`miri` test complains about `Arena::get2_mut`

[toyboot4e]

miri test result:

$ cargo +nightly miri test
    Finished test [unoptimized + debuginfo] target(s) in 0.01s
     Running unittests (target/miri/x86_64-apple-darwin/debug/deps/thunderdome-fba4034cefd759f7)
error: Undefined Behavior: trying to reborrow for Unique at alloc84249+0x8, but parent tag <217107> does not have an appropriate item in the borrow stack
   --> src/arena.rs:254:48
    |
254 |         let item1 = unsafe { item1_ptr.map(|x| &mut *x) };
    |                                                ^^^^^^^ trying to reborrow for Unique at alloc84249+0x8, but parent tag <217107> does not have an appropriate item in the borrow stack
    |
    = help: this indicates a potential bug in the program: it performed an invalid operation, but the 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

    = note: inside closure at src/arena.rs:254:48
    = note: inside `std::option::Option::<*mut i32>::map::<&mut i32, [closure@src/arena.rs:254:44: 254:55]>` at /Users/tbm/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/library/core/src/option.rs:836:29
note: inside `arena::Arena::<i32>::get2_mut` at src/arena.rs:254:30
   --> src/arena.rs:254:30
    |
254 |         let item1 = unsafe { item1_ptr.map(|x| &mut *x) };
    |                              ^^^^^^^^^^^^^^^^^^^^^^^^^^
note: inside `arena::test::get2_mut` at src/arena.rs:600:40
   --> src/arena.rs:600:40
    |
600 |         let (foo_handle, bar_handle) = arena.get2_mut(foo, bar);
    |                                        ^^^^^^^^^^^^^^^^^^^^^^^^
note: inside closure at src/arena.rs:595:5
   --> src/arena.rs:595:5
    |
594 |       #[test]
    |       ------- in this procedural macro expansion
595 | /     fn get2_mut() {
596 | |         let mut arena = Arena::new();
597 | |         let foo = arena.insert(100);
598 | |         let bar = arena.insert(500);
...   |
607 | |         assert_eq!(arena.get(bar), Some(&505));
608 | |     }
    | |_____^
    = note: inside `<[closure@src/arena.rs:595:5: 608:6] as std::ops::FnOnce<()>>::call_once - shim` at /Users/tbm/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/library/core/src/ops/function.rs:227:5
    = note: inside `<fn() as std::ops::FnOnce<()>>::call_once - shim(fn())` at /Users/tbm/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/library/core/src/ops/function.rs:227:5
    = note: inside `test::__rust_begin_short_backtrace::<fn()>` at /Users/tbm/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/library/test/src/lib.rs:578:5
    = note: inside closure at /Users/tbm/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/library/test/src/lib.rs:569:30
    = note: inside `<[closure@test::run_test::{closure#2}] as std::ops::FnOnce<()>>::call_once - shim(vtable)` at /Users/tbm/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/library/core/src/ops/function.rs:227:5
    = note: inside `<std::boxed::Box<dyn std::ops::FnOnce() + std::marker::Send> as std::ops::FnOnce<()>>::call_once` at /Users/tbm/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/library/alloc/src/boxed.rs:1572:9
    = note: inside `<std::panic::AssertUnwindSafe<std::boxed::Box<dyn std::ops::FnOnce() + std::marker::Send>> as std::ops::FnOnce<()>>::call_once` at /Users/tbm/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/library/core/src/panic/unwind_safe.rs:271:9
    = note: inside `std::panicking::r#try::do_call::<std::panic::AssertUnwindSafe<std::boxed::Box<dyn std::ops::FnOnce() + std::marker::Send>>, ()>` at /Users/tbm/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/library/std/src/panicking.rs:401:40
    = note: inside `std::panicking::r#try::<(), std::panic::AssertUnwindSafe<std::boxed::Box<dyn std::ops::FnOnce() + std::marker::Send>>>` at /Users/tbm/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/library/std/src/panicking.rs:365:19
    = note: inside `std::panic::catch_unwind::<std::panic::AssertUnwindSafe<std::boxed::Box<dyn std::ops::FnOnce() + std::marker::Send>>, ()>` at /Users/tbm/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/library/std/src/panic.rs:129:14
    = note: inside `test::run_test_in_process` at /Users/tbm/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/library/test/src/lib.rs:601:18
