Box::into_raw: make Miri understand that this is a box-to-raw cast

tests/fail/both_borrows/newtype_pair_retagging.stack.stderr

@@ -6,7 +6,7 @@ LL |         Box(unsafe { Unique::new_unchecked(raw) }, alloc)
    |
    = help: this indicates a potential bug in the program: it performed an invalid operation, but the Stacked Borrows rules it violated are still experimental
    = help: see https://github.com/rust-lang/unsafe-code-guidelines/blob/master/wip/stacked-borrows.md for further information
-help: <TAG> was created by a Unique retag at offsets [0x0..0x4]
+help: <TAG> was created by a SharedReadWrite retag at offsets [0x0..0x4]
   --> $DIR/newtype_pair_retagging.rs:LL:CC
    |
 LL |     let ptr = Box::into_raw(Box::new(0i32));

tests/fail/both_borrows/newtype_retagging.stack.stderr

@@ -6,7 +6,7 @@ LL |         Box(unsafe { Unique::new_unchecked(raw) }, alloc)
    |
    = help: this indicates a potential bug in the program: it performed an invalid operation, but the Stacked Borrows rules it violated are still experimental
    = help: see https://github.com/rust-lang/unsafe-code-guidelines/blob/master/wip/stacked-borrows.md for further information
-help: <TAG> was created by a Unique retag at offsets [0x0..0x4]
+help: <TAG> was created by a SharedReadWrite retag at offsets [0x0..0x4]
   --> $DIR/newtype_retagging.rs:LL:CC
    |
 LL |     let ptr = Box::into_raw(Box::new(0i32));

tests/pass/issues/issue-miri-3473.rs

@@ -0,0 +1,28 @@
+//@revisions: stack tree
+//@[tree]compile-flags: -Zmiri-tree-borrows
+use std::cell::UnsafeCell;
+
+#[repr(C)]
+#[derive(Default)]
+struct Node {
+    _meta: UnsafeCell<usize>,
+    value: usize,
+}
+
+impl Node {
+    fn value(&self) -> &usize {
+        &self.value
+    }
+}
+
+/// This used to cause Stacked Borrows errors because of trouble around conversion
+/// from Box to raw pointer.
+fn main() {
+    unsafe {
+        let a = Box::into_raw(Box::new(Node::default()));
+        let ptr = &*a;
+        *UnsafeCell::raw_get(a.cast::<UnsafeCell<usize>>()) = 2;
+        assert_eq!(*ptr.value(), 0);
+        drop(Box::from_raw(a));
+    }
+}

tests/pass/stacked-borrows/stacked-borrows.rs

@@ -20,6 +20,7 @@ fn main() {
     wide_raw_ptr_in_tuple();
     not_unpin_not_protected();
     write_does_not_invalidate_all_aliases();
+    box_into_raw_allows_interior_mutable_alias();
 }
 
 // Make sure that reading from an `&mut` does, like reborrowing to `&`,
@@ -263,3 +264,14 @@ fn write_does_not_invalidate_all_aliases() {
     other::lib2();
     assert_eq!(*x, 1337); // oops, the value changed! I guess not all pointers were invalidated
 }
+
+fn box_into_raw_allows_interior_mutable_alias() { unsafe {
+    let b = Box::new(std::cell::Cell::new(42));
+    let raw = Box::into_raw(b);
+    let c = &*raw;
+    let d = raw.cast::<i32>(); // bypassing `Cell` -- only okay in Miri tests
+    // `c` and `d` should permit arbitrary aliasing with each other now.
+    *d = 1;
+    c.set(2);
+    drop(Box::from_raw(raw));
+} }