1. bits-and-bytes.rs

pub fn i64_as_bytes_be(n: i64) -> Vec<u8> {
    let mut vec = Vec::with_capacity(8);
    vec.push(((n & 0xff00000000000000) >> 56) as u8);
    vec.push(((n & 0x00ff000000000000) >> 48) as u8);
    vec.push(((n & 0x0000ff0000000000) >> 40) as u8);
    vec.push(((n & 0x000000ff00000000) >> 32) as u8);
    vec.push(((n & 0x00000000ff000000) >> 24) as u8);
    vec.push(((n & 0x0000000000ff0000) >> 16) as u8);
    vec.push(((n & 0x000000000000ff00) >> 8) as u8);
    vec.push(((n & 0x00000000000000ff)) as u8);
    vec
}

pub fn i64_as_bytes_le(n: i64) -> Vec<u8> {
    let mut vec = Vec::with_capacity(8);
    vec.push(((n & 0x00000000000000ff)) as u8);
    vec.push(((n & 0x000000000000ff00) >> 8) as u8);
    vec.push(((n & 0x0000000000ff0000) >> 16) as u8);
    vec.push(((n & 0x00000000ff000000) >> 24) as u8);
    vec.push(((n & 0x000000ff00000000) >> 32) as u8);
    vec.push(((n & 0x0000ff0000000000) >> 40) as u8);
    vec.push(((n & 0x00ff000000000000) >> 48) as u8);
    vec.push(((n & 0xff00000000000000) >> 56) as u8);
    vec
}

fn bytes_as_i64_be(n: &[u8]) -> i64 {
    n[0] as i64 << 56
        | n[1] as i64 << 48
        | n[2] as i64 << 40
        | n[3] as i64 << 32
        | n[4] as i64 << 24
        | n[5] as i64 << 16
        | n[6] as i64 << 8
        | n[7] as i64
}

fn bytes_as_i64_le(n: &[u8]) -> i64 {
    n[7] as i64 << 56
        | n[6] as i64 << 48
        | n[5] as i64 << 40
        | n[4] as i64 << 32
        | n[3] as i64 << 24
        | n[2] as i64 << 16
        | n[1] as i64 << 8
        | n[0] as i64
}

fn main() {
    let n = 0x1122334455667788_i64;
    let be_bytes = i64_as_bytes_be(n);
    let le_bytes = i64_as_bytes_le(n);
    println!("{}", n);
    println!("{}", be_bytes);
    println!("{}", le_bytes);
    assert_eq!(n, bytes_as_i64_be(be_bytes.as_slice()));
    assert_eq!(n, bytes_as_i64_le(le_bytes.as_slice()));
}

2.output.txt

1234605616436508552
[17, 34, 51, 68, 85, 102, 119, 136]
[136, 119, 102, 85, 68, 51, 34, 17]