Return UTF-8 strings when replacing with a string

When replacing with a string pattern, we will implicitly create an RE2
pattern from the string with the default options meaning it will assume
and produce UTF-8 results.

This could potentially be a breaking change for users who rely on the
string pattern's encoding but the behaviour has been misleading (and
passing anything except ISO-8859-1 or UTF-8 to RE2 is undefined).

ext/re2/re2.cc

@@ -39,18 +39,9 @@ using std::vector;
       rb_enc_associate_index(_string, _enc); \
       _string; \
     })
-  #define ENCODED_STR_NEW2(str, length, str2) \
-    ({ \
-      VALUE _string = rb_str_new(str, length); \
-      int _enc = rb_enc_get_index(str2); \
-      rb_enc_associate_index(_string, _enc); \
-      _string; \
-    })
 #else
   #define ENCODED_STR_NEW(str, length, encoding) \
     rb_str_new((const char *)str, (long)length)
-  #define ENCODED_STR_NEW2(str, length, str2) \
-    rb_str_new((const char *)str, (long)length)
 #endif
 
 #ifdef HAVE_RB_STR_SUBLEN
@@ -1419,8 +1410,8 @@ static VALUE re2_Replace(VALUE self, VALUE str, VALUE pattern,
     RE2::Replace(&str_as_string, StringValuePtr(pattern),
         StringValuePtr(rewrite));
 
-    return ENCODED_STR_NEW2(str_as_string.data(), str_as_string.size(),
-        pattern);
+    return ENCODED_STR_NEW(str_as_string.data(), str_as_string.size(),
+        "UTF-8");
   }
 
 }
@@ -1458,8 +1449,8 @@ static VALUE re2_GlobalReplace(VALUE self, VALUE str, VALUE pattern,
     RE2::GlobalReplace(&str_as_string, StringValuePtr(pattern),
                        StringValuePtr(rewrite));
 
-    return ENCODED_STR_NEW2(str_as_string.data(), str_as_string.size(),
-        pattern);
+    return ENCODED_STR_NEW(str_as_string.data(), str_as_string.size(),
+        "UTF-8");
   }
 }
 

spec/re2_spec.rb

@@ -57,10 +57,10 @@
       expect(replacement.encoding).to eq(Encoding::ISO_8859_1)
     end
 
-    it "returns strings in the encoding of the given String pattern" do
+    it "returns UTF-8 strings when given a String pattern" do
       replacement = RE2.Replace("Foo", "oo".encode("Shift_JIS"), "ah")
 
-      expect(replacement.encoding).to eq(Encoding::Shift_JIS)
+      expect(replacement.encoding).to eq(Encoding::UTF_8)
     end
 
     it "raises a Type Error for input that can't be converted to String" do
@@ -134,10 +134,10 @@
       expect(replacement.encoding).to eq(Encoding::ISO_8859_1)
     end
 
-    it "returns strings in the encoding of the given String pattern" do
+    it "returns UTF-8 strings when given a String pattern" do
       replacement = RE2.GlobalReplace("Foo", "oo".encode("Shift_JIS"), "ah")
 
-      expect(replacement.encoding).to eq(Encoding::Shift_JIS)
+      expect(replacement.encoding).to eq(Encoding::UTF_8)
     end
 
     it "raises a Type Error for input that can't be converted to String" do