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Printing panics in Rust

2017-12-05 16:19

This blog post is not about about dealing with normal runtime errors, you should really use the Result Type for that. This is about the case where some component might panic, but that shouldn’t bring the whole system to halt.

I was debugging some issue in the Node.js binding for Noise. It is using the noise_search crate which might panic if there’s an unrecoverable error. Though the Node.js binding should of course not crash, but handle it in a more graceful way. Hence it is catching the panics.

The existing code was only printing that there was some panic, but it didn’t contain the actual cause. I wanted to improve that.

I thought it would be easy and I could just print the debug version of the panic. So I changed the println!() to:

println!("panic happend: {:?}", result)

But that resulted only in a:

panic happened: Err(Any)

Which isn’t really that meaningful either. In the documentation about catch_unwind I read

…and will return Err(cause) if the closure panics. The cause returned is the object with which panic was originally invoked.

I didn’t really understand what this meant. Is the object that invokes the panic the function where the panic happens? I wanted the text I was putting into the panic!() call.

Thanks to rkruppe on IRC I learnt that panic!() can take any object, not just strings. Now the documentation made sense. He also mentioned that I can downcast Any if I know that type. As I always only use strings for panics that was easy:

if let Err(panic) = result {
    match panic.downcast::<String>() {
        Ok(panic_msg) => {
            println!("panic happened: {}", panic_msg);
        }
        Err(_) => {
            println!("panic happened: unknown type.");
        }
    }
}

If you want to play a bit around with it, I’ve created a minimal example for the Rust Playground. Happy panicking!

Categories: en, Noise, Rust

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By Volker Mische

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