Error out early if there is no app configured

lib/puma/cli.rb

@@ -419,6 +419,11 @@ module Puma
 
       @binder.parse @options[:binds], self
 
+      unless @config.app_configured?
+        error "No application configured, nothing to run"
+        exit 1
+      end
+
       if @options[:daemon]
         Process.daemon(true, @io_redirected)
       end
@@ -651,6 +656,11 @@ module Puma
 
       @binder.parse @options[:binds], self
 
+      unless @config.app_configured?
+        error "No application configured, nothing to run"
+        exit 1
+      end
+
       read, write = Puma::Util.pipe
 
       Signal.trap "SIGCHLD" do

lib/puma/configuration.rb

@@ -67,6 +67,17 @@ module Puma
       end
     end
 
+    # Indicate if there is a properly configured app
+    #
+    def app_configured?
+      return true if @options[:app]
+      if path = @options[:rackup]
+        return File.exists?(path)
+      end
+
+      false
+    end
+
     # Load the specified rackup file, pull an options from
     # the rackup file, and set @app.
     #