use std::mem;
#[derive(Debug)]
struct Foo {
    a: u8,
    b: u8,
}
#[macro_export]
macro_rules! parse_bytes {
    ( $buf:expr; ) => {};
    ( $buf:expr; $( $t:tt => $field:expr ),+ ) => {
        let mut index: usize = 0;
        $(
            _parse_field!($t => $buf; $field; index);
        )+
    }
}

macro_rules! _parse_field {
    ( u8 => $buf:expr; $field:expr; $index:expr ) => {
        $field = u8::from_le_bytes([$buf[$index]]);
        $index += 1;
    };
    ( u16 => $buf:expr; $field:expr; $index:expr ) => {
        $field = u16::from_le_bytes([$buf[$index], $buf[$index+1]]);
        $index += 2;
    };
    ( u32 => $buf:expr; $field:expr; $index:expr ) => {
        $field = u16::from_le_bytes([$buf[$index], $buf[$index+1], $buf[$index+2], $buf[$index+3]]);
        $index += 4;
    };
}

fn main() {
    let mut x = Foo {a:0, b:0};
    let buf: &[u8] = &[1, 2];
    parse_bytes!{buf;
        u8 => x.a,
        u8 => x.b
    }
    println!("{:?}", x);
}