comment tweak

Author: RalfJung

src/borrow_tracker/tree_borrows/mod.rs

@@ -355,8 +355,11 @@ trait EvalContextPrivExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
             // Store initial permissions.
             for (_loc_range, loc) in perms_map.iter_mut(range.start, range.size) {
                 let sifa = perm.strongest_idempotent_foreign_access(protected);
-                // NOTE: `access` is false if and only if `perm` is Cell, so this `if`
-                // doesn't not change whether any code is UB or not.
+                // NOTE: Currently, `access` is false if and only if `perm` is Cell, so this `if`
+                // doesn't not change whether any code is UB or not. We could just always use
+                // `new_accessed` and everything would stay the same. But that seems conceptually
+                // odd, so we keep the initial "accessed" bit of the `LocationState` in sync with whether
+                // a read access is performed below.
                 if access {
                     *loc = LocationState::new_accessed(perm, sifa);
                 } else {