Merge pull request #25690 from robin850/speed-up-rdoc

Speed up RDoc generation when it's already been done once

railties/lib/rails/api/task.rb

@@ -121,6 +121,19 @@ def configure_rdoc_files
             rdoc_files.exclude("#{cdr}/#{pattern}")
           end
         end
+
+        # Only generate documentation for files that have been
+        # changed since the API was generated.
+        if Dir.exist?('doc/rdoc') && !ENV['ALL']
+          last_generation = DateTime.rfc2822(File.open('doc/rdoc/created.rid', &:readline))
+
+          rdoc_files.keep_if do |file|
+            File.mtime(file).to_datetime > last_generation
+          end
+
+          # Nothing to do
+          exit(0) if rdoc_files.empty?
+        end
       end
 
       def setup_horo_variables