TB: optimize accesses on large trees by ignoring subtrees if the acce…

…ss would mostly be a NOP

src/borrow_tracker/tree_borrows/perms.rs

@@ -237,6 +237,10 @@ impl Permission {
     pub fn is_active(&self) -> bool {
         self.inner == Active
     }
+    /// Check if `self` is the never-allow-writes-again state of a pointer (is `Frozen`).
+    pub fn is_frozen(&self) -> bool {
+        self.inner == Frozen
+    }
 
     /// Default initial permission of the root of a new tree at inbounds positions.
     /// Must *only* be used for the root, this is not in general an "initial" permission!

src/borrow_tracker/tree_borrows/tree.rs

@@ -153,8 +153,31 @@ impl LocationState {
     ) -> ContinueTraversal {
         if rel_pos.is_foreign() {
             let happening_now = IdempotentForeignAccess::from_foreign(access_kind);
-            let new_access_noop =
+            let mut new_access_noop =
                 self.idempotent_foreign_access.can_skip_foreign_access(happening_now);
+            if self.permission.is_disabled() {
+                // A foreign access to a `Disabled` tag will have almost no observable effect.
+                // It's a theorem that `Disabled` node have no protected initialized children,
+                // and so this foreign access will never trigger any protector.
+                // (Intuition: You're either protected initialized, and thus can't become Disabled
+                // or you're already Disabled protected, but not initialized, and then can't
+                // become initialized since that requires a child access, which Disabled blocks.)
+                // Further, the children will never be able to read or write again, since they
+                // have a `Disabled` parent. So this only affects diagnostics, such that the
+                // blocking write will still be identified directly, just at a different tag.
+                new_access_noop = true;
+            }
+            if self.permission.is_frozen() && access_kind == AccessKind::Read {
+                // A foreign read to a `Frozen` tag will have almost no observable effect.
+                // It's a theorem that `Frozen` nodes have no active children, so all children
+                // already survive foreign reads. Foreign reads in general have almost no
+                // effect, the only further thing they could do is make protected `Reserved`
+                // nodes become conflicted, i.e. make them reject child writes for the further
+                // duration of their protector. But such a child write is already rejected
+                // because this node is frozen. So this only affects diagnostics, but the
+                // blocking read will still be identified directly, just at a different tag.
+                new_access_noop = true;
+            }
             if new_access_noop {
                 // Abort traversal if the new access is indeed guaranteed
                 // to be noop.

tests/fail/tree_borrows/subtree_traversal_skipping_diagnostics.stderr

@@ -5,18 +5,18 @@ LL |     *m = 42;
    |     ^^^^^^^ write access through <TAG> at ALLOC[0x0] is forbidden
    |
    = help: this indicates a potential bug in the program: it performed an invalid operation, but the Tree Borrows rules it violated are still experimental
-   = help: the accessed tag <TAG> has state Reserved (conflicted) which forbids this child write access
-help: the accessed tag <TAG> was created here, in the initial state Reserved
+   = help: the accessed tag <TAG> is a child of the conflicting tag <TAG>
+   = help: the conflicting tag <TAG> has state Frozen which forbids this child write access
+help: the accessed tag <TAG> was created here
   --> tests/fail/tree_borrows/subtree_traversal_skipping_diagnostics.rs:LL:CC
    |
 LL | fn write_to_mut(m: &mut u8, other_ptr: *const u8) {
    |                 ^
-help: the accessed tag <TAG> later transitioned to Reserved (conflicted) due to a foreign read access at offsets [0x0..0x1]
+help: the conflicting tag <TAG> was created here, in the initial state Frozen
   --> tests/fail/tree_borrows/subtree_traversal_skipping_diagnostics.rs:LL:CC
    |
-LL |         std::hint::black_box(*other_ptr);
-   |                              ^^^^^^^^^^
-   = help: this transition corresponds to a temporary loss of write permissions until function exit
+LL |         let intermediary = &root;
+   |                            ^^^^^
    = note: BACKTRACE (of the first span):
    = note: inside `write_to_mut` at tests/fail/tree_borrows/subtree_traversal_skipping_diagnostics.rs:LL:CC
 note: inside `main`