    = note: inside closure at /Users/tbm/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/library/test/src/lib.rs:493:39
    = note: inside `test::run_test::run_test_inner` at /Users/tbm/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/library/test/src/lib.rs:531:13
    = note: inside `test::run_test` at /Users/tbm/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/library/test/src/lib.rs:565:28
    = note: inside `test::run_tests::<[closure@test::run_tests_console::{closure#2}]>` at /Users/tbm/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/library/test/src/lib.rs:306:17
    = note: inside `test::run_tests_console` at /Users/tbm/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/library/test/src/console.rs:290:5
    = note: inside `test::test_main` at /Users/tbm/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/library/test/src/lib.rs:123:15
    = note: inside `test::test_main_static` at /Users/tbm/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/library/test/src/lib.rs:142:5
    = note: inside `main`
    = note: inside `<fn() as std::ops::FnOnce<()>>::call_once - shim(fn())` at /Users/tbm/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/library/core/src/ops/function.rs:227:5
    = note: inside `std::sys_common::backtrace::__rust_begin_short_backtrace::<fn(), ()>` at /Users/tbm/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/library/std/src/sys_common/backtrace.rs:125:18
    = note: inside closure at /Users/tbm/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/library/std/src/rt.rs:63:18
    = note: inside `std::ops::function::impls::<impl std::ops::FnOnce<()> for &dyn std::ops::Fn() -> i32 + std::marker::Sync + std::panic::RefUnwindSafe>::call_once` at /Users/tbm/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/library/core/src/ops/function.rs:259:13
    = note: inside `std::panicking::r#try::do_call::<&dyn std::ops::Fn() -> i32 + std::marker::Sync + std::panic::RefUnwindSafe, i32>` at /Users/tbm/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/library/std/src/panicking.rs:401:40
    = note: inside `std::panicking::r#try::<i32, &dyn std::ops::Fn() -> i32 + std::marker::Sync + std::panic::RefUnwindSafe>` at /Users/tbm/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/library/std/src/panicking.rs:365:19
    = note: inside `std::panic::catch_unwind::<&dyn std::ops::Fn() -> i32 + std::marker::Sync + std::panic::RefUnwindSafe, i32>` at /Users/tbm/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/library/std/src/panic.rs:129:14
    = note: inside closure at /Users/tbm/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/library/std/src/rt.rs:45:48
    = note: inside `std::panicking::r#try::do_call::<[closure@std::rt::lang_start_internal::{closure#2}], isize>` at /Users/tbm/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/library/std/src/panicking.rs:401:40
    = note: inside `std::panicking::r#try::<isize, [closure@std::rt::lang_start_internal::{closure#2}]>` at /Users/tbm/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/library/std/src/panicking.rs:365:19
    = note: inside `std::panic::catch_unwind::<[closure@std::rt::lang_start_internal::{closure#2}], isize>` at /Users/tbm/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/library/std/src/panic.rs:129:14
    = note: inside `std::rt::lang_start_internal` at /Users/tbm/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/library/std/src/rt.rs:45:20
    = note: inside `std::rt::lang_start::<()>` at /Users/tbm/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/library/std/src/rt.rs:62:5
    = note: this error originates in the attribute macro `test` (in Nightly builds, run with -Z macro-backtrace for more info)

error: aborting due to previous error

error: test failed, to rerun pass '--lib'

running 25 tests
test arena::test::get2_mut ... 

[toyboot4e]

I think it doesn't matter in real case, but I'm not an unsafe export

[LPGhatguy]

Hm, it's totally possible that we aren't doing something right here. I'm not an unsafety expert either here. This code does work in a lot of configurations but I'm inclined to believe miri.