Simplification rule in PVL program triggers ColToSilver$NotSupported exception

[wandernauta]

The specification grammar allows providing axioms at the top level, which are then treated as SimplificationRules. However, trying to add such a simplification rule to the input being verified (so not simplify.pvl), as in the following PVL input, triggers an exception:

axiom add { 2 + 2 == 4 }

This kind of node (SimplificationRule) is not supported by silver directly. Is there a rewrite missing?

The same applies to the C, C++ and Java frontends.

[INFO] Done: VerCors (at 16:19:57, duration: 00:00:03)
viper.api.transform.ColToSilver$NotSupported: ======================================
9 }
   10 
     [------------------------
   11 axiom Add { 2 + 2 == 4 }
      ------------------------]
   12 
   13 void `type`() {
--------------------------------------
This kind of node (SimplificationRule) is not supported by silver directly. Is there a rewrite missing?
======================================
	at viper.api.transform.ColToSilver.$qmark$qmark(ColToSilver.scala:57)
	at viper.api.transform.ColToSilver.collect(ColToSilver.scala:221)
	at viper.api.transform.ColToSilver.$anonfun$transform$16(ColToSilver.scala:176)
	at viper.api.transform.ColToSilver.$anonfun$transform$16$adapted(ColToSilver.scala:176)
	at scala.collection.immutable.List.foreach(List.scala:333)
	at viper.api.transform.ColToSilver.transform(ColToSilver.scala:176)
	at viper.api.transform.ColToSilver$.transform(ColToSilver.scala:26)
	at viper.api.backend.SilverBackend.$anonfun$transform$1(SilverBackend.scala:91)
	at hre.progress.task.NameSequenceTask.scope(NameSequenceTask.scala:16)
	at hre.progress.Progress$.stages(Progress.scala:47)
	at viper.api.backend.SilverBackend.transform(SilverBackend.scala:90)
	at viper.api.backend.SilverBackend.transform$(SilverBackend.scala:81)
	at viper.api.backend.silicon.Silicon.transform(Silicon.scala:34)
	at vct.main.stages.SilverBackend.transform(Backend.scala:196)
	at vct.main.stages.Backend.$anonfun$run$3(Backend.scala:152)
	at hre.progress.Progress$.$anonfun$map$2(Progress.scala:35)
	at hre.progress.task.AbstractTask.frame(AbstractTask.scala:166)
	at hre.progress.Progress$.$anonfun$map$1(Progress.scala:35)
	at scala.collection.parallel.IterableSplitter$Mapped.next(RemainsIterator.scala:463)
	at scala.collection.immutable.List.prependedAll(List.scala:153)
	at scala.collection.immutable.List$.from(List.scala:684)
	at scala.collection.immutable.List$.from(List.scala:681)
	at scala.collection.SeqFactory$Delegate.from(Factory.scala:306)
	at scala.collection.immutable.Seq$.from(Seq.scala:42)
	at scala.collection.immutable.Seq$.from(Seq.scala:39)
	at scala.collection.IterableFactory$ToFactory.fromSpecific(Factory.scala:274)
	at scala.collection.IterableOnceOps.to(IterableOnce.scala:1310)
	at scala.collection.IterableOnceOps.to$(IterableOnce.scala:1310)
	at scala.collection.parallel.IterableSplitter$Mapped.to(RemainsIterator.scala:460)
	at vct.main.stages.Backend.$anonfun$run$1(Backend.scala:153)
	at hre.progress.task.NameSequenceTask.scope(NameSequenceTask.scala:16)
	at hre.progress.Progress$.stages(Progress.scala:47)
	at vct.main.stages.Backend.run(Backend.scala:145)
	at vct.main.stages.Backend.run$(Backend.scala:137)
	at vct.main.stages.SilverBackend.run(Backend.scala:184)
	at vct.main.stages.SilverBackend.run(Backend.scala:184)
	at hre.stages.Stages.$anonfun$run$3(Stages.scala:104)
	at hre.stages.Stages.$anonfun$run$3$adapted(Stages.scala:101)
	at scala.collection.IterableOnceOps.foreach(IterableOnce.scala:576)
	at scala.collection.IterableOnceOps.foreach$(IterableOnce.scala:574)
	at scala.collection.AbstractIterable.foreach(Iterable.scala:933)
	at scala.collection.IterableOps$WithFilter.foreach(Iterable.scala:903)
	at hre.stages.Stages.$anonfun$run$1(Stages.scala:101)
	at hre.progress.task.NameSequenceTask.scope(NameSequenceTask.scala:16)
	at hre.progress.Progress$.stages(Progress.scala:47)
	at hre.stages.Stages.run(Stages.scala:98)
	at hre.stages.Stages.run$(Stages.scala:95)
	at hre.stages.StagesPair.run(Stages.scala:145)
	at vct.main.modes.Verify$.verifyWithOptions(Verify.scala:64)
	at vct.main.modes.Verify$.$anonfun$runOptions$3(Verify.scala:99)
	at scala.runtime.java8.JFunction0$mcI$sp.apply(JFunction0$mcI$sp.scala:17)
	at hre.util.Time$.logTime(Time.scala:23)
	at vct.main.modes.Verify$.runOptions(Verify.scala:99)
	at vct.main.Main$.runMode(Main.scala:107)
	at vct.main.Main$.$anonfun$runOptions$3(Main.scala:100)
	at scala.runtime.java8.JFunction0$mcI$sp.apply(JFunction0$mcI$sp.scala:17)
	at hre.middleware.Middleware$.using(Middleware.scala:78)
	at vct.main.Main$.$anonfun$runOptions$2(Main.scala:100)
	at scala.runtime.java8.JFunction0$mcI$sp.apply(JFunction0$mcI$sp.scala:17)
	at hre.io.Watch$.booleanWithWatch(Watch.scala:58)
	at vct.main.Main$.$anonfun$runOptions$1(Main.scala:100)
	at scala.runtime.java8.JFunction0$mcI$sp.apply(JFunction0$mcI$sp.scala:17)
	at hre.middleware.Middleware$.using(Middleware.scala:78)
	at vct.main.Main$.runOptions(Main.scala:95)
	at vct.main.Main$.main(Main.scala:50)
	at vct.main.Main.main(Main.scala)
[ERROR] !*!*!*!*!*!*!*!*!*!*!*!
[ERROR] ! VerCors has crashed !
[ERROR] !*!*!*!*!*!*!*!*!*!*!*!

Version: 2bd3bca (dev branch).

This issue was found by fuzzing.