More consistency in diagnostics

src/stacked_borrows.rs

@@ -346,7 +346,7 @@ impl<'tcx> Stack {
         // Step 1: Find granting item.
         let granting_idx = self.find_granting(access, tag).ok_or_else(|| {
             err_sb_ub(format!(
-                "no item granting {} to tag {:?} over {}[{:#x}..{:#x}] found in borrow stack.",
+                "no item granting {} to tag {:?} exists in the borrow stack for memory range {}[{:#x}..{:#x}].",
                 access,
                 tag,
                 alloc_id,
@@ -485,23 +485,33 @@ impl<'tcx> Stack {
             alloc_range.start.bytes(),
             alloc_range.end().bytes()
         );
-        if derived_from == SbTag::Untagged {
+        // Find all permissions that this tag grants. When we are not doing raw pointer tagging,
+        // there may be many untagged items in the borrow stack, and we want to report the most
+        // permissions that the tag grants.
+        let mut available_permissions = self
+            .borrows
+            .iter()
+            .filter_map(|item| (item.tag == derived_from).then(|| item.perm))
+            .filter(|perm| *perm != Permission::Disabled) // Dont't want to say "parent tag grants Disabled"
+            .collect::<Vec<_>>();
+        available_permissions.sort_by_key(|perm| {
+            match perm {
+                Permission::Unique => 0,
+                Permission::SharedReadWrite => 1,
+                Permission::SharedReadOnly => 2,
+                Permission::Disabled => unreachable!(),
+            }
+        });
+        if let Some(max_granted) = available_permissions.get(0) {
             err_sb_ub(format!(
-                "{}, but parent tag {:?} does not grant the required permission over memory range",
-                action, derived_from
+                "{}, but parent tag {:?} grants insufficient permission: {:?}",
+                action, derived_from, max_granted
             ))
         } else {
-            if let Some(parent) = self.borrows.iter().rev().find(|item| derived_from == item.tag) {
-                err_sb_ub(format!(
-                    "{}, but parent tag {:?} grants insufficient permission: {:?}",
-                    action, derived_from, parent.perm
-                ))
-            } else {
-                err_sb_ub(format!(
-                    "{}, but parent tag {:?} does not exist in the borrow stack for this memory range",
-                    action, derived_from
-                ))
-            }
+            err_sb_ub(format!(
+                "{}, but parent tag {:?} does not exist in the borrow stack for this memory range",
+                action, derived_from
+            ))
         }
     }
 }