Common Rust Lifetime Misconceptions (2020) (opens in new tab)

> who probably learned Rust the wrong way (by just trying to build stuff as if it were C/node.js and run into problems instead of slowing down + reading):

This is the right way to learn. I'm quite familiar with lifetimes and whatnot, but when I didn't bother with them much at all when learning - I just "clone"'d.

This allowed me to learn 95% of the language, and then once I did that, learn the last 5% (lifetimes).

Highly recommend.

I think there's a lot to be gained by not writing Rust like C, to the extent that it might be worth taking some time to pick up another language (maybe a lisp variant?) first.

2.) Be careful because &String is not &str, although in many cases you can pretend thanks to magic of AsRef/AsDeref.

4.) If you find yourself calling is_none, rethink things a bit. Pattern matching and the various destructuring (e.g. if let) features are some of the most powerful tools at your disposal with Rust. This is where experience with something else first (e.g. Elixir) can be helpful.

  I rarely introduce a lifetime to a struct/impl.

IMO the big use case here is &str.

  Arc kind of bails you out of a lot

While that's totally reasonable it's good to remember that you're essentially trading compile time guarantees for runtime ones. If you can figure out how to beat the borrow checker into submission that's one less thing to debug at runtime.

  primitive like u32 and it's borrowed and you had to dereference it or clone it.

The primitive types should all implement Copy which means you should (almost?) never have to explicitly clone them. Dereferencing is another story tho.

This is how my team writes production Rust code. Knowing which one to use and when is important, but there's nothing wrong with using the tools available to you.

Non-lexical lifetimes are, in my experience, pretty uncommon in most non-library code. You don't really need them until you really need them.

Good advice, though I'd recommend Rc<RefCell<T>> instead of Arc<Mutex<T>> if you're not sharing the data between threads, to avoid synchronization overhead. I use Arc pretty rarely.

Using `clone` etc when it's easier is actually common advice, it's perfectly OK to not have borrowing everywhere if you don't need it. My usual starting point / default / rule of thumb is to take references as parameters and return owned values (for example, take `&str` as an argument, return an owned `String`).

This is totally valid. Lifetimes can be an optimization you go back and add later as needed; in structs, especially, they require a ton of code changes to add and a ton of code changes to change your mind about later, so they should be used judiciously

Other things I would add to this list:

- For structs that don't own anything on the heap, derive Copy. Then you can just pass them around willy-nilly without explicit clone()s

- Using a functional style where it makes sense to helps a lot; it can be really easy to pass things by reference when they only need to be temporarily, immutably used by that one function. And if you make Copyable structs, you can pass owned structs and return owned structs and not worry about any of it

I know there was a thread involving a rust team member saying that clone / to_owned is ok to start with The memory copying just nags and distracts me from moving on

Cloning things which are Copy (such as u32) is futile and Clippy will tell you not to bother where it can see this is definitely Copy. If you don't use Clippy, I'd suggest trying it for a while.

Rc will be faster (if that matters to you) than Arc but it can't cross threads. (Safe) Rust will check you didn't get this wrong, so there's no danger but obviously knowing ahead of time avoids writing an Rc that you then need to be an Arc instead.

Sometimes it's tidier to write the borrow in the type of a pattern match e.g. if let Some(&foo) = ... Means you won't need to dereference foo inside the block.

Once I started taking this advice, Rust became manageable to me! Currently, Trait Implementations have been more of a stumbling block for me than the borrow checker

This is the way. The real misconception of lifetimes is that people have to use them often. You don’t unless your are writing libraries or system code.

For the edit: if you have an idea what the output should look like instead, please file a ticket.

> as Rust is many times faster than other languages even with liberal cloning

Have you really compared? I have. Rust was faster for "small input", but quickly got beaten by Java and other languages I tried because the cost of doing things this way grows exponentially. I suggest you run benchmarks before you make your mind up and start throwing opinions around.

Me too. I also do this with .unwrap() - when I'm pretty sure of the happy path - while I'm prototyping. It's pretty easy afterwards to run back through and replace .unwrap() calls with better error handling. But as I improve, I'm getting more in the habit of using matching directly. Understanding lifetimes is probably really helpful for understanding library errors though and I need to go deeper there for sure

Another technique for dodging [manual] lifetimes is not store references in struct fields etc.

`static is actually the superlifetime of all lifetimes, but & is contravariant in its lifetime parameter (and covariant in its type parameter).

Could you elaborate? I find this unintuitive.

“Applied it when told to do so.”

This is my beginner-level experience with Rust. It’s amazing that the compiler can be so specific about what’s wrong. But taking the error and getting explanations that even I can understand has been tricky.

Same. I wrote a lot of Rust, even some open source libraries that are being used and generally feel comfortable using it but I learned a lot out of this very nice write-up.

A new edition is not a free pass to break source compatibility; it's more like a fire axe hidden behind breakable glass. You really, really want to avoid breaking changes because your users will now have to know about the difference between the compiler editions used by each of their projects.

In short, yes, but be very wary.

Technically yes, but editions are more like different "flavors" of Rust than actual breaking changes. A 2021 crate is supposed to be able to import a 2018 crate, likewise for 2015 and 2024. Because they all need to work together, edition changes still need to be somewhat compatible with older versions.

Because you can pass closures into functions across crate boundaries, which requires consistent lifetime semantics, I find it unlikely that this will be implemented.

Most likely yes.

However, if you don't specify the argument type, it compiles fine. It's rare that you need to specify argument or return types on a closure, so it's not actually a large issue.

Wouldn't fixing 10 just mean allowing things that were previously not allowed? That's not a breaking change. What am I missing?

I wholeheartedly agree and also extend this to software I write. I try very hard to have booleans never be called e.g. disableFoo. It isn't always easy, but you glue enough logic together and then try to say it becomes very difficult. This becomes even more true when you have a language that has unset or null type values and then configurations for those settings where the logic gets even nastier because usually you want unset to be false. If you have a negatively declared variable in this case, you have to check if it is true OR it is unset, whereas if it is a positively named thing, simply checking for a truthy value is adequate, more concise, and less error prone.

> have some consistent and unmistakable typography for the "false statements" (highlighting or color, etc)

THIS ^^

I stopped reading after about paragraph two, when I realized that it was very unclear that the code and table that I was reading was in the "this is a false belief" part of the explanation.

Don't do that.

Communicate more clearly and the article will be useful to more people, longer.