`AtomicCell<Option<Box<T>>>` triggers undefined behavior under Miri

[brunocodutra]

Pointer has no provenance

assert!(AtomicCell::<Option<Box<i32>>>::is_lock_free());
let cell = AtomicCell::new(None);
cell.store(Some(Box::new(1)));

error: Undefined Behavior: dereferencing pointer failed: 0x2c5ad8[noalloc] is a dangling pointer (it has no provenance)
   --> /home/user/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/ptr/mod.rs:487:1
    |
487 | pub unsafe fn drop_in_place<T: ?Sized>(to_drop: *mut T) {
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ dereferencing pointer failed: 0x2c5ad8[noalloc] is a dangling pointer (it has no provenance)
    |
    = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
    = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information
    = note: BACKTRACE:
    = note: inside `core::ptr::drop_in_place::<alloc::boxed::Box<i32>> - shim(Some(alloc::boxed::Box<i32>))` at /home/user/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/ptr/mod.rs:487:1
    = note: inside `core::ptr::drop_in_place::<core::option::Option<alloc::boxed::Box<i32>>> - shim(Some(core::option::Option<alloc::boxed::Box<i32>>))` at /home/user/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/ptr/mod.rs:487:1
    = note: inside `core::ptr::mut_ptr::<impl *mut core::option::Option<alloc::boxed::Box<i32>>>::drop_in_place` at /home/user/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/ptr/mut_ptr.rs:1424:18
    = note: inside `<crossbeam_utils::atomic::AtomicCell<core::option::Option<alloc::boxed::Box<i32>>> as core::ops::Drop>::drop` at /home/user/.cargo/registry/src/github.com-1ecc6299db9ec823/crossbeam-utils-0.8.11/src/atomic/atomic_cell.rs:319:17
    = note: inside `core::ptr::drop_in_place::<crossbeam_utils::atomic::AtomicCell<core::option::Option<alloc::boxed::Box<i32>>>> - shim(Some(crossbeam_utils::atomic::AtomicCell<core::option::Option<alloc::boxed::Box<i32>>>))` at /home/user/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/ptr/mod.rs:487:1

Dangling box

assert!(AtomicCell::<Option<Box<i32>>>::is_lock_free());
let cell = AtomicCell::new(Some(Box::new(0)));
cell.store(Some(Box::new(1)));

error: Undefined Behavior: constructing invalid value at .value.<enum-variant(Some)>.0: encountered a dangling box (address 0x2c5a20 is unallocated)
    --> /home/user/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/mem/mod.rs:1068:18
     |
1068 | ... { ptr::read(src as *const T as *const U) }
     |       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ constructing invalid value at .value.<enum-variant(Some)>.0: encountered a dangling box (address 0x2c5a20 is unallocated)
     |
     = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
     = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information
     = note: BACKTRACE:
     = note: inside `core::mem::transmute_copy::<u64, core::option::Option<alloc::boxed::Box<i32>>>` at /home/user/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/mem/mod.rs:1068:18
     = note: inside `crossbeam_utils::atomic::atomic_cell::atomic_swap::<core::option::Option<alloc::boxed::Box<i32>>>` at /home/user/.cargo/registry/src/github.com-1ecc6299db9ec823/crossbeam-utils-0.8.11/src/atomic/atomic_cell.rs:1053:23
     = note: inside `crossbeam_utils::atomic::AtomicCell::<core::option::Option<alloc::boxed::Box<i32>>>::swap` at /home/user/.cargo/registry/src/github.com-1ecc6299db9ec823/crossbeam-utils-0.8.11/src/atomic/atomic_cell.rs:167:18
     = note: inside `crossbeam_utils::atomic::AtomicCell::<core::option::Option<alloc::boxed::Box<i32>>>::store` at /home/user/.cargo/registry/src/github.com-1ecc6299db9ec823/crossbeam-utils-0.8.11/src/atomic/atomic_cell.rs:145:18

[taiki-e]

Duplicate of #748 and #847
Closing in favor of #744 and #748. See also #744 (comment) and subsequent comment. That issue is about a different topic, but the solution is the same (AtomicMaybeUninit).