How to implement bitwise operations on a bitflags enum? - rust

Question

I have an enum that looks like this:
#[repr(u8)]
pub enum PublicFlags {
PublicFlagVersion = 0x01,
PublicFlagReset = 0x02,
NoncePresent = 0x04,
IdPresent = 0x08,
PktNumLen4 = 0x30,
PktNumLen2 = 0x20,
PktNumLen1 = 0x10,
Multipath = 0x40,
}
I want to do a bitwise operation on several of the enum values. However, the Rust compiler complains:
an implementation of `std::ops::BitAnd` might be missing for `PublicFlags`.

Answer 1

An enum in Rust is not intended to be used as bit flags. PublicFlags can only take the values given in the enum (and not a combination). So for instance, the following match statement is exhaustive:
let flags: PublicFlags;
...
match flags {
PublicFlagVersion => {...}
PublicFlagReset => {...}
NoncePresent => {...}
IdPresent => {...}
PktNumLen4 => {...}
PktNumLen2 => {...}
PktNumLen1 => {...}
Multipath => {...}
}
There is no way to have a PublicFlags variable with a combination of the flags.
The solution is to actually store the value as a u8, then use constants to store the value of each flag. This can be cumbersome, but thankfully the bitflags crate wraps all the boilerplate up in a macro for you. Here is an example how you would create your struct using bitflags:
#[macro_use]
extern crate bitflags;
bitflags! {
flags PublicFlags: u8 {
const PUBLIC_FLAG_VERSION = 0x01,
const PUBLIC_FLAG_RESET = 0x02,
const NONCE_PRESENT = 0x04,
const ID_PRESENT = 0x08,
const PKT_NUM_LEN_4 = 0x30,
const PKT_NUM_LEN_2 = 0x20,
const PKT_NUM_LEN_1 = 0x10,
const MULTIPATH = 0x40,
}
}
fn main() {
let flag = PUBLIC_FLAG_VERSION | ID_PRESENT;
assert!((flag & MULTIPATH).is_empty());
assert!(flag.contains(ID_PRESENT));
}

Answer 2

This could work as an alternative answer without new dependencies.
pub mod PublicFlags {
pub const PublicFlagVersion: u8 = 0x01;
pub const PublicFlagReset: u8 = 0x02;
pub const NoncePresent: u8 = 0x04;
pub const IdPresent: u8 = 0x08;
pub const PktNumLen4: u8 = 0x30;
pub const PktNumLen2: u8 = 0x20;
pub const PktNumLen1: u8 = 0x10;
pub const Multipath: u8 = 0x40;
}
You can refer to the values just like with an enum with PublicFlags::PublicFlagVersion, or add use PublicFlags::*; if it is cleaner to reference the values without specifying the namespace.