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🍄 bitfield utilities, courtesy of Mycelium.

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what is it?

This library provides utilities for defining structured bitfields in Rust. It consists of a set of types for defining ranges that can be packed and unpacked from an integer value, and a bitfield! macro for generating bitfield types automatically using the packing types. These components are modular: it’s possible to use the packing spec types to hand-write all of the code that the bitfield! macro would generate.

This crate was originally implemented for usage in the Mycelium operating system, although it is usable in other projects and does not depend on any Mycelium-specific libraries.

comparison with other crates

There are several other crates implementing bitfields or bitfield-related utilities in Rust. These crates offer different, but sometimes overlapping, functionality relative to mycelium-bitfield. In particular, the most directly comparable crates that I’m currently aware of are the modular-bitfield and bitflags libraries.

Note This crate exists primarily because I thought it would be fun to write my own bitfield crate, not because the existing libraries were deficient. It is possible that I have a somewhat perverse conception of “fun”…

The modular-bitfield crate, in particular, can do most of the same things as mycelium-bitfield. However, there are some differences between the two libraries which may be interesting to consider.

  • bitflags: The bitflags crate provides a declarative macro for generating a structured type representing a set of bitflags.

    The critical difference between bitflagsbitflags! macro and mycelium-bitfield’s bitfield! macro is that the bitflags crate only implements bitflags, not bitfields. It is not possible to define multi-bit structured ranges using bitflags; only single-bit flags can be set and unset.

    However, the bitflags crate is widely used, has been around for a long time, and is relatively simple and lightweight. If all you need is a set of boolean, single-bit flags, it’s a very solid choice. But, if you need mycelium-bitfield’s additional functionality for working with multi-bit ranges, note that it can also do most most of what the bitflags crate can do.

  • modular-bitfield: The modular-bitfield crate provides a procedural macro for generating typed structured bitfields.

    The functionality implemented by modular-bitfield is broadly very similar to mycelium-bitfield — the two libraries can do most of the same things.

    The primary difference is that modular-bitfield is implemented using a procedural macro attribute, while mycelium-bitfield’s bitfield! macro is a declarative macro. In my opinion, this isn’t a reason to prefer mycelium-bitfield over modular-bitfield in most use cases. I decided to try to write the whole thing using a declarative macro because I thought it would be a fun challenge, not because it’s better (in fact, it would probably have been much easier to implement the bitfield type generation using a procedural macro). However, users who need to reduce or avoid procedural macros for some reason may want to consider choosing mycelium-bitfield for that reason.

    The other primary difference between mycelium-bitfield and modular-bitfield is that mycelium-bitfield also provides the pack module with packing spec types. These types can be used to build bitfield types “by hand”, in cases where different behavior from the macro-generated code is needed. modular-bitfield only provides a procedural macro, and does not have an equivalent to this lower-level interface.

    On the other hand, modular-bitfield provides nicer validation that a typed value used as part of a bitfield actually fits in that bitfield. mycelium-bitfield cannot currently do this kind of compile-time checking, and relies on implementations of the FromBits trait for user-provided types being correct.


This crate’s API consists of three primary components, the packing spec types, the bitfield! macro, and the FromBits trait.

packing spec types

The pack module defines a set of types that can be used to pack and unpack ranges from integer values of various sizes, such as Pack64 for packing and unpacking a range from a u64 value.

These packing spec types have const fn constructors that allow them to be defined in relationship with each other. For example:

use mycelium_bitfield::Pack64;

// Defines a packing spec for the least-significant 12 bits of a 64-bit value.
const LOW: Pack64 = Pack64::least_significant(12);
// Defines a packing spec for the next 8 more-significant bits after `LOW`.
const MID: Pack64 = LOW.next(8);
// Defines a packing spec for the next 4 more-significant bits after `MID`.
const HIGH: Pack64 = MID.next(4);

// Wrap an integer value to pack it using method calls.
let coffee = Pack64::pack_in(0)
    // pack the 12 bits of `0xfee` at the range specified by `LOW`.
    .pack(0xfee, &LOW)
    // pack the 4 bits `0xc` at the range specified by `HIGH`.
    .pack(0xc, &HIGH)
    // pack `0xf` in the 8 bits specified by `MID`.
    .pack(0xf, &MID)
    // unwrap the packing value back into a `u64`.

assert_eq!(coffee, 0xc0ffee); // i want c0ffee

A majority of the functions in the pack module are const fns, allowing the use of packing specs in const contexts.

