Tell the user about stacked borrow debugging flags

src/stacked_borrows.rs

@@ -4,6 +4,7 @@
 use std::cell::RefCell;
 use std::collections::{HashMap, HashSet};
 use std::fmt;
+use std::fmt::Write;
 use std::num::NonZeroU64;
 use std::rc::Rc;
 
@@ -278,10 +279,13 @@ impl<'tcx> Stack {
         if let Some(call) = item.protector {
             if global.is_active(call) {
                 if let Some(tag) = tag {
-                    throw_ub!(UbExperimental(format!(
-                        "not granting access to tag {:?} because incompatible item is protected: {:?}",
-                        tag, item
-                    )));
+                    return Err(err_ub_experimental(
+                        tag,
+                        format!(
+                            "not granting access to tag {:?} because incompatible item is protected: {:?}",
+                            tag, item
+                        ),
+                    ));
                 } else {
                     throw_ub!(UbExperimental(format!(
                         "deallocating while item is protected: {:?}",
@@ -300,10 +304,10 @@ impl<'tcx> Stack {
 
         // Step 1: Find granting item.
         let granting_idx = self.find_granting(access, tag).ok_or_else(|| {
-            err_ub!(UbExperimental(format!(
-                "no item granting {} to tag {:?} found in borrow stack",
-                access, tag,
-            )))
+            err_ub_experimental(
+                tag,
+                format!("no item granting {} to tag {:?} found in borrow stack.", access, tag),
+            )
         })?;
 
         // Step 2: Remove incompatible items above them.  Make sure we do not remove protected
@@ -344,10 +348,11 @@ impl<'tcx> Stack {
     fn dealloc(&mut self, tag: Tag, global: &GlobalState) -> InterpResult<'tcx> {
         // Step 1: Find granting item.
         self.find_granting(AccessKind::Write, tag).ok_or_else(|| {
-            err_ub!(UbExperimental(format!(
+            err_ub_experimental(
+                tag,format!(
                 "no item granting write access for deallocation to tag {:?} found in borrow stack",
                 tag,
-            )))
+            ))
         })?;
 
         // Step 2: Remove all items.  Also checks for protectors.
@@ -369,9 +374,14 @@ impl<'tcx> Stack {
         // Now we figure out which item grants our parent (`derived_from`) this kind of access.
         // We use that to determine where to put the new item.
         let granting_idx = self.find_granting(access, derived_from)
-            .ok_or_else(|| err_ub!(UbExperimental(format!(
-                "trying to reborrow for {:?}, but parent tag {:?} does not have an appropriate item in the borrow stack", new.perm, derived_from,
-            ))))?;
+            .ok_or_else(||
+            err_ub_experimental(
+                derived_from,
+                format!(
+                    "trying to reborrow for {:?}, but parent tag {:?} does not have an appropriate item in the borrow stack",
+                    new.perm, derived_from,
+                ),
+            ))?;
 
         // Compute where to put the new item.
         // Either way, we ensure that we insert the new item in a way such that between
@@ -638,3 +648,11 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
         Ok(())
     }
 }
+
+fn err_ub_experimental(tag: Tag, mut msg: String) -> InterpErrorInfo<'static> {
+    if let Tag::Tagged(id) = tag {
+        // FIXME: do not add this message when the flag is already set
+        write!(msg, " Rerun with `-Zmiri-track-pointer-tag={}` for more information", id).unwrap();
+    }
+    err_ub!(UbExperimental(msg)).into()
+}