How to unify error types for Sender::send and JoinHandle::join?

Question

The signatures for Sender::send and JoinHandle::join are, respectively:

pub fn send(&self, t: T) -> Result<(), SendError<T>>
pub fn join(self) -> Result<T, Box<dyn Any + Send + 'static>>

Since SendError implements Send and Any and owns its content, this should work:

pub fn finish(
    tx: std::sync::mpsc::Sender<()>,
    jh: std::thread::JoinHandle<()>,
) -> Result<(), Box<dyn std::any::Any + Send + 'static>> {
    tx.send(())?;
    jh.join()?;
    Ok(())
}

but it fails with the trait From<SendError<()>> is not implemented for Box<dyn Any + Send>.

Why is there a constraint failure and how does the function signature need to change to have it compile?

Would like to not use external libraries.

It may be better to unwrap the result of calling join. This will only be Err if the thread panicked, and in that case it may make more sense to propagate the panic as a panic anyway. — cdhowie
– cdhowie, Commented Jan 27 at 20:55

orlp · Accepted Answer · 2025-01-27 20:43:11Z

0

You can do it explicitly by wrapping the error in a Box (giving concrete type Box<SendError>) and then decaying it to Box<dyn Any> with an explicit as cast:

pub fn finish(
    tx: std::sync::mpsc::Sender<()>,
    jh: std::thread::JoinHandle<()>,
) -> Result<(), Box<dyn std::any::Any + Send + 'static>> {
    tx.send(()).map_err(|e| Box::new(e) as Box<_>)?;
    jh.join()?;
    Ok(())
}

answered Jan 27 at 20:43

orlp

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3 Comments

twig-froth Jan 27 at 22:53

I tried adding .map_err(Box::new), but didn't cast. What's happening with this cast?

kmdreko Jan 28 at 5:37

@twig-froth you need to induce a coercion, the as is doing the work of coercing a Box<SendError<()>> into a Box<dyn Any + Send>

orlp Jan 28 at 9:01

@twig-froth For what it's worth I have a lot of experience with Rust and I still find it confusing the Rust compiler doesn't infer and coerce the right thing automatically here without the explicit as cast.

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