All makes sense, but I don't agree with:

> This is not always desirable because you may want to catch and recover from panics, particularly in long-running servers.

IMHO a panic implies that execution cannot continue under any circumstances, and even any attempts for a graceful shutdown might be futile (if recovery is possible it shouldn't be a panic but done through regular error handling).

For a server process the best reaction to a panic would mean abort and clean restart.

> A single cgu will significantly increase the compilation time, while allowing a bit more optimization.

Increased build time is acceptable for release mode IMHO.

> IMHO a panic implies that execution cannot continue under any circumstances, and even any attempts for a graceful shutdown might be futile (if recovery is possible it shouldn't be a panic but done through regular error handling).

Rust panic is just a C++ exception in its implementation, and not every C++ programmer would terminate a process when an exception is thrown. Of course Rust panic is more resillient because Rust provides a memory safety and the logic error can be reasonably bounded as a result.

> Increased build time is acceptable for release mode IMHO.

I think the slowest possible configuration is at least 3x slower than the default, and that's too slow to be acceptable for most people. But you can always tune them up if you want---please note that this issue is all about defaults.

I remember my disappointment when I found out panics could be “caught” and that I was still living in the world of exceptions

Panics have to be catchable because it's undefined behavior for Rust to unwind through C code, so a Rust library that exposes a C API has to stop panics at the FFI boundary and propagate errors in a C-compatible way. It's not "catching" in the Java sense; nobody is using catch_panic to implement resumable error-handling.

Oh no, I didn't know this. I'm adding panic = "abort" to all my projects. If I wanted exceptions I wouldn't be using rust.

That's actually not too bad if you know you don't need to recover from panic. I kinda hope that `panic = "abort"` is indeed default unless overridden by dependencies, say, tokio...

You can catch them locally (https://doc.rust-lang.org/std/panic/fn.catch_unwind.html) or with a global exception handler (https://doc.rust-lang.org/std/panic/fn.set_hook.html)

You aren't. They're only caught in special cases, e.g. if a thread panics and you want to propagate that to other threads, to catch panics in unit tests, to avoid unwinding across FFI, etc.

You shouldn't use them as a general exception mechanism. They aren't the same, even if under the hood they both use stack unwinding.

Naively, what is the difference between panics and exceptions? Is it the philosophy on when/how to use them?

Yes, and also exceptions can have arbitrary value associated with them. You aren't going to get far using panics as a general error handling mechanism because there's only one type of panic.

>For a server process the best reaction to a panic would mean abort and clean restart.

I suppose you could have a panic in one thread whilst others are still running fine. You may just want to shut those other threads down gracefully.

> For a server process the best reaction to a panic would mean abort and clean restart.

That's often infeasible, and creates a major DoS/reliability problem.

The server may be processing hundreds of different requests at the same time, and panic=abort will kill all of them. That creates a visible failure to other unrelated clients of the server, not just the offending request.

If you try to fix that and retry aborted requests, you'll retry the panic-inducing request and cause another failure (and it'll take several restarts to bisect out the offending request).

Plus a restart may be costly, require loading data, warming up caches, etc.

It's just way cheaper to catch a panic and return 500 to the offending request. Rust guarantees to panic before anything terrible memory-corrupting happens. Even if you don't trust it and would prefer to restart anyway, you have an option to gracefully hand over the traffic before the restart.