[Truffle] Support String coercion in String#encode.

Author: nirvdrum

spec/truffle/tags/core/string/encode_tags.txt

@@ -1,7 +1,6 @@
 fails:String#encode when passed no options transcodes to Encoding.default_internal when set
 fails:String#encode when passed no options transcodes a 7-bit String despite no generic converting being available
 fails:String#encode when passed no options raises an Encoding::ConverterNotFoundError when no conversion is possible
-fails:String#encode when passed to encoding calls #to_str to convert the object to an Encoding
 fails:String#encode when passed to encoding transcodes a 7-bit String despite no generic converting being available
 fails:String#encode when passed to encoding raises an Encoding::ConverterNotFoundError when no conversion is possible
 fails:String#encode when passed to encoding raises an Encoding::ConverterNotFoundError for an invalid encoding

truffle/src/main/java/org/jruby/truffle/nodes/core/StringNodes.java

@@ -796,6 +796,8 @@ public boolean empty(RubyString string) {
     @CoreMethod(names = "encode", required = 1)
     public abstract static class EncodeNode extends CoreMethodNode {
 
+        @Child private ToStrNode toStrNode;
+
         public EncodeNode(RubyContext context, SourceSection sourceSection) {
             super(context, sourceSection);
         }
@@ -825,6 +827,18 @@ public RubyString encode(RubyString string, RubyEncoding encoding) {
 
             return getContext().toTruffle(jrubyTranscoded);
         }
+
+        @Specialization(guards = { "!isRubyString(arguments[1])", "!isRubyEncoding(arguments[1])" })
+        public RubyString encode(VirtualFrame frame, RubyString string, Object encoding) {
+            notDesignedForCompilation();
+
+            if (toStrNode == null) {
+                CompilerDirectives.transferToInterpreter();
+                toStrNode = insert(ToStrNodeFactory.create(getContext(), getSourceSection(), null));
+            }
+
+            return encode(string, toStrNode.executeRubyString(frame, encoding));
+        }
     }
 
     @CoreMethod(names = "encoding")