adding path to io error in rust, with a trait to ease the error transformation

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.display(), source)},
    // other kinds of error would go there
}

trait ToPathIOErr<T> {
    fn with_path(self: Self, path: &Path) -> Result<T, ProgramError>;
}

impl<T> ToPathIOErr<T> for io::Result<T> {
    fn with_path(self: Self, path: &Path) -> Result<T, ProgramError> {
        self.map_err(|e| ProgramError::Io {
            source: e,
            path: path.to_path_buf(),
        })
    }
}

fn file_size(path: &Path) -> Result<u64, ProgramError> {
    Ok(metadata(path).with_path(path)?.len())
}

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