Data Race Unsoundness

[WalterSmuts]

The following code results in a bug from miri:

use std::{cell::RefCell, ops::AddAssign};

use num_traits::{One, Zero};

fn generic_call_counter<T: Zero + One + Copy + AddAssign + 'static>() -> T {
    let mut count = generic_singleton::get_or_init!(|| RefCell::new(T::zero())).borrow_mut();
    *count += T::one();
    *count
}

fn main() {
    let mut handles = Vec::new();
    for _ in 0..2 {
        handles.push(std::thread::spawn(|| {
            println!("{}", generic_call_counter::<i32>())
        }));
    }
    for handle in handles {
        handle.join().unwrap();
    }
}

BUG:

[walter@cuddles generic_singleton_user]$ cargo miri run
Preparing a sysroot for Miri (target: x86_64-unknown-linux-gnu)... done
    Finished dev [unoptimized + debuginfo] target(s) in 0.01s
     Running `/home/walter/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/bin/cargo-miri runner target/miri/x86_64-unknown-linux-gnu/debug/generic_singleton_user`
1
error: Undefined Behavior: Data race detected between Read on thread `<unnamed>` and Write on thread `<unnamed>` at alloc3852
   --> /home/walter/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/cell.rs:452:18
    |
452 |         unsafe { *self.value.get() }
    |                  ^^^^^^^^^^^^^^^^^ Data race detected between Read on thread `<unnamed>` and Write on thread `<unnamed>` at alloc3852
    |
    = 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 `std::cell::Cell::<isize>::get` at /home/walter/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/cell.rs:452:18
    = note: inside `std::cell::BorrowRefMut::<'_>::new` at /home/walter/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/cell.rs:1682:15
    = note: inside `std::cell::RefCell::<i32>::try_borrow_mut` at /home/walter/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/cell.rs:987:15
note: inside `generic_call_counter::<i32>` at src/main.rs:6:21
   --> src/main.rs:6:21
    |
6   |     let mut count = generic_singleton::get_or_init!(|| RefCell::new(T::zero())).borrow_mut();
    |                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
note: inside closure at src/main.rs:15:28
   --> src/main.rs:15:28
    |
15  |             println!("{}", generic_call_counter::<i32>())
    |                            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

note: some details are omitted, run with `MIRIFLAGS=-Zmiri-backtrace=full` for a verbose backtrace

error: aborting due to previous error

The bug makes sense. Not sure why generic_singleton allows this usage :/

[WalterSmuts]

Fixed by e6110fa