Allow struct members to be [de]serialised in a certain number of bits

[aka-demik]

It is often necessary for binary protocols.

A la:

#[derive(RustcEncodable, RustcDecodable, PartialEq)]
struct Entity {
    #[bits(4)]
    x: u8,
    #[bits(4)]
    y: u8,
}

[TyOverby]

This is practically impossible because bincode needs to work with rustc_serialize and serde; both of which are general enough that it would hard to get that information through them and back into the reader/writer in bincode.

I suggest that you do something like this

struct Entity {
    xy: u8,
}

impl Entity {
    fn x(&self) -> u8 {
        self.xy & 0xF
    }

    fn y(&self) -> u8 {
        self.xy & 0xF0
    }
}