Avoid miri error in slice::sort under Stacked Borrows

See comment in code.

Fixes: #131065

Author: Voultapher

library/alloctests/tests/sort/tests.rs

@@ -362,6 +362,13 @@ fn sort_vs_sort_by_impl<S: Sort>() {
     assert_eq!(input_sort_by, expected);
 }
 
+pub fn box_value_impl<S: Sort>() {
+    for len in [3, 9, 35, 56, 132] {
+        test_is_sorted::<Box<i32>, S>(len, Box::new, patterns::random);
+        test_is_sorted::<Box<i32>, S>(len, Box::new, |len| patterns::random_sorted(len, 80.0));
+    }
+}
+
 gen_sort_test_fns_with_default_patterns!(
     correct_i32,
     |len, pattern_fn| test_is_sorted::<i32, S>(len, |val| val, pattern_fn),
@@ -967,6 +974,7 @@ define_instantiate_sort_tests!(
     [miri_yes, fixed_seed_rand_vec_prefix],
     [miri_yes, int_edge],
     [miri_yes, sort_vs_sort_by],
+    [miri_yes, box_value],
     [miri_yes, correct_i32_random],
     [miri_yes, correct_i32_random_z1],
     [miri_yes, correct_i32_random_d2],

library/core/src/slice/sort/stable/quicksort.rs

@@ -42,7 +42,10 @@ pub fn quicksort<T, F: FnMut(&T, &T) -> bool>(
         // self-modifications via `is_less` would not be observed and this would
         // be unsound. Our temporary copy does not escape this scope.
         let pivot_copy = unsafe { ManuallyDrop::new(ptr::read(&v[pivot_pos])) };
-        let pivot_ref = (!has_direct_interior_mutability::<T>()).then_some(&*pivot_copy);
+        // Stacked Borrows does not allow the access pattern for `pivot_copy`, but Tree Borrows
+        // does. FIXME: Remove `!cfg!(miri)` once/if Tree Borrows becomes the standard.
+        let pivot_ref =
+            (!has_direct_interior_mutability::<T>() && !cfg!(miri)).then_some(&*pivot_copy);
 
         // We choose a pivot, and check if this pivot is equal to our left
         // ancestor. If true, we do a partition putting equal elements on the