Borrowing as mutable with prior immutable references

help
#1

Hey all, I'm trying to write a program to go through a zip archive and parse all JSON files, returning an error if one occurs. My current code looks like this:

fn main() -> Result<(), String> {
    let path = PathBuf::from("file.zip");

    let file = File::open(&path).unwrap();
    let mut archive = zip::ZipArchive::new(file).unwrap();

    for i in 0..archive.len() {
        let mut zip_file = archive.by_index(i).unwrap();

        let zip_file_path = zip_file.enclosed_name().unwrap();

        if !zip_file_path.ends_with(".json") {
            continue;
        }

        let mut buffer = String::new();
        // cannot borrow `zip_file` as mutable because it is also borrowed as immutable
        zip_file.read_to_string(&mut buffer).map_err(|e| {
            format!("Error reading file {}: {}", zip_file_path.display(), e)
        })?;

        let json = serde_json::from_str::<String>(&buffer).map_err(|e| {
            format!("Error parsing file {}: {}", zip_file_path.display(), e)
        })?;

        // Continue using json...
    }

    Ok(())
}

My main issue here is making use of the zip_file_path variable. When I use it, I can a "cannot borrow zip_file as mutable because it is also borrowed as immutable" error, but when I replace usages of the variable with the value itself, the error no longer appears, i.e.

fn main() -> Result<(), String> {
    let path = PathBuf::from("file.zip");

    let file = File::open(&path).unwrap();
    let mut archive = zip::ZipArchive::new(file).unwrap();

    for i in 0..archive.len() {
        let mut zip_file = archive.by_index(i).unwrap();

        let zip_file_path = zip_file.enclosed_name().unwrap();

        if !zip_file_path.ends_with(".json") {
            continue;
        }

        let mut buffer = String::new();
        // cannot borrow `zip_file` as mutable because it is also borrowed as immutable
        zip_file.read_to_string(&mut buffer).map_err(|e| {
            format!("Error reading file {}: {}", zip_file.enclosed_name().unwrap().display(), e)
        })?;

        let json = serde_json::from_str::<String>(&buffer).map_err(|e| {
            format!("Error parsing file {}: {}", zip_file.enclosed_name().unwrap().display(), e)
        })?;

        // Continue using json...
    }

    Ok(())
}

I'd much prefer having a variable I can use to pass into my errors, so I'm wondering what the idiomatic solution to my problem is. Thanks in advance for taking the time to help!

#2

This line returns an &Path which points to memory within the ZipFile structure. This prevents you taking an exclusive (&mut) reference to zip_file because the compiler can't guarantee that the target bytes of zip_file_path won't change.

To fix this, you need to make your own copy of the path, so that it unlocks zip_file:

       let zip_file_path = zip_file.enclosed_name().unwrap().to_path_buf();
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