Return only commands when completing help command's argument

The command only takes command names as arguments, so we should only
return command names as candidates.

This will help users find a command faster as completion will be
another useful hint too.

lib/irb/completion.rb

@@ -86,8 +86,8 @@ def retrieve_files_to_require_from_load_path
         )
     end
 
-    def command_completions(preposing, target)
-      if preposing.empty? && !target.empty?
+    def command_candidates(target)
+      if !target.empty?
         IRB::Command.command_names.select { _1.start_with?(target) }
       else
         []
@@ -111,8 +111,17 @@ def inspect
     end
 
     def completion_candidates(preposing, target, _postposing, bind:)
-      commands = command_completions(preposing, target)
+      preposing ||= ''
+      commands = command_candidates(target)
+
+      # When completing the argument of `help` command, only commands should be candidates
+      return commands if preposing.match?(/\Ahelp\s+/)
+
+      # It doesn't make sense to propose commands with other preposing
+      commands = [] unless preposing.empty?
+
       result = ReplTypeCompletor.analyze(preposing + target, binding: bind, filename: @context.irb_path)
+
       return commands unless result
 
       commands | result.completion_candidates.map { target + _1 }
@@ -191,8 +200,18 @@ def completion_candidates(preposing, target, postposing, bind:)
         result = complete_require_path(target, preposing, postposing)
         return result if result
       end
-      commands = command_completions(preposing || '', target)
-      commands | retrieve_completion_data(target, bind: bind, doc_namespace: false).compact.map{ |i| i.encode(Encoding.default_external) }
+
+      preposing ||= ''
+      commands = command_candidates(target)
+
+      # When completing the argument of `help` command, only commands should be candidates
+      return commands if preposing.match?(/\Ahelp\s+/)
+
+      # It doesn't make sense to propose commands with other preposing
+      commands = [] unless preposing.empty?
+
+      completion_data = retrieve_completion_data(target, bind: bind, doc_namespace: false).compact.map{ |i| i.encode(Encoding.default_external) }
+      commands | completion_data
     end
 
     def doc_namespace(_preposing, matched, _postposing, bind:)

test/irb/test_completion.rb

@@ -16,8 +16,14 @@ def doc_namespace(target, bind)
 
     class CommandCompletionTest < CompletionTest
       def test_command_completion
-        assert_include(IRB::RegexpCompletor.new.completion_candidates('', 'show_s', '', bind: binding), 'show_source')
-        assert_not_include(IRB::RegexpCompletor.new.completion_candidates(';', 'show_s', '', bind: binding), 'show_source')
+        completor = IRB::RegexpCompletor.new
+        binding.eval("some_var = 1")
+        # completion for help command's argument should only include command names
+        assert_include(completor.completion_candidates('help ', 's', '', bind: binding), 'show_source')
+        assert_not_include(completor.completion_candidates('help ', 's', '', bind: binding), 'some_var')
+
+        assert_include(completor.completion_candidates('', 'show_s', '', bind: binding), 'show_source')
+        assert_not_include(completor.completion_candidates(';', 'show_s', '', bind: binding), 'show_source')
       end
     end
 

test/irb/test_type_completor.rb

@@ -56,6 +56,11 @@ def test_empty_completion
     end
 
     def test_command_completion
+      binding.eval("some_var = 1")
+      # completion for help command's argument should only include command names
+      assert_include(@completor.completion_candidates('help ', 's', '', bind: binding), 'show_source')
+      assert_not_include(@completor.completion_candidates('help ', 's', '', bind: binding), 'some_var')
+
       assert_include(@completor.completion_candidates('', 'show_s', '', bind: binding), 'show_source')
       assert_not_include(@completor.completion_candidates(';', 'show_s', '', bind: binding), 'show_source')
     end