miri failure in `entry`

[droundy]

dashmap 5.4.0 has a miri failure when its entry method is called with a Box value. Here is a minimal reproducible example:

#[test]
fn just_dashmap() {
    let m: DashMap<Box<&'static str>, ()> = DashMap::new();
    match m.entry(Box::new("hello")) {
        Entry::Vacant(e) => {
            e.insert(());
        }
        Entry::Occupied(_) => {
            panic!("Should not exist yet");
        }
    };
}

I run this with

cargo +nightly miri test just_dashmap

and get the following error:

$ cargo +nightly miri test --features arc just_dashmap
Preparing a sysroot for Miri (target: x86_64-unknown-linux-gnu)... done
    Finished test [unoptimized + debuginfo] target(s) in 0.03s
     Running unittests src/lib.rs (target/miri/x86_64-unknown-linux-gnu/debug/deps/internment-70bb0e72db6e1fd0)

running 1 test
test arc::just_dashmap ... error: Undefined Behavior: trying to retag from <250988> for SharedReadOnly permission at alloc105584[0x0], but that tag does not exist in the borrow stack for this location
    --> /home/droundy/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/alloc/src/boxed.rs:1383:9
     |
1383 |         (**self).hash(state);
     |         ^^^^^^^^^^^^^^^^^^^^
     |         |
     |         trying to retag from <250988> for SharedReadOnly permission at alloc105584[0x0], but that tag does not exist in the borrow stack for this location
     |         this error occurs as part of retag at alloc105584[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: <250988> was created by a Unique retag at offsets [0x0..0x10]
    --> src/arc.rs:506:13
     |
506  |             e.insert(());
     |             ^^^^^^^^^^^^
help: <250988> was later invalidated at offsets [0x0..0x10] by a Unique retag
    --> src/arc.rs:506:13
     |
506  |             e.insert(());
     |             ^^^^^^^^^^^^
     = note: BACKTRACE:
     = note: inside `<std::boxed::Box<&str> as std::hash::Hash>::hash::<std::collections::hash_map::DefaultHasher>` at /home/droundy/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/alloc/src/boxed.rs:1383:9
     = note: inside `hashbrown::map::make_hash::<std::boxed::Box<&str>, std::boxed::Box<&str>, std::collections::hash_map::RandomState>` at /home/droundy/.cargo/registry/src/github.com-1ecc6299db9ec823/hashbrown-0.12.3/src/map.rs:253:5
     = note: inside `hashbrown::HashMap::<std::boxed::Box<&str>, dashmap::util::SharedValue<()>, std::collections::hash_map::RandomState>::get_inner::<std::boxed::Box<&str>>` at /home/droundy/.cargo/registry/src/github.com-1ecc6299db9ec823/hashbrown-0.12.3/src/map.rs:1270:24
     = note: inside `hashbrown::HashMap::<std::boxed::Box<&str>, dashmap::util::SharedValue<()>, std::collections::hash_map::RandomState>::get_key_value::<std::boxed::Box<&str>>` at /home/droundy/.cargo/registry/src/github.com-1ecc6299db9ec823/hashbrown-0.12.3/src/map.rs:1255:15
     = note: inside `dashmap::mapref::entry::VacantEntry::<'_, std::boxed::Box<&str>, ()>::insert` at /home/droundy/.cargo/registry/src/github.com-1ecc6299db9ec823/dashmap-5.4.0/src/mapref/entry.rs:106:26
note: inside `arc::just_dashmap` at src/arc.rs:506:13
    --> src/arc.rs:506:13
     |
506  |             e.insert(());
     |             ^^^^^^^^^^^^
note: inside closure at src/arc.rs:502:19
    --> src/arc.rs:502:19
     |
501  | #[test]
     | ------- in this procedural macro expansion
502  | fn just_dashmap() {
     |                   ^
     = 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

error: aborting due to previous error

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

Caused by:
  process didn't exit successfully: `/home/droundy/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/bin/cargo-miri runner /home/droundy/src/internment/target/miri/x86_64-unknown-linux-gnu/debug/deps/internment-70bb0e72db6e1fd0 just_dashmap` (exit status: 1)

[xacrimon]

I'll look into this but it seems new, the rules violated also seem new and experimental so it'll be interesting.

[olivereanderson]

Does anyone know what consequences this error could have in terms of undefined behavior when compiling with the latest stable rustc?