String)

Author: eregon

spec/ruby/core/string/gsub_spec.rb

@@ -594,6 +594,14 @@ def obj.to_s() "ok" end
   end
 end
 
+describe "String#gsub with a string pattern" do
+  it "handles multibyte characters" do
+    "é".gsub("é", "â").should == "â"
+    "aé".gsub("é", "â").should == "aâ"
+    "éa".gsub("é", "â").should == "âa"
+  end
+end
+
 describe "String#gsub! with pattern and replacement" do
   it "modifies self in place and returns self" do
     a = "hello"

src/main/java/org/truffleruby/core/string/StringNodes.java

@@ -4528,11 +4528,7 @@ protected RubyNode coercePatternToRope(RubyNode pattern) {
             return ToRopeNodeGen.create(pattern);
         }
 
-        @Specialization(
-                guards = {
-                        "offset >= 0",
-                        "singleByteOptimizableNode.execute(stringRope)",
-                        "!patternFits(stringRope, patternRope, offset)" })
+        @Specialization(guards = { "offset >= 0", "!patternFits(stringRope, patternRope, offset)" })
         protected Object stringByteIndexPatternTooLarge(Rope stringRope, Rope patternRope, int offset) {
             return nil;
         }
@@ -4590,7 +4586,7 @@ protected Object stringByteIndex(Rope stringRope, Rope patternRope, int offset,
 
             final Encoding enc = stringRope.getEncoding();
             final CodeRange cr = stringRope.getCodeRange();
-            int c = 0;
+            int c;
 
             for (; p < l; p += c) {
                 c = calculateCharacterLengthNode.characterLength(enc, cr, Bytes.fromRange(stringBytes, p, e));