make the new option actually do something

README.md

@@ -180,6 +180,8 @@ Several `-Z` flags are relevant for Miri:
   is popped from a borrow stack (which is where the tag becomes invalid and any
   future use of it will error).  This helps you in finding out why UB is
   happening and where in your code would be a good place to look for it.
+* `-Zmiri-track-alloc-id=<id>` shows a backtrace when the given allocation is
+  being allocated.  This helps in debugging memory leaks.
 
 Moreover, Miri recognizes some environment variables:
 

src/diagnostics.rs

@@ -6,6 +6,7 @@
 /// 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 @@ fn process_diagnostics(&self) {
         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 @@ fn init_allocation_extra<'b>(
         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) =