This isn't the right advice to give, and is only going to confuse beginners. Panicking via unwrap/expect is perfectly fine, whether in a library or an application, only when the panic represents a bug. For example, accessing an element of a Vec with an incorrect index because of a logic error. If a panic occurs, then it should reflect a bug that is intended to be fixed.

Stated differently, it should be impossible for an end user to cause a Rust application to panic. If they can, then it's a bug.

This advice permits use of unwrap/expect anywhere, so long as its occurrence is never expected.

As an example it’s quite easy to fall astray of taking your perspective to say read a packet off the network, and panic on malformed data. I know you’d agree that would be a inappropriate time to use panic, as that would crash any upstream program with trivial DOS exploits. But this is easy to do if you say have an API that takes a value, translates it to something else and panics on invalid inputs.

My advice is generally in line with the GP, pornel—use the tools at hand for avoiding panics where it’s easy to do so, such as ?, map, etc. Only panic/unwrap on bad library API usage (which I think is what you’re suggesting). Feel free to panic anywhere in main.

People often translate “ use of unwrap/expect anywhere, so long as its occurrence is never expected” in ways that can create major bugs as the software goes into more use.

> but in practice I think using unwrap or panicking in libraries is usually wrong

Very strongly disagree. I've created dozens of Rust libraries, and probably all of them have dozens of code paths that can panic. Other core libraries do the same. Just a simple slice access, e.g., `slice[i]`, is a line of code that could panic. (Since it's just a shorter way of writing `slice.get(i).unwrap()`.) I don't think we should be giving advice that runs contrary to how important libraries actually work. A lot of people learn to code by reading others' code, and when we give advice like "don't use unwrap/expect," they get confused when they see that virtually every widely used piece of Rust code violates it.

The key here is really that one shouldn't panic unless the panic itself is indicative of a bug somewhere. Blanket advice like "don't use unwrap/panic in libraries" is bad because---as I argued in the Clippy issue I posted in a sibling comment---it's effectively a prohibition against runtime invariants themselves. As a programmer, you insert a panic when you've failed (for any number of reasons) to capture the invariant in the type system. Blanket advice saying that one shouldn't use unwrap/expect in these circumstances leads one toward a path of much more complex APIs with error types that are never constructed unless there is a bug in the code. (It's likely that actually adding all of those error types will be so annoying that it's plausible the programmer will give up on Rust.)

> As an example it’s quite easy to fall astray of taking your perspective to say read a packet off the network, and panic on malformed data. I know you’d agree that would be a inappropriate time to use panic, as that would crash any upstream program with trivial DOS exploits. But this is easy to do if you say have an API that takes a value, translates it to something else and panics on invalid inputs.

I think this is a reflection of error handling being hard. It's just as easy to code a program with the mistaken assumption that a particular file path will always point to a valid and readable file. It takes a bit of learning to understand which things you can rely on never happening and which you can't. In the mean time, I don't think we should be giving advice that is very easy to misinterpret into a conclusion that just isn't tenable. Personally, I think it's much easier to talk about end user behavior. If there's a panic, then it's a bug that ought to be fixed. If you follow that, it's hard to go wrong.

You could rephrase it with, "If there's a reachable code path that panics, then the code path should be removed." You could then talk about what "reachable" means, i.e., code paths that are determine based on data that the program doesn't control vs code paths that are enforced for all inputs to the program.

Because the problem that I'm seeing in Swift code (which also has a "crash on logic errors" approach) is that bugs can bring your whole system down, which is especially bad on server-side apps with multiple threads. Yes, you can use supervisord and/or load balancers, but you still lose in-flight requests.

By contrast, in a language with a runtime like Java or Ruby, you can catch almost everything at the top level, so you could just have some logic that generates a "whoops something went wrong" response in case of a serious error.

The point here is resiliency. I agree that you want to catch bugs early, and you should immediately abort execution once bad things happen; I also fundamentally agree that you don't want to litter your code with error types or other constructs that are supposed to never actually be used. But faults do happen in practice, because we write buggy code, and in such a case, it's good if you can isolate the fault and recover at a higher level. Languages like Erlang take this idea to an extreme.

This is currently a real problem for us with Swift, so I was wondering whether Rust has a better solution here.

Yes: https://doc.rust-lang.org/std/panic/fn.catch_unwind.html

However, this comes with a critical caveat: unwinding can be disabled when compiling an application, which means one cannot guarantee that catch_unwind will actually work. (If unwinding is disabled, then panics turn into unrecoverable aborts.)

