Base64 encoder written in Rust

main.rs

const CHARS: [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 encode_bytes(chunk: &[u8]) -> [char; 4] {
    let combined: u32 = ((chunk[0] as u32) << 16) + ((chunk[1] as u32) << 8) + (chunk[2] as u32);

    let part_1 = ((combined & 0b111111000000000000000000) >> 18) as usize;
    let part_2 = ((combined & 0b000000111111000000000000) >> 12) as usize;
    let part_3 = ((combined & 0b000000000000111111000000) >> 6) as usize;
    let part_4 = (combined & 0b000000000000000000111111) as usize;

    [CHARS[part_1], CHARS[part_2], CHARS[part_3], CHARS[part_4]]
}

fn get_input() -> String {
    let mut input = String::new();
    std::io::stdin().read_line(&mut input).unwrap();
    input.trim().to_string()
}

fn get_size(input: &[u8]) -> (usize, usize) {
    let size = input.len();
    let reminder = size % 3;
    (size, size + ((3 - reminder) % 3))
}

fn pad_bytes(input: &[u8], size: usize) -> Vec<u8> {
    let mut padded_input: Vec<u8> = input.to_vec();

    for _ in (input.len())..size {
        padded_input.push(0);
    }

    padded_input
}

fn main() {
    let input = get_input();
    let input = input.as_bytes();
    let (original_size, padded_size) = get_size(input);
    let input = pad_bytes(input, padded_size);
    let mut output = String::new();

    for chunk in input.chunks(3) {
        for c in encode_bytes(&chunk) {
            output.push(c);
        }
    }

    for _ in original_size..padded_size {
        output.pop();
    }

    for _ in original_size..padded_size {
        output.push('=');
    }

    println!("{}", output);
}

Random thought: it would probably be more efficient if you did all the byte manipulations on the vector of u8's and converted that vector to String only at the very very end. This way you could just replace the last few elements of the array with = instead of doing pop/push or truncate/extend.