Bitfield Struct

Procedural macro for bitfields that allows specifying bitfields as structs. As this library provides a procedural macro, it has no runtime dependencies and works for no-std environments.

• Supports bