Miri test failure: Stacked Borrow rules violated

[phil-opp]

Our scheduled CI jobs discovered a new set of Miri errors on the latest nightlies. The full logs are:

running 26 tests
error: Undefined Behavior: trying to retag from <202439> for SharedReadOnly permission at alloc86155[0x0], but that tag does not exist in the borrow stack for this location
   --> /home/runner/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/ptr/non_null.rs:385:18
    |
385 |         unsafe { &*self.as_ptr() }
    |                  ^^^^^^^^^^^^^^^
    |                  |
    |                  trying to retag from <202439> for SharedReadOnly permission at alloc86155[0x0], but that tag does not exist in the borrow stack for this location
    |                  this error occurs as part of retag at alloc86155[0x0..0x10]
    |
    = 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: <202439> was created by a SharedReadWrite retag at offsets [0x0..0x3e8]
   --> src/test.rs:49:35
    |
49  |     let heap = unsafe { Heap::new(data.as_mut_ptr().cast(), data.len()) };
    |                                   ^^^^^^^^^^^^^^^^^
help: <202439> was later invalidated at offsets [0x0..0x400] by a Unique retag
   --> src/test.rs:53:16
    |
53  |       let drop = move || {
    |  ________________^
54  | |         let _ = heap_space;
55  | |     };
    | |_____^
    = note: BACKTRACE:
    = note: inside `core::ptr::NonNull::<hole::Hole>::as_ref::<'_>` at /home/runner/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/ptr/non_null.rs:385:18
note: inside `hole::Cursor::current` at src/hole.rs:50:18
   --> src/hole.rs:50:18
    |
50  |         unsafe { self.hole.as_ref() }
    |                  ^^^^^^^^^^^^^^^^^^
note: inside `hole::Cursor::split_current` at src/hole.rs:69:29
   --> src/hole.rs:69:29
    |
69  |             let hole_size = self.current().size;
    |                             ^^^^^^^^^^^^^^
note: inside `hole::HoleList::allocate_first_fit` at src/hole.rs:411:19
   --> src/hole.rs:411:19
    |
411 |             match cursor.split_current(aligned_layout) {
    |                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
note: inside `Heap::allocate_first_fit` at src/lib.rs:196:15
   --> src/lib.rs:196:15
    |
196 |         match self.holes.allocate_first_fit(layout) {
    |               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
note: inside `hole::test::aff` at src/hole.rs:723:17
   --> src/hole.rs:723:17
    |
723 |         let _ = heap.allocate_first_fit(reqd).unwrap();
    |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
note: inside closure at src/hole.rs:720:14
   --> src/hole.rs:720:14
    |
719 |     #[test]
    |     ------- in this procedural macro expansion
720 |     fn aff() {
    |              ^
    = note: this error originates in the attribute macro `test` (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

cc @jamesmunns

[jamesmunns]

Thanks for tagging me! I'll look into these today and report back :)