this post was submitted on 31 Jan 2026
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Programmer Humor

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Me, after carefully reading Rust's ownership and borrow checker rules (lemmy.world)
submitted 3 weeks ago by chromeleon@lemmy.world to c/programmer_humor@programming.dev
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[–] Ephera@lemmy.ml 46 points 3 weeks ago (3 children)

The rule of thumb I always tell people is that they should generally put owned data into struct fields and references into function parameters.

[–] calcopiritus@lemmy.world 17 points 3 weeks ago

Good rule of thumb. As long as it's not followed blindly of course.

Structs with lifetimes are often quite convenient. Especially for performance.

[–] joyjoy@lemmy.zip 10 points 3 weeks ago (1 children)

I recently learned you can pass a &String to a &str parameter, so that's neat. 

[–] Ephera@lemmy.ml 8 points 3 weeks ago (1 children)

Ah yeah, via deref coercion, which is also called "auto-dereferencing" at times. Not to be confused with "auto-referencing", which is also a thing^1.

You can do some wild shit with deref coercion. And when I say "wild", I guess, I'm talking about the most normal thing for Java devs, because well, it's a lot like inheritance. 😅

Basically, this concept of being able to pass &String into a parameter that takes &str also applies to the self parameter. Or in other words, methods implemented on str can also be called on String, as if String extends str.
And well, obviously you can also make use of that yourself, by writing your own wrapper type. You can even "override" existing methods in a sense by re-defining them in the wrapper type.

I had to play around a bit with it myself, so here's a playground: https://play.rust-lang.org/?version=stable&mode=debug&edition=2024&gist=af65ed396dec88c8406163acaa1f8f8d

[–] Ephera@lemmy.ml 6 points 3 weeks ago

Welp, I posted my hot take that impl Deref is similar to inheritance as a meme in !rust@lemmy.ml: https://lemmy.ml/post/42514248

Now, let's see how many feathers get ruffled. 🙃

[–] harrowhawk 5 points 3 weeks ago

I like this rule of thumb and it goes nicely with the “if you have more than 3 arguments to a function, consider making a struct to pass the arguments in” rule of thumb because

You can have the struct use a named lifetime (or several) for the different parameters so it doesn’t own them!

Best of both worlds.