Lazy can cause UB when initialization fails, and program retries initialization

[KamilaBorowska]

The following program causes UB:

use once_cell::sync::Lazy;

static HI: Lazy<i32> = Lazy::new(|| {
    panic!();
});

fn main() {
    let _ = std::panic::catch_unwind(|| println!("{}", *HI));
    println!("{}", *HI);
}

This is because of incorrect use of unreachable_unchecked when Lazy structure is poisoned.

        pub fn force(this: &Lazy<T, F>) -> &T {
            // Safe because closure is guaranteed to be called at most once
            // so we only call `F` once, this also guarantees no race conditions
            this.cell.get_or_init(|| unsafe {
                match (*this.init.get()).take() {
                    Some(f) => f(),
                    None => unreachable_unchecked(),
                }
            })
        }

This should panic instead of causing unreachable UB.

[matklad]

Ouch, excellent catch!