Remove an optimistic integrity check we can no longer do

Author: saethlin

src/stacked_borrows/stack.rs

@@ -75,9 +75,8 @@ impl Stack {
 
         #[cfg(feature = "stack-cache")]
         if let Some(first_removed) = first_removed {
-            // Base tags are Unique or SharedReadOnly.
-            // When the base tag is SharedReadOnly, we do nothing. When the base tag is Unique,
-            // the start of this range is still precise.
+            // Either end of unique_range may have shifted, all we really know is that we can't
+            // have introduced a new Unique.
             if !self.unique_range.is_empty() {
                 self.unique_range = 0..self.len();
             }
@@ -159,14 +158,11 @@ impl<'tcx> Stack {
 
         // Check that the unique_range is a valid index into the borrow stack.
         // This asserts that the unique_range's start <= end.
-        let uniques = &self.borrows[self.unique_range.clone()];
+        let _uniques = &self.borrows[self.unique_range.clone()];
 
-        // Check that the start of the unique_range is precise.
-        if let Some(first_unique) = uniques.first() {
-            assert_eq!(first_unique.perm(), Permission::Unique);
-        }
-        // We cannot assert that the unique range is exact on the upper end.
-        // When we pop items within the unique range, setting the end of the range precisely
+        // We cannot assert that the unique range is precise.
+        // Both ends may shift around when `Stack::retain` is called. Additionally,
+        // when we pop items within the unique range, setting the end of the range precisely
         // requires doing a linear search of the borrow stack, which is exactly the kind of
         // operation that all this caching exists to avoid.
     }