add test to demonstrate the effect of #4008

src/tools/miri/tests/fail/tree_borrows/subtree_traversal_skipping_diagnostics.rs

@@ -0,0 +1,29 @@
+//@compile-flags: -Zmiri-tree-borrows -Zmiri-provenance-gc=0
+
+// Shows the effect of the optimization of #4008.
+// The diagnostics change, but not the error itself.
+
+// When this method is called, the tree will be a single line and look like this,
+// with other_ptr being the root at the top
+// other_ptr = root : Active
+// intermediary     : Frozen // an intermediary node
+// m                : Reserved
+fn write_to_mut(m: &mut u8, other_ptr: *const u8) {
+    unsafe {
+        std::hint::black_box(*other_ptr);
+    }
+    // In line 17 above, m should have become Reserved (protected) so that this write is impossible.
+    // However, that does not happen because the read above is not forwarded to the subtree below
+    // the Frozen intermediary node. This does not affect UB, however, because the Frozen that blocked
+    // the read already prevents any child writes.
+    *m = 42; //~ERROR: /write access through .* is forbidden/
+}
+
+fn main() {
+    let root = 42u8;
+    unsafe {
+        let intermediary = &root;
+        let data = &mut *(core::ptr::addr_of!(*intermediary) as *mut u8);
+        write_to_mut(data, core::ptr::addr_of!(root));
+    }
+}

src/tools/miri/tests/fail/tree_borrows/subtree_traversal_skipping_diagnostics.stderr

@@ -0,0 +1,31 @@
+error: Undefined Behavior: write access through <TAG> at ALLOC[0x0] is forbidden
+  --> tests/fail/tree_borrows/subtree_traversal_skipping_diagnostics.rs:LL:CC
+   |
+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
+  --> 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]
+  --> 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
+   = 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`
+  --> tests/fail/tree_borrows/subtree_traversal_skipping_diagnostics.rs:LL:CC
+   |
+LL |         write_to_mut(data, core::ptr::addr_of!(root));
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+note: some details are omitted, run with `MIRIFLAGS=-Zmiri-backtrace=full` for a verbose backtrace
+
+error: aborting due to 1 previous error
+