For this reason, and because the standard error handling mechanism is done through return values, panicking is not an acceptable way to do robust error handling in Rust. Recovering from panics is useful in niche scenarios, like keeping a web server running even if a request causes a panic or in tests for ensuring that all tests run even if one panics. Which basically matches your key concern here.

(To be clear, I think this is mostly orthogonal to my original comment in this thread. :-))

But in general, exception handling is hard. There is value in having locally-unrecoverable, but globally-recoverable faults, stuff like that is probably where languages like Erlang shine.

You’ve said don’t panic for code paths that the program doesn’t control. That is nuanced, and often is not always obvious and easy to reason about, but the language makes those exceptional cases obvious, so why not use that to your advantage?

edit: and by the way, I didn't say that on indexes, I totally agree with you. Though, sometimes it is valuable to go to the extreme and prevent malicious index values from untrusted sources crashing your program...

I think we are disagreeing mostly over pedagogy.

> That is nuanced, and often is not always obvious and easy to reason about

Right. I think this is what I meant by "error handling is hard." But this really boils down to understanding what a runtime invariant is, and that takes time to learn. It's hard to do good error handling without internalizing that.

> but the language makes those exceptional cases obvious, so why not use that to your advantage?

I think you are, specifically by using unwrap/expect. In many such cases, the unwrap/expect has a comment explaining why it's impossible to panic.

> edit: and by the way, I didn't say that on indexes, I totally agree with you. Though, sometimes it is valuable to go to the extreme and prevent malicious index values from untrusted sources crashing your program...

Well, `&slice[i]` and `slice.get(i).unwrap()` are equivalent. So if we give blanket advice like "don't use unwrap/expect in libraries" then the latter gets caught up in that advice while the former doesn't, even though they are exactly equivalent. To me, this reveals the problem in that pedagogy because it focuses too much on one particularly common manifestation rather than the thing that actually matters: a panic visible to an end user is always a bug.

To be clear, the thing I am disagreeing with is the advice to "not use unwrap/expect; use case analysis instead." I can appreciate a pedagogy that simplifies things upfront with the cost of getting some corner cases wrong. But unwrap/expect are used too much in too many valid cases IMO for that type of strategy to be effective.

I’m not sure that was the best example but I agree with your overall point.

Also, Clippy's defaults are developed to be quite opinionated. It makes many suggestions that I (and many others) disagree with. So Clippy's defaults should not be used as a bludgeon for what's idiomatic and correct.

    pub fn new(mut streams: Vec<TokenStream>) -> TokenStream {
        match streams.len() {
            0 => TokenStream::empty(),
            1 => streams.pop().unwrap(),
            _ => TokenStream::Stream(Lrc::new(streams)),
        }
    }

But I agree, most of the time using `match` (or its simplified form `if let`) or one of these combinators instead of `unwrap()` is better.

The message helps a bit more, and I also want to avoid debug unwraps that were forgotten about.

Though, the more I think about it the more a macro makes sense to me, purely for code search. If we used a macro, something like `impossible!(streams.pop())` then I can code search for `.unwrap` and `.expect`.

Hmm

IIUC, panicking via unwrap() or expect() IS graceful error handling. It unwinds the stack and releases resources. We should be clearer because it's confusing to people less familiar with Rust who will confuse this with actually crashing and immediately terminating.

Personally, I think if there's nothing you can actually do to handle this error then unwrap() or expect() are the correct choices. There's no point writing more code just to bubble an error that you can't handle.

AFAIK, only with the default panic=unwind mode. With the panic=abort mode, it aborts the whole process instead of unwinding.

This is often poor advice, because continuing in a corrupt state can often be worse (and harder to debug) than a clean panic. E.g. it's better to crash than to overwrite a save file with corrupt data, or transfer money to the wrong account, or show one user data belonging to another user.

Let's say you have CLI tool - instead of message "XX is a directory - file expected" your tool will just fail silently.

There are much more examples. Graceful shutdown is what we have to do, even laziest of us.

.expect(format!("{} is a directory - file expected")) gives exactly the behaviour you've described for a CLI tool.

Graceful shutdown means releasing locks, deleting PID files etc when encountering an unrecoverable error. Not just any error.

When language allows you to make all errors "expected", it's really stupid to don't use it. Only reason is laziness.

Many languages don't have Result types to return so there you can't do much sometimes to prevent data corruption or something worse. But in languages where you can return more than just "false", it's not acceptable.