You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: main.rs
    path of file: ./repos/texture-synthesis/cli/src
    
    the code of the file until where you have to start completion: // BEGIN - Embark standard lints v5 for Rust 1.55+
// do not change or add/remove here, but one can add excep
    
fn parse_mask(input: &str) -> Result<ChannelMask, String> {
    let mask = match &input.to_lowercase()[..] {
        "r" => ChannelMask::R,
        "g" => ChannelMask::G,
        "b" => ChannelMask::B,
        "a" => ChannelMask::A,
        mask => {
            return Err(format!(
                "unknown mask '{}', must be one of 'a', 'r', 'g', 'b'",
                mask
            ))
        }
    };

    Ok(mask)
}