Fix miri issue 4579 by checking if the strong protector is actually "active".

Where "active" means that the accessed bit is set.

This also reverts miri PR 3831.

Author: JoJoDeveloping

src/tools/miri/src/borrow_tracker/tree_borrows/tree.rs

@@ -756,6 +756,8 @@ impl<'tcx> Tree {
                             // Don't check for protector if it is a Cell (see `unsafe_cell_deallocate` in `interior_mutability.rs`).
                             // Related to https://github.com/rust-lang/rust/issues/55005.
                             && !perm.permission().is_cell()
+                            // Only trigger UB if the accessed bit is set, i.e. if the protector is actually protecting this offset. See #4579.
+                            && perm.is_accessed()
                         {
                             Err(TransitionError::ProtectedDealloc)
                         } else {

src/tools/miri/tests/fail/both_borrows/zero-sized-protected.stack.stderr

@@ -3,16 +3,13 @@
 use std::alloc::{Layout, alloc, dealloc};
 
 // `x` is strongly protected but covers zero bytes.
-// Let's see if deallocating the allocation x points to is UB:
-// in TB, it is UB, but in SB it is not.
+// This should never be UB.
 fn test(_x: &mut (), ptr: *mut u8, l: Layout) {
-    unsafe { dealloc(ptr, l) }; //~[tree] ERROR: /deallocation .* is forbidden/
+    unsafe { dealloc(ptr, l) };
 }
 
 fn main() {
     let l = Layout::from_size_align(1, 1).unwrap();
     let ptr = unsafe { alloc(l) };
     unsafe { test(&mut *ptr.cast::<()>(), ptr, l) };
-    // In SB the test would pass if it weren't for this line.
-    unsafe { std::hint::unreachable_unchecked() }; //~[stack] ERROR: unreachable
 }