btree_map::OccupiedEntry: Send regression

[ghost]

The code below does not compile on new versions of rustc (bisect below)

use std::collections::{btree_map::Entry, BTreeMap};
use std::thread::spawn;

fn main() {
    let map = BTreeMap::<i32, i32>::new();
    let map = Box::leak(Box::new(map));

    match map.entry(1) {
        Entry::Occupied(entry) => {
            spawn(move || {
                entry.get();
            });
        }
        _ => (),
    }
}

error[E0277]: `NonNull<BTreeMap<i32, i32>>` cannot be sent between threads safely
   --> src/main.rs:10:13
    |
10  |               spawn(move || {
    |  _____________^^^^^_-
    | |             |
    | |             `NonNull<BTreeMap<i32, i32>>` cannot be sent between threads safely
11  | |                 entry.get();
12  | |             });
    | |_____________- within this `[closure@src/main.rs:10:19: 12:14 entry:std::collections::btree_map::OccupiedEntry<'_, i32, i32>]`
    | 
   ::: /home/antek/.local/opt/rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/thread/mod.rs:596:8
    |
596 |       F: Send + 'static,
    |          ---- required by this bound in `spawn`
    |
    = help: within `[closure@src/main.rs:10:19: 12:14 entry:std::collections::btree_map::OccupiedEntry<'_, i32, i32>]`, the trait `Send` is not implemented for `NonNull<BTreeMap<i32, i32>>`
    = note: required because it appears within the type `alloc::collections::btree::borrow::DormantMutRef<'_, BTreeMap<i32, i32>>`
    = note: required because it appears within the type `std::collections::btree_map::OccupiedEntry<'_, i32, i32>`
    = note: required because it appears within the type `[closure@src/main.rs:10:19: 12:14 entry:std::collections::btree_map::OccupiedEntry<'_, i32, i32>]`

Bisect

searched nightlies: from nightly-2020-09-11 to nightly-2020-09-12
regressed nightly: nightly-2020-09-12
searched commits: from a1947b3 to 9911160
regressed commit: ee04f9a

bisected with cargo-bisect-rustc v0.5.2

Host triple: x86_64-unknown-linux-gnu
Reproduce with:

cargo bisect-rustc --start 2020-09-11 

[jonas-schievink]

cc @ssomers @Mark-Simulacrum

[Mark-Simulacrum]

We probably just need to unsafe impl<T> Send for DormantMutRef<T> where &mut T: Send {} and likewise for Sync. Conceptually it's no different to a &mut T, so that should be reasonable.

[spastorino]

@ssommers can we assign the issue to you given that you've already provided a PR?

[Mark-Simulacrum]

I'm going to self-assign, I don't expect to hit any roadblocks with the PR though, it's already bors-ready.