Add UI test for protector end write semantics

src/tools/miri/tests/fail/tree_borrows/protector-write-lazy.rs

@@ -0,0 +1,35 @@
+//@compile-flags: -Zmiri-tree-borrows
+// This test tests that TB's protector end semantics correctly ensure
+// that protected activated writes can be reordered.
+fn the_other_function(ref_to_fst_elem: &mut i32, ptr_to_vec: *mut i32) -> *mut i32 {
+    // Activate the reference. Afterwards, we should be able to reorder arbitrary writes.
+    *ref_to_fst_elem = 0;
+    // Here is such an arbitrary write.
+    // It could be moved down after the retag, in which case the `funky_ref` would be invalidated.
+    // We need to ensure that the `funky_ptr` is unusable even if the write to `ref_to_fst_elem`
+    // happens before the retag.
+    *ref_to_fst_elem = 42;
+    // this creates a reference that is Reserved Lazy on the first element (offset 0).
+    // It does so by doing a proper retag on the second element (offset 1), which is fine
+    // since nothing else happens at that offset, but the lazy init mechanism means it's
+    // also reserved at offset 0, but not initialized.
+    let funky_ptr_lazy_on_fst_elem =
+        unsafe { (&mut *(ptr_to_vec.wrapping_add(1))) as *mut i32 }.wrapping_sub(1);
+    // If we write to `ref_to_fst_elem` here, then any further access to `funky_ptr_lazy_on_fst_elem` would
+    // definitely be UB. Since the compiler ought to be able to reorder the write of `42` above down to
+    // here, that means we want this program to also be UB.
+    return funky_ptr_lazy_on_fst_elem;
+}
+
+fn main() {
+    let mut v = vec![0, 1];
+    // get a pointer to the root of the allocation
+    // note that it's not important it's the actual root, what matters is that it's a parent
+    // of both references that will be created
+    let ptr_to_vec = v.as_mut_ptr();
+    let ref_to_fst_elem = unsafe { &mut *ptr_to_vec };
+    let funky_ptr_lazy_on_fst_elem = the_other_function(ref_to_fst_elem, ptr_to_vec);
+    // now we try to use the funky lazy pointer.
+    // It should be UB, since the write-on-protector-end should disable it.
+    unsafe { println!("Value of funky: {}", *funky_ptr_lazy_on_fst_elem) } //~ ERROR: /reborrow through .* is forbidden/
+}

src/tools/miri/tests/fail/tree_borrows/protector-write-lazy.stderr

@@ -0,0 +1,27 @@
+error: Undefined Behavior: reborrow through <TAG> at ALLOC[0x0] is forbidden
+  --> $DIR/protector-write-lazy.rs:LL:CC
+   |
+LL |     unsafe { println!("Value of funky: {}", *funky_ptr_lazy_on_fst_elem) }
+   |                                             ^^^^^^^^^^^^^^^^^^^^^^^^^^^ reborrow 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 Disabled which forbids this reborrow (acting as a child read access)
+help: the accessed tag <TAG> was created here, in the initial state Reserved
+  --> $DIR/protector-write-lazy.rs:LL:CC
+   |
+LL |         unsafe { (&mut *(ptr_to_vec.wrapping_add(1))) as *mut i32 }.wrapping_sub(1);
+   |                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+help: the accessed tag <TAG> later transitioned to Disabled due to a protector release (acting as a foreign write access) on every location previously accessed by this tag
+  --> $DIR/protector-write-lazy.rs:LL:CC
+   |
+LL | }
+   |  ^
+   = help: this transition corresponds to a loss of read and write permissions
+   = note: BACKTRACE (of the first span):
+   = note: inside `main` at $DIR/protector-write-lazy.rs:LL:CC
+   = note: this error originates in the macro `$crate::format_args_nl` which comes from the expansion of the macro `println` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+note: some details are omitted, run with `MIRIFLAGS=-Zmiri-backtrace=full` for a verbose backtrace
+
+error: aborting due to 1 previous error
+