[Truffle] Move String#starts_with? out to Ruby.

While the savings looks small, it'll be much greater once we support more encoding compatibility checks and taintedness.

Author: nirvdrum

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

@@ -0,0 +1 @@
+fails:String#start_with? works for multibyte strings

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

@@ -1542,37 +1542,6 @@ private RubyArray splitHelper(RubyString string, String sep) {
         }
     }
 
-    @CoreMethod(names = "start_with?", argumentsAsArray = true)
-    public abstract static class StartWithNode extends CoreMethodNode {
-
-        @Child private ToStrNode toStrNode;
-
-        public StartWithNode(RubyContext context, SourceSection sourceSection) {
-            super(context, sourceSection);
-            toStrNode = insert(ToStrNodeFactory.create(getContext(), getSourceSection(), null));
-        }
-
-        public StartWithNode(StartWithNode prev) {
-            super(prev);
-            toStrNode = prev.toStrNode;
-        }
-
-        @Specialization
-        public boolean startWith(VirtualFrame frame, RubyString string, Object... prefixes) {
-            notDesignedForCompilation();
-
-            for (Object prefix : prefixes) {
-                final RubyString coerced = toStrNode.executeRubyString(frame, prefix);
-
-                if (string.toString().startsWith(coerced.toString())) {
-                    return  true;
-                }
-            }
-
-            return false;
-        }
-    }
-
     @CoreMethod(names = "sub", required = 2)
     public abstract static class SubNode extends RegexpNodes.EscapingNode {
 

truffle/src/main/ruby/jruby/truffle/core/rubinius/kernel/common/string.rb

@@ -47,4 +47,15 @@ def chomp(separator=$/)
     str.chomp!(separator) || str
   end
 
+  def start_with?(*prefixes)
+    prefixes.each do |original_prefix|
+      prefix = Rubinius::Type.check_convert_type original_prefix, String, :to_str
+      unless prefix
+        raise TypeError, "no implicit conversion of #{original_prefix.class} into String"
+      end
+      return true if self[0, prefix.length] == prefix
+    end
+    false
+  end
+
 end