Make dry-run smart about potential file task executions. Fixes ruby#92

lib/rake/file_task.rb

@@ -17,6 +17,10 @@ def needed?
       ! File.exist?(name) || out_of_date?(timestamp) || @application.options.build_all
     end
 
+    def potentially_needed?
+      needed? || prerequisite_tasks.any?(&:potentially_needed?)
+    end
+
     # Time stamp for file task.
     def timestamp
       if File.exist?(name)

lib/rake/task.rb

@@ -192,14 +192,23 @@ def invoke_with_call_chain(task_args, invocation_chain) # :nodoc:
         return if @already_invoked
         @already_invoked = true
         invoke_prerequisites(task_args, new_chain)
-        execute(task_args) if needed?
+
+        if needed?
+          execute(task_args)
+        elsif application.options.dryrun && potentially_needed?
+          application.trace "** May execute (dry run) #{name}"
+        end
       end
     rescue Exception => ex
       add_chain_to(ex, new_chain)
       raise ex
     end
     protected :invoke_with_call_chain
 
+    def potentially_needed?
+      needed?
+    end
+
     def add_chain_to(exception, new_chain) # :nodoc:
       exception.extend(InvocationExceptionMixin) unless
         exception.respond_to?(:chain)

test/test_rake_functional.rb

@@ -243,6 +243,29 @@ def test_dry_run
     refute_match %r{OTHER}, @out
   end
 
+  def test_dry_run_smart_about_potential_file_executions
+    FileUtils.touch("fileA")
+    FileUtils.touch("fileB")
+    FileUtils.touch("fileC")
+    FileUtils.touch("fileD")
+
+    rakefile <<-RAKEFILE
+      file "fileA" do; end
+      file "fileB" => ['fileA'] do; end
+      file "fileC" => ['fileB'] do; end
+      file "fileD" => ['fileC'] do; end
+    RAKEFILE
+
+    sleep 1
+    FileUtils.touch("fileA")
+
+    rake "-n", "fileD"
+
+    assert_match(/Execute \(dry run\) fileB/, @err)
+    assert_match(/May execute \(dry run\) fileC/, @err)
+    assert_match(/May execute \(dry run\) fileD/, @err)
+  end
+
   # Test for the trace/dry_run bug found by Brian Chandler
   def test_dry_run_bug
     rakefile_dryrun