make the new option actually do something

src/diagnostics.rs

@@ -6,6 +6,7 @@ use crate::*;
 /// Miri specific diagnostics
 pub enum NonHaltingDiagnostic {
     PoppedTrackedPointerTag(Item),
+    CreatedAlloc(AllocId),
 }
 
 /// Emit a custom diagnostic without going through the miri-engine machinery
@@ -97,9 +98,12 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
         let this = self.eval_context_ref();
         DIAGNOSTICS.with(|diagnostics| {
             for e in diagnostics.borrow_mut().drain(..) {
+                use NonHaltingDiagnostic::*;
                 let msg = match e {
-                    NonHaltingDiagnostic::PoppedTrackedPointerTag(item) =>
+                    PoppedTrackedPointerTag(item) =>
                         format!("popped tracked tag for item {:?}", item),
+                    CreatedAlloc(AllocId(id)) =>
+                        format!("created allocation with id {}", id),
                 };
                 report_msg(this, msg, false);
             }

src/machine.rs

@@ -334,6 +334,10 @@ impl<'mir, 'tcx> Machine<'mir, 'tcx> for Evaluator<'tcx> {
         alloc: Cow<'b, Allocation>,
         kind: Option<MemoryKind<Self::MemoryKinds>>,
     ) -> (Cow<'b, Allocation<Self::PointerTag, Self::AllocExtra>>, Self::PointerTag) {
+        if Some(id) == memory_extra.tracked_alloc_id {
+            register_diagnostic(NonHaltingDiagnostic::CreatedAlloc(id));
+        }
+
         let kind = kind.expect("we set our STATIC_KIND so this cannot be None");
         let alloc = alloc.into_owned();
         let (stacks, base_tag) =