Do not rely on exceptions in Struct#dig

spec/ruby/core/struct/dig_spec.rb

@@ -10,6 +10,12 @@
     @instance.dig(:a, :a).should == { b: [1, 2, 3] }
   end
 
+  it "returns the value by the index" do
+    instance = Struct.new(:a, :b).new(:one, :two)
+    instance.dig(0).should == :one
+    instance.dig(1).should == :two
+  end
+
   it "returns the nested value specified if the sequence includes an index" do
     @instance.dig(:a, :a, :b, 0).should == 1
   end

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

@@ -252,13 +252,22 @@ def initialize_copy(other)
     _attrs[var]
   end
 
-  def dig(key, *more)
-    result = nil
-    begin
-      result = self[key]
-    rescue IndexError, NameError
-      nil # nothing found with key
+  private def read_or_nil(var)
+    case var
+    when Symbol, String
+      return unless _attrs.include?(var.to_sym)
+    else
+      var = Integer(var)
+      a_len = _attrs.length
+      return if (var > a_len - 1) || (var < -a_len)
+      var = _attrs[var]
     end
+
+    Primitive.object_hidden_var_get(self, var)
+  end
+
+  def dig(key, *more)
+    result = read_or_nil(key)
     if Primitive.nil?(result) || more.empty?
       result
     else

src/main/ruby/truffleruby/core/truffle/diggable.rb

@@ -29,11 +29,7 @@ def self.dig(obj, idxs)
       when Array
         obj = obj.at(idx)
       when Struct
-        begin
-          obj = obj[idx]
-        rescue IndexError, NameError
-          obj = nil
-        end
+        obj = obj.__send__(:read_or_nil, idx)
       end
 
       return nil if Primitive.nil?(obj)