Revert "TruffleString.ConcatNode currently needs both sides to be of the same encoding if one side is MutableTruffleString"

* This reverts commit d3f95ab.
* Not needed anymore.

Author: eregon

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

@@ -591,9 +591,7 @@ protected RubyString stringAppend(Object string, Object other,
                 @Cached RubyStringLibrary libString,
                 @Cached RubyStringLibrary libOther,
                 @Cached EncodingNodes.CheckStringEncodingNode checkEncodingNode,
-                @Cached TruffleString.ConcatNode concatNode,
-                @Cached TruffleString.ForceEncodingNode forceEncodingNodeLeft,
-                @Cached TruffleString.ForceEncodingNode forceEncodingNodeRight) {
+                @Cached TruffleString.ConcatNode concatNode) {
 
             var left = libString.getTString(string);
             var leftEncoding = libString.getEncoding(string);
@@ -603,11 +601,7 @@ protected RubyString stringAppend(Object string, Object other,
             final RubyEncoding compatibleEncoding = checkEncodingNode.executeCheckEncoding(left, leftEncoding,
                     right, rightEncoding);
 
-            var result = concatNode.execute(
-                    forceEncodingNodeLeft.execute(left, leftEncoding.tencoding, compatibleEncoding.tencoding),
-                    forceEncodingNodeRight.execute(right, rightEncoding.tencoding, compatibleEncoding.tencoding),
-                    compatibleEncoding.tencoding,
-                    true);
+            var result = concatNode.execute(left, right, compatibleEncoding.tencoding, true);
             return createString(result, compatibleEncoding);
         }
     }

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

@@ -4008,12 +4008,10 @@ protected Object splicePrepend(
                 @Cached RubyStringLibrary libString,
                 @Cached RubyStringLibrary libOther,
                 @Cached TruffleString.SubstringByteIndexNode prependSubstringNode,
-                @Cached TruffleString.ConcatNode prependConcatNode,
-                @Cached TruffleString.ForceEncodingNode forceEncodingNode) {
+                @Cached TruffleString.ConcatNode prependConcatNode) {
             var original = string.tstring;
             var originalTEncoding = libString.getTEncoding(string);
-            var left = forceEncodingNode.execute(libOther.getTString(other), libOther.getTEncoding(other),
-                    rubyEncoding.tencoding);
+            var left = libOther.getTString(other);
             var right = prependSubstringNode.execute(original, byteCountToReplace,
                     original.byteLength(originalTEncoding) - byteCountToReplace, originalTEncoding, true);
 
@@ -4029,13 +4027,9 @@ protected Object spliceAppend(
                 @Cached RubyStringLibrary libString,
                 @Cached RubyStringLibrary libOther,
                 @Cached TruffleString.ConcatNode appendConcatNode,
-                @Cached TruffleString.ForceEncodingNode forceEncodingNodeLeft,
-                @Cached TruffleString.ForceEncodingNode forceEncodingNodeRight,
                 @Bind("libString.byteLength(string)") int byteLength) {
-            var left = forceEncodingNodeLeft.execute(string.tstring, libString.getTEncoding(string),
-                    rubyEncoding.tencoding);
-            var right = forceEncodingNodeRight.execute(libOther.getTString(other), libOther.getTEncoding(other),
-                    rubyEncoding.tencoding);
+            var left = string.tstring;
+            var right = libOther.getTString(other);
 
             var concatResult = appendConcatNode.execute(left, right, rubyEncoding.tencoding, true);
             string.setTString(concatResult, rubyEncoding);
@@ -4055,7 +4049,6 @@ protected RubyString splice(
                 @Cached TruffleString.ConcatNode leftConcatNode,
                 @Cached TruffleString.ConcatNode rightConcatNode,
                 @Cached TruffleString.ForceEncodingNode forceEncodingNode,
-                @Cached TruffleString.ForceEncodingNode forceEncodingNodeOther,
                 @Bind("libString.byteLength(string)") int byteLength) {
             var sourceTEncoding = libString.getTEncoding(string);
             var resultTEncoding = rubyEncoding.tencoding;
@@ -4071,9 +4064,7 @@ protected RubyString splice(
             if (insertStringIsEmptyProfile.profile(insert.isEmpty())) {
                 joinedLeft = forceEncodingNode.execute(splitLeft, sourceTEncoding, resultTEncoding);
             } else {
-                joinedLeft = leftConcatNode.execute(splitLeft,
-                        forceEncodingNodeOther.execute(insert, libOther.getTEncoding(other), resultTEncoding),
-                        resultTEncoding, true);
+                joinedLeft = leftConcatNode.execute(splitLeft, insert, resultTEncoding, true);
             }
 
             final TruffleString joinedRight; // always in resultTEncoding

src/main/ruby/truffleruby/core/string.rb

@@ -894,7 +894,7 @@ def scrub(replace = nil, &block)
       begin
         replace = "\xEF\xBF\xBD".encode(self.encoding, :undef => :replace, :replace => '?')
       rescue Encoding::ConverterNotFoundError
-        replace = '?'.encode(self.encoding)
+        replace = '?'.encode(encoding)
       end
     end
 
@@ -907,7 +907,7 @@ def scrub(replace = nil, &block)
       if str.ascii_only? ? !encoding.ascii_compatible? : encoding != str.encoding
         raise Encoding::CompatibilityError, 'incompatible character encodings'
       end
-      str.dup.force_encoding(self.encoding)
+      str
     }
 
     if replace