[ruby/irb] Introduce cd command

(https://github.com/ruby/irb/pull/971)

It's essentially a combination of pushws and popws commands that are
easier to use.

Help message:

```
Usage: cd ([target]|..)

IRB uses a stack of workspaces to keep track of context(s), with `pushws` and `popws` commands to manipulate the stack.
The `cd` command is an attempt to simplify the operation and will be subject to change.

When given:
- an object, cd will use that object as the new context by pushing it onto the workspace stack.
- "..", cd will leave the current context by popping the top workspace off the stack.
- no arguments, cd will move to the top workspace on the stack by popping off all workspaces.

Examples:

  cd Foo
  cd Foo.new
  cd @ivar
  cd ..
  cd
```

https://github.com/ruby/irb/commit/4a0e0e89b7

1 files changed, 65 insertions, 0 deletions

diff --git a/test/irb/command/test_cd.rb b/test/irb/command/test_cd.rb
new file mode 100644
index 0000000000..4537286f73
--- /dev/null
+++ b/test/irb/command/test_cd.rb
@@ -0,0 +1,65 @@
+require "tempfile"
+require_relative "../helper"
+
+module TestIRB
+  class CDTest < IntegrationTestCase
+    def setup
+      super
+
+      write_ruby <<~'RUBY'
+        class Foo
+          class Bar
+            def bar
+              "this is bar"
+            end
+          end
+
+          def foo
+            "this is foo"
+          end
+        end
+
+        binding.irb
+      RUBY
+    end
+
+    def test_cd
+      out = run_ruby_file do
+        type "cd Foo"
+        type "ls"
+        type "cd Bar"
+        type "ls"
+        type "cd .."
+        type "exit"
+      end
+
+      assert_match(/irb\(Foo\):002>/, out)
+      assert_match(/Foo#methods: foo/, out)
+      assert_match(/irb\(Foo::Bar\):004>/, out)
+      assert_match(/Bar#methods: bar/, out)
+      assert_match(/irb\(Foo\):006>/, out)
+    end
+
+    def test_cd_moves_top_level_with_no_args
+      out = run_ruby_file do
+        type "cd Foo"
+        type "cd Bar"
+        type "cd"
+        type "exit"
+      end
+
+      assert_match(/irb\(Foo::Bar\):003>/, out)
+      assert_match(/irb\(main\):004>/, out)
+    end
+
+    def test_cd_with_error
+      out = run_ruby_file do
+        type "cd Baz"
+        type "exit"
+      end
+
+      assert_match(/Error: uninitialized constant Baz/, out)
+      assert_match(/irb\(main\):002>/, out) # the context should not change
+    end
+  end
+end