See the module-level docs for pack for details.

bitfield! macro

The bitfield! macro allows defining a structured bitfield type declaratively. The macro will generate code that uses the pack module’s packing spec APIs to represent a bitfield type.

For example:

mycelium_bitfield::bitfield! {
    /// Bitfield types can have doc comments.
    #[derive(Eq, PartialEq)] // ...and attributes
    pub struct MyBitfield<u16> {
        /// Generates a packing spec named `HELLO` for the first 6
        /// least-significant bits.
        pub const HELLO = 6;

        // Fields with names starting with `_` can be used to mark bits as
        // reserved.
        const _RESERVED = 4;

        /// Generates a packing spec named `WORLD` for the next 3 bits.
        pub const WORLD = 3;

        /// A boolean value will generate a packing spec for a single bit.
        pub const FLAG: bool;

// Bitfield types can be cheaply constructed from a raw numeric
// representation:
let bitfield = MyBitfield::from_bits(0b10100_0011_0101);

// `get` methods can be used to unpack fields from a bitfield type:
assert_eq!(bitfield.get(MyBitfield::HELLO), 0b11_0101);
assert_eq!(bitfield.get(MyBitfield::WORLD), 0b0101);

// `with` methods can be used to pack bits into a bitfield type by
// value:
let bitfield2 = MyBitfield::new()
    .with(MyBitfield::HELLO, 0b11_0101)
    .with(MyBitfield::WORLD, 0b0101);

assert_eq!(bitfield, bitfield2);

// `set` methods can be used to mutate a bitfield type in place:
let mut bitfield3 = MyBitfield::new();

    .set(MyBitfield::HELLO, 0b011_0101)
    .set(MyBitfield::WORLD, 0b0101);

assert_eq!(bitfield, bitfield3);

See the bitfield! macro’s documentation for details on the macro’s usage and the code it generates.

FromBits trait

The FromBits trait can be implemented for user-defined types which can be used as subfields of a bitfield!-generated structured bitfield type.

For example:

use mycelium_bitfield::{bitfield, FromBits};

// An enum type can implement the `FromBits` trait if it has a
// `#[repr(uN)]` attribute.
#[derive(Debug, Eq, PartialEq)]
enum MyEnum {
    Foo = 0b00,
    Bar = 0b01,
    Baz = 0b10,

impl FromBits<u32> for MyEnum {
    // Two bits can represent all possible `MyEnum` values.
    const BITS: u32 = 2;
    type Error = &'static str;

    fn try_from_bits(bits: u32) -> Result<Self, Self::Error> {
        match bits as u8 {
            bits if bits == Self::Foo as u8 => Ok(Self::Foo),
            bits if bits == Self::Bar as u8 => Ok(Self::Bar),
            bits if bits == Self::Baz as u8 => Ok(Self::Baz),
            _ => Err("expected one of 0b00, 0b01, or 0b10"),

    fn into_bits(self) -> u32 {
        self as u8 as u32

bitfield! {
    pub struct TypedBitfield<u32> {
        /// Use the first two bits to represent a typed `MyEnum` value.
        const ENUM_VALUE: MyEnum;

        /// Typed values and untyped raw bit fields can be used in the
        /// same bitfield type.
        pub const SOME_BITS = 6;

        /// The `FromBits` trait is also implemented for `bool`, which
        /// can be used to implement bitflags.
        pub const FLAG_1: bool;
        pub const FLAG_2: bool;

        /// `FromBits` is also implemented by (signed and unsigned) integer
        /// types. This will allow the next 8 bits to be treated as a `u8`.
        pub const A_BYTE: u8;

// Unpacking a typed value with `get` will return that value, or panic if
// the bit pattern is invalid:
let my_bitfield = TypedBitfield::from_bits(0b0011_0101_1001_1110);

assert_eq!(my_bitfield.get(TypedBitfield::ENUM_VALUE), MyEnum::Baz);
assert_eq!(my_bitfield.get(TypedBitfield::FLAG_1), true);
assert_eq!(my_bitfield.get(TypedBitfield::FLAG_2), false);

// The `try_get` method will return an error rather than panicking if an
// invalid bit pattern is encountered:

let invalid = TypedBitfield::from_bits(0b0011);

// There is no `MyEnum` variant for 0b11.

See the FromBits trait documentation for details on implementing FromBits for user-defined types.


pub use self::pack::*;



An example of the code generated by the bitfield! macro.

Packing spec types.


Generates a typed bitfield struct.


Trait implemented by values which can be converted to and from raw bits.