Add in-tree mspec and ruby/spec

* For easier modifications of ruby/spec by MRI developers.
* .gitignore: track changes under spec.
* spec/mspec, spec/rubyspec: add in-tree mspec and ruby/spec.
  These files can therefore be updated like any other file in MRI.
  Instructions are provided in spec/README.
  [Feature #13156] [ruby-core:79246]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@58595 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

spec/rubyspec/core/array/dig_spec.rb

@@ -0,0 +1,54 @@
+require File.expand_path('../../../spec_helper', __FILE__)
+
+ruby_version_is '2.3' do
+  describe "Array#dig" do
+
+    it "returns #at with one arg" do
+      ['a'].dig(0).should == 'a'
+      ['a'].dig(1).should be_nil
+    end
+
+    it "recurses array elements" do
+      a = [ [ 1, [2, '3'] ] ]
+      a.dig(0, 0).should == 1
+      a.dig(0, 1, 1).should == '3'
+      a.dig(0, -1, 0).should == 2
+    end
+
+    it "returns the nested value specified if the sequence includes a key" do
+      a = [42, { foo: :bar }]
+      a.dig(1, :foo).should == :bar
+    end
+
+    it "raises a TypeError for a non-numeric index" do
+      lambda {
+        ['a'].dig(:first)
+      }.should raise_error(TypeError)
+    end
+
+    it "raises a TypeError if any intermediate step does not respond to #dig" do
+      a = [1, 2]
+      lambda {
+        a.dig(0, 1)
+      }.should raise_error(TypeError)
+    end
+
+    it "raises an ArgumentError if no arguments provided" do
+      lambda {
+        [10].dig()
+      }.should raise_error(ArgumentError)
+    end
+
+    it "returns nil if any intermediate step is nil" do
+      a = [[1, [2, 3]]]
+      a.dig(1, 2, 3).should == nil
+    end
+
+    it "calls #dig on the result of #at with the remaining arguments" do
+      h = [[nil, [nil, nil, 42]]]
+      h[0].should_receive(:dig).with(1, 2).and_return(42)
+      h.dig(0, 1, 2).should == 42
+    end
+
+  end
+end