Add safe invocation CLI syntax

SUch syntax is useful for projects like buildpacks
and other automated CI/CD tools.

It would allow these tools to very easily invoke a task
if it exists, without failing if it doesn't.

lib/rake/application.rb

@@ -155,19 +155,21 @@ def thread_pool             # :nodoc:
 
     # Invokes a task with arguments that are extracted from +task_string+
     def invoke_task(task_string) # :nodoc:
-      name, args = parse_task_string(task_string)
+      name, args, safe_call = parse_task_string(task_string)
+      return if safe_call && !lookup(name)
+
       t = self[name]
       t.invoke(*args)
     end
 
     def parse_task_string(string) # :nodoc:
-      /^([^\[]+)(?:\[(.*)\])$/ =~ string.to_s
-
+      /^([^\[]+)(?:\[(.*)\])(\?)?$/ =~ string.to_s
       name           = $1
       remaining_args = $2
+      safe_call      = !!$~.begin(3)
 
-      return string, [] unless name
-      return name,   [] if     remaining_args.empty?
+      return string, [], safe_call unless name
+      return name,   [], safe_call if remaining_args.empty?
 
       args = []
 
@@ -178,7 +180,7 @@ def parse_task_string(string) # :nodoc:
         args << $1.gsub(/\\(.)/, '\1')
       end while remaining_args
 
-      return name, args
+      return name, args, safe_call
     end
 
     # Provide standard exception handling for the given block.

test/test_rake_application.rb

@@ -439,6 +439,26 @@ def test_good_run
     assert_equal "DEFAULT\n", out
   end
 
+  def test_safe_run
+    ran = false
+
+    @app.options.silent = true
+
+    @app.instance_eval do
+      intern(Rake::Task, "default").enhance { ran = true }
+    end
+
+    rakefile_default
+
+    out, err = capture_io do
+      @app.run %w[--rakelib="" default missing?]
+    end
+
+    assert ran
+    assert_empty err
+    assert_equal "DEFAULT\n", out
+  end
+
   def test_display_task_run
     ran = false
     @app.last_description = "COMMENT"