In Rust/wasm, is there a way to say: run { ... this block of code } and, if it crashes, do web_sys::console::log_1(....) Use case, I have a function foo that is called many many times. Way too many times for me to add a println! to every call. Once in a while, something that foo calls panics!() .. and I would love to know what the argument to foo was, when something that foo called ends up panicing.

It sounds like you are looking for catch_unwind.

Based on the documentation, std::panic::catch_unwind is indeed what I want. However, it appears to be a no-op in Rust/wasm32.

@quinedot : Definitely informative. Thanks! If I'm reading that github thread correctly: std::panic::catch_unwind requires panic='unwind' and does not work with panic='abort' wasm32-unknown-unknown only supports panic='abort', blocking on some wasm exception handling spec @bjorn3: Sorry to bother you again. Since you answered in other thread. Can you please confirm of the two above statements are true / false ?

If the only panic behavior available is abort, I think there is no way to catch it.

From the Rust side, certainly. I have no experience with WASM, but I expect the host JS environment has some way to tell if the program has halted; you could build something around that.

This isn't possible on WebAssembly at the moment. The unwinding process in a browser and VMs like Wasmer will just pop WebAssembly stack frames with no concern for the data they contain. From memory, there is a proposal for destructors which would let you use RAII to make sure things are cleaned up during an exception, but I don't know if it's been implemented by any runtimes (browser or otherwise) yet. [image] anon80458984: Once in a while, something that foo calls panics!() .. and I would love to know what the argument to foo was, when something that foo called ends up panicing. Have you thought of wiring something like tracing up? You could add a #[tracing::instrument] annotation to the function (probably at the TRACE level so it doesn't spam you unnecessarily), and the last log message before the panic will tell you the function's arguments.

Thank you everyone for your help. I ended up finding the bug in my code (by luck), but not solving the general problem (i.e. how to catch panics in wasm). If anyone else has insights on catching panic in wasm, I am happy to listen. Thanks!

Unfortunately, the answer is simply that it can't be done.

If (this block of code panics) { println ... }

quinedot July 31, 2022, 4:23am 4

I'm pretty ignorant of this topic, but here's a previous thread regarding catch_unwind on wasm.

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If (this block of code panics) { println ... }

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