Raise error if incomplete trigger block is defined

lib/vagrant/errors.rb

@@ -780,6 +780,10 @@ module Vagrant
       error_key(:triggers_guest_not_running)
     end
 
+    class TriggersNoBlockGiven < VagrantError
+      error_key(:triggers_no_block_given)
+    end
+
     class TriggersNoStageGiven < VagrantError
       error_key(:triggers_no_stage_given)
     end

plugins/kernel_v2/config/trigger.rb

@@ -49,6 +49,9 @@ module VagrantPlugins
           # so the last element should be the "config block"
           # and the rest are commands for the trigger
           blk = command.pop
+        elsif !block_given?
+          raise Vagrant::Errors::TriggersNoBlockGiven,
+            command: command
         end
 
         command.each do |cmd|
@@ -71,6 +74,9 @@ module VagrantPlugins
           # so the last element should be the "config block"
           # and the rest are commands for the trigger
           blk = command.pop
+        elsif !block_given?
+          raise Vagrant::Errors::TriggersNoBlockGiven,
+            command: command
         end
 
         command.each do |cmd|

templates/locales/en.yml

@@ -1414,6 +1414,9 @@ en:
         Trigger run failed
       triggers_guest_not_running: |-
         Could not run remote script on %{machine_name} because its state is %{state}
+      triggers_no_block_given: |-
+        There was an error parsing the Vagrantfile:
+        No config was given for the trigger(s) %{command}.
       triggers_no_stage_given: |-
         The incorrect stage was given to the trigger plugin:
         Guest: %{guest_name}