this post was submitted on 26 Sep 2024
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Rust Programming

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[newbie] one-liner to avoid "temporary value dropped while borrowed"? (lemmy.ml)
submitted 1 month ago by [email protected] to c/[email protected]
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(I'm just starting off with rust, so please be patient)

Is there an idiomatic way of writing the following as a one-liner, somehow informing rustc that it should keep the PathBuf around?

// nevermind the fully-qualified names
// they are there to clarify the code
// (that's what I hope at least)

let dir: std::path::PathBuf = std::env::current_dir().unwrap();
let dir: &std::path::Path   = dir.as_path();

// this won't do:
// let dir = std::env::current_dir().unwrap().as_path();

I do understand why rust complains that "temporary value dropped while borrowed" (I mean, the message says it all), but, since I don't really need the PathBuf for anything else, I was wondering if there's an idiomatic to tell rust that it should extend its life until the end of the code block.

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[–] [email protected] 1 points 1 month ago* (last edited 1 month ago) (1 children)

There is a general mechanism in Rust that allows language users to add their own sugar. It's called macros 😉

macro_rules! keep {
    (let $id:ident = $expr:expr => $($tt:tt)+) => {
        let $id = $expr;
        let $id = $id$($tt)+;
    }
}

fn main() {
    keep!{ let path = std::env::current_dir().unwrap() => .as_path() };
    println!("{path:?}");
}

You can remove let from the macro's fragment specifier and invocation.

[–] [email protected] 11 points 1 month ago (1 children)

While macros are cool and it's good to keep them as an option in the back of the mind, it should be clarified that you're not supposed to immediately reach for macros for small things you don't quite like about the language.

Excessive macro use makes it impossible for others (including your future self) to read your code and there's often good reasons why it's designed like it is.