Undefined behavior when getting a value twice, and from another thread in between

[Kestrer]

The following program:

let tls = std::sync::Arc::new(ThreadLocal::new());

let s = tls.get_or(|| "Hello World".to_owned());

let thread_tls = tls.clone();
std::thread::spawn(move || {
    thread_tls.get_or(String::new);
})
.join()
.unwrap();

tls.get().unwrap();

s;

Produces undefined behavior when run through Miri:

error: Undefined Behavior: trying to reborrow for SharedReadOnly, but parent tag <234426> does not have an appropriate item in the borrow stack
   --> src/main.rs:17:5
   |
17 |     s;
   |     ^ trying to reborrow for SharedReadOnly, but parent tag <234426> 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

I think this is caused by moving the data to the current hash map in the get_slow path. If I do not move it to the current hash map and instead just get it directly, this UB does not occur.