frequency = io::stdin()
    .lock()
    .lines()
    .map(|x| {
        let x = x.map_err(|e| e.to_string())?;
        x.parse().map_err(|e: ParseIntError| e.to_string())
    })
    .inspect(|r| {
        if r.is_err() {
            println!("Couldn't parse line as a number");
        }
    })
    .filter_map(|x| x.ok())
    .next()
    .unwrap();

$ rustup component add rustfmt --toolchain nightly-x86_64-pc-windows-gnu
error: component 'rustfmt' for target 'x86_64-pc-windows-gnu' is unavailable for download for channel nightly
Sometimes not all components are available in any given nightly.

$ rustup component add rustfmt --toolchain nightly-x86_64-pc-windows-gnu
error: component 'rustfmt' for target 'x86_64-pc-windows-gnu' is unavailable for download for channel nightly
Sometimes not all components are available in any given nightly.