State change during `on_exit` causes infinite loop

[hayashi-stl]

Bevy version

0.6.1

Operating system & version

Ubuntu 20.04

What you did

I changed the state during a system that runs on exiting a state:

use bevy::prelude::*;

#[derive(Clone, Copy, Debug, PartialEq, Eq, Hash)]
enum AppState {
    A,
    B,
}

fn main() {
    App::new()
        .add_state(AppState::A)
        .add_system_set(SystemSet::on_update(AppState::A).with_system(change_state))
        .add_system_set(SystemSet::on_exit(AppState::A).with_system(change_state))
        .run();
}

fn change_state(mut state: ResMut<State<AppState>>) {
    println!("Changing state to B");
    state.set(AppState::B).unwrap();
}

What you expected to happen

A panic, probably, since either the state transition didn't happen yet (in which case there's a state transition waiting to happen, so set should panic), or the state transition did happen (in which case we're already in state B, so set should panic).

What actually happened

Changing state to B prints forever