Provide slightly better notes when tracking a pointer tag

Author: saethlin

src/diagnostics.rs

@@ -7,6 +7,7 @@ use log::trace;
 use rustc_middle::ty::{self, TyCtxt};
 use rustc_span::{source_map::DUMMY_SP, Span, SpanData, Symbol};
 
+use crate::stacked_borrows::{AccessKind, SbTag};
 use crate::*;
 
 /// Details of premature program termination.
@@ -58,7 +59,7 @@ impl MachineStopType for TerminationInfo {}
 /// Miri specific diagnostics
 pub enum NonHaltingDiagnostic {
     CreatedPointerTag(NonZeroU64),
-    PoppedPointerTag(Item),
+    PoppedPointerTag(Item, AccessKind, Option<SbTag>),
     CreatedCallId(CallId),
     CreatedAlloc(AllocId),
     FreedAlloc(AllocId),
@@ -321,7 +322,16 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
                 use NonHaltingDiagnostic::*;
                 let msg = match e {
                     CreatedPointerTag(tag) => format!("created tag {:?}", tag),
-                    PoppedPointerTag(item) => format!("popped tracked tag for item {:?}", item),
+                    PoppedPointerTag(item, access, tag) =>
+                        match tag {
+                            None => format!("popped tracked tag for item {:?}", item),
+                            Some(tag) => {
+                                format!(
+                                    "popped tracked tag for item {:?} due to {:?} access for {:?}",
+                                    item, access, tag
+                                )
+                            }
+                        },
                     CreatedCallId(id) => format!("function call with id {}", id),
                     CreatedAlloc(AllocId(id)) => format!("created allocation with id {}", id),
                     FreedAlloc(AllocId(id)) => format!("freed allocation with id {}", id),

src/stacked_borrows.rs

@@ -111,7 +111,7 @@ pub struct GlobalState {
 pub type MemoryExtra = RefCell<GlobalState>;
 
 /// Indicates which kind of access is being performed.
-#[derive(Copy, Clone, Hash, PartialEq, Eq)]
+#[derive(Copy, Clone, Hash, PartialEq, Eq, Debug)]
 pub enum AccessKind {
     Read,
     Write,
@@ -300,10 +300,15 @@ impl<'tcx> Stack {
         item: &Item,
         tag: Option<SbTag>,
         global: &GlobalState,
+        cause: AccessKind,
     ) -> InterpResult<'tcx> {
         if let SbTag::Tagged(id) = item.tag {
             if Some(id) == global.tracked_pointer_tag {
-                register_diagnostic(NonHaltingDiagnostic::PoppedPointerTag(item.clone()));
+                register_diagnostic(NonHaltingDiagnostic::PoppedPointerTag(
+                    item.clone(),
+                    cause,
+                    tag,
+                ));
             }
         }
         if let Some(call) = item.protector {
@@ -348,7 +353,7 @@ impl<'tcx> Stack {
             let first_incompatible_idx = self.find_first_write_incompatible(granting_idx);
             for item in self.borrows.drain(first_incompatible_idx..).rev() {
                 trace!("access: popping item {:?}", item);
-                Stack::check_protector(&item, Some(tag), global)?;
+                Stack::check_protector(&item, Some(tag), global, access)?;
             }
         } else {
             // On a read, *disable* all `Unique` above the granting item.  This ensures U2 for read accesses.
@@ -363,7 +368,7 @@ impl<'tcx> Stack {
                 let item = &mut self.borrows[idx];
                 if item.perm == Permission::Unique {
                     trace!("access: disabling item {:?}", item);
-                    Stack::check_protector(item, Some(tag), global)?;
+                    Stack::check_protector(item, Some(tag), global, access)?;
                     item.perm = Permission::Disabled;
                 }
             }
@@ -391,7 +396,7 @@ impl<'tcx> Stack {
 
         // Step 2: Remove all items.  Also checks for protectors.
         for item in self.borrows.drain(..).rev() {
-            Stack::check_protector(&item, None, global)?;
+            Stack::check_protector(&item, None, global, AccessKind::Write)?;
         }
 
         Ok(())