Fix bug where successful selective verification is still cached

In general, when skipping verification, we introduced a so called
fake_error to make sure the compiler does not just cache this
as successful verification and then skip later verifications even
if that's not intended. Same issue occured for selective verification,
when verifying a single method that is "correct", this would be cached.
Now we throw a fake error in this case too, to avoid this.

prusti/src/callbacks.rs

@@ -152,20 +152,24 @@ impl prusti_rustc_interface::driver::Callbacks for PrustiCompilerCalls {
                         procedures: annotated_procedures,
                         types,
                     };
-                    verify(env, def_spec, verification_task);
+                    verify(&env, def_spec, verification_task);
                 } else {
                     let target_def_path = config::selective_verify().unwrap();
                     let procedures = annotated_procedures
                         .into_iter()
                         .filter(|x| env.name.get_unique_item_name(*x) == target_def_path)
                         .collect();
                     let selective_task = VerificationTask { procedures, types };
-                    verify(env, def_spec, selective_task);
+                    // fake_error because otherwise a verification-success 
+                    // (for a single method for example) will cause this result
+                    // to be cached by compiler at the moment
+                    verify(&env, def_spec, selective_task);
+                    fake_error(&env);
                 }
             } else if config::show_ide_info() && config::skip_verification() && !config::no_verify()
             {
                 // add a fake error, reason explained in issue #1261
-                fake_error(env)
+                fake_error(&env)
             }
         });
 

prusti/src/ide_helper/fake_error.rs

@@ -5,7 +5,7 @@ use prusti_rustc_interface::{errors::MultiSpan, span::DUMMY_SP};
 /// when we just collect information for prusti-assistant), because then
 /// a successful result will be cached and subsequent actual verifications succeed
 /// (see issue #1261)
-pub fn fake_error(env: Environment) {
+pub fn fake_error(env: &Environment) {
     let sp = MultiSpan::from_span(DUMMY_SP);
     let message = String::from("[Prusti: FakeError]");
     let help = None;

prusti/src/verifier.rs

@@ -10,7 +10,7 @@ use prusti_interface::{
 use prusti_viper::verifier::Verifier;
 
 pub fn verify<'tcx>(
-    env: Environment<'tcx>,
+    env: &Environment<'tcx>,
     def_spec: typed::DefSpecificationMap,
     verification_task: VerificationTask<'tcx>,
 ) {