Code shared from the Rust Playground

playground.rs

const BASE64CHARS: [char; 64] = [
    'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I', 'J', 'K', 'L', 'M', 'N', 'O', 'P', 'Q', 'R', 'S',
    'T', 'U', 'V', 'W', 'X', 'Y', 'Z', 'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l',
    'm', 'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z', '0', '1', '2', '3', '4',
    '5', '6', '7', '8', '9', '+', '/',
];

fn main() {
    println!("{:?}", format_base64_string("foz=="));
}

fn format_base64_string(base64input: &str) -> Result<&str, &str> {
    for ch in base64input.trim_right_matches('=').chars() {
        if !BASE64CHARS.contains(&ch) {
            return Err("Error.. An invalid base 64 string was input!");
        }
    }
    
    Ok(base64input)
}