`hint::select_unpredictable` forgets to drop one argument if dropping another panics.

[theemathas]

I tried this code:

struct Thing {
    label: &'static str,
    panic_on_drop: bool,
}

impl Drop for Thing {
    fn drop(&mut self) {
        println!("Dropping: {}", self.label);
        if self.panic_on_drop {
            println!("Panicking...");
            panic!("Drop panicked");
        } else {
            println!("Not panicking.")
        }
    }
}

fn main() {
    let a = Thing {
        label: "a",
        panic_on_drop: false,
    };
    let b = Thing {
        label: "b",
        panic_on_drop: true,
    };
    let _ = std::hint::select_unpredictable(true, a, b);
}

I expected b to be dropped, at which point a panic occurs, then a is dropped. Instead, a is not dropped at all.

Dropping: b
Panicking...

thread 'main' (133912) panicked at src/main.rs:11:13:
Drop panicked
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

Meta

rustc --version --verbose:

rustc 1.91.0-nightly (de3efa79f 2025-08-08)
binary: rustc
commit-hash: de3efa79f95852c7427587f1d535bfea7c0d6779
commit-date: 2025-08-08
host: aarch64-apple-darwin
release: 1.91.0-nightly
LLVM version: 21.1.0