Segfault when using downcast_ref through Layer

[jonhoo]

Bug Report

Version

tracing 0.1.10
tracing-core 0.1.7
tracing-subscriber 0.2.0-alpha.1

Description

The following program segfaults:

use std::sync::{mpsc, Mutex};
use tracing_subscriber::{registry::Registry, Layer};

struct X(Mutex<mpsc::Receiver<()>>);
impl<S> tracing_subscriber::Layer<S> for X
where
    S: tracing::Subscriber,
    Self: 'static,
{
}

fn main() {
    let (tx, rx) = mpsc::channel();
    let d = tracing::Dispatch::new(X(Mutex::new(rx)).with_subscriber(Registry::default()));
    let mutex = &d.downcast_ref::<X>().unwrap().0;
    mutex.lock().unwrap().try_recv().unwrap_err();
    drop(tx);
}

There is no unsafe in this code, so it should at least not segfault. I believe the program should run even correctly (as in, no unwrap call should fail) given the documented behavior of downcast_ref.