Rust: What the fuck is a borrow checker and why is it yelling at me?

The Something Awful Forums > Discussion > Serious Hardware/Software Crap > The Cavern of COBOL > Rust: What the fuck is a borrow checker and why is it yelling at me?

fritz: Jul 26, 2003

I'm having trouble with BufWriter living inside an option. I'm creating it as:

code:

// cli.outfile is an Option<String>
let fh: Option<File> = match cli.outfile {
    None => None,
    Some(s) => {
	let file : Result<File> = File::create(&s);
        match file {
		Err(_) => None.
		Ok(f) => Some(f)
        }
}

What I can't figure out is how to borrow in such a way that I can write to the thing inside 'fh' and not consume it.

Something like

code:

match &fh {
	None => None,
	Some(&f) => writeln!(f, "output");
}

but this consumes fh in a way I can't figure out how to avoid.

I've been trying a number of things but they're getting increasingly convoluted in a way that makes me think I'm fighting the languge.

# ¿ Aug 24, 2022 21:12

Adbot: ADBOT LOVES YOU

# ¿ May 12, 2024 15:19

fritz: Jul 26, 2003

gonadic io posted:

You want `match fs.as_mut()`. That and `as_ref` seem pretty unknown but are one of my most-used Option methods tbh

That worked! Thanks!

# ¿ Aug 24, 2022 21:41

fritz: Jul 26, 2003

gonadic io posted:

Once your code is more settled you'll find that clippy will tell you that lots of your matches are equivalent to functions like `map` and `ok` and so on, make the code much easier to read. No worries when just messing around though.

Yeah I'm already a big fan of clippy, but I wasn't even able to get the thing to compile so it wasn't helping.

# ¿ Aug 25, 2022 17:50

fritz: Jul 26, 2003

Had a failure of intuition today.

code:

struct Dummy {
   pub data : Vec<f64>
}

let mut data : Vec< (usize, Dummy) > = Vec::new();
let mut table : HashMap< usize, Dummy > = HashMap::new();
// data gets filled somewhere along the way
for (i,j) in data {

   table.entry(i)
   .and_modify(|v| *v = j)
   .or_insert(j)
}

rustc errors at the or_insert because j is already moved in the and_modify. I'm confused because I would have thought that it wasn't possible for both code paths to execute, i.e. if it goes thru .and_modify, it won't go thru .or_insert, and vice versa.

# ¿ Oct 21, 2022 20:51

fritz: Jul 26, 2003

Thanks for the wisdom everybody, I think I get it now.

# ¿ Oct 23, 2022 12:37

Adbot: ADBOT LOVES YOU

# ¿ May 12, 2024 15:19

fritz: Jul 26, 2003

A "pointer" can be something that indexes into an array. You could have a data structure that contains a next_index and prev_index and update those as you iterate thru.

# ¿ Dec 11, 2023 19:32

The Something Awful Forums > Discussion > Serious Hardware/Software Crap > The Cavern of COBOL > Rust: What the fuck is a borrow checker and why is it yelling at me?