adding path to io error in rust

main.rs

extern crate custom_error;

use custom_error::custom_error;
use std::fs::metadata;
use std::io;
use std::path::Path;
use std::path::PathBuf;
use std::result::Result;

custom_error! {ProgramError
    Io {
        source: io::Error, // we also wrap the source error
        path: PathBuf
    } = @{format!("{path}: {source}", source=source, path=path.display())},
    // other kinds of error would go there
}

fn file_size(path: &Path) -> Result<u64, ProgramError> {
    match metadata(path) {
        Ok(md) => Ok(md.len()),
        Err(e) => Err(ProgramError::Io {
            source: e,
            path: path.to_path_buf(),
        }),
    }
}

fn main() {
    if let Err(err) = file_size(&PathBuf::from("/not/there")) {
        eprintln!("{}", err);
    }
}

see https://stackoverflow.com/questions/53934888/how-to-include-the-file-path-in-an-io-error-in-rust