fix ICE in pointer tracking

Author: RalfJung

src/borrow_tracker/stacked_borrows/diagnostics.rs

@@ -455,23 +455,18 @@ impl<'history, 'ecx, 'mir, 'tcx> DiagnosticCx<'history, 'ecx, 'mir, 'tcx> {
         if !global.tracked_pointer_tags.contains(&item.tag()) {
             return;
         }
-        let summary = match self.operation {
-            Operation::Dealloc(_) => None,
-            Operation::Access(AccessOp { kind, tag, .. }) => Some((tag, kind)),
+        let cause = match self.operation {
+            Operation::Dealloc(_) => format!(" due to deallocation"),
+            Operation::Access(AccessOp { kind, tag, .. }) =>
+                format!(" due to {kind:?} access for {tag:?}"),
             Operation::Retag(RetagOp { orig_tag, permission, .. }) => {
-                let kind = match permission
-                    .expect("start_grant should set the current permission before popping a tag")
-                {
-                    Permission::SharedReadOnly => AccessKind::Read,
-                    Permission::Unique => AccessKind::Write,
-                    Permission::SharedReadWrite | Permission::Disabled => {
-                        panic!("Only SharedReadOnly and Unique retags can pop tags");
-                    }
-                };
-                Some((orig_tag, kind))
+                let permission = permission
+                    .expect("start_grant should set the current permission before popping a tag");
+                format!(" due to {permission:?} retag from {orig_tag:?}")
             }
         };
-        self.machine.emit_diagnostic(NonHaltingDiagnostic::PoppedPointerTag(*item, summary));
+
+        self.machine.emit_diagnostic(NonHaltingDiagnostic::PoppedPointerTag(*item, cause));
     }
 }
 

src/diagnostics.rs

@@ -6,7 +6,7 @@ use log::trace;
 use rustc_span::{source_map::DUMMY_SP, SpanData, Symbol};
 use rustc_target::abi::{Align, Size};
 
-use crate::borrow_tracker::{stacked_borrows::diagnostics::TagHistory, AccessKind};
+use crate::borrow_tracker::stacked_borrows::diagnostics::TagHistory;
 use crate::*;
 
 /// Details of premature program termination.
@@ -67,9 +67,8 @@ pub enum NonHaltingDiagnostic {
     ///
     /// new_kind is `None` for base tags.
     CreatedPointerTag(NonZeroU64, Option<String>, Option<(AllocId, AllocRange, ProvenanceExtra)>),
-    /// This `Item` was popped from the borrow stack, either due to an access with the given tag or
-    /// a deallocation when the second argument is `None`.
-    PoppedPointerTag(Item, Option<(ProvenanceExtra, AccessKind)>),
+    /// This `Item` was popped from the borrow stack. The string explains the reason.
+    PoppedPointerTag(Item, String),
     CreatedCallId(CallId),
     CreatedAlloc(AllocId, Size, Align, MemoryKind<MiriMemoryKind>),
     FreedAlloc(AllocId),
@@ -399,15 +398,7 @@ impl<'mir, 'tcx> MiriMachine<'mir, 'tcx> {
                 format!(
                     "created tag {tag:?} for {kind} at {alloc_id:?}{range:?} derived from {orig_tag:?}"
                 ),
-            PoppedPointerTag(item, tag) =>
-                match tag {
-                    None => format!("popped tracked tag for item {item:?} due to deallocation",),
-                    Some((tag, access)) => {
-                        format!(
-                            "popped tracked tag for item {item:?} due to {access:?} access for {tag:?}",
-                        )
-                    }
-                },
+            PoppedPointerTag(item, cause) => format!("popped tracked tag for item {item:?}{cause}"),
             CreatedCallId(id) => format!("function call with id {id}"),
             CreatedAlloc(AllocId(id), size, align, kind) =>
                 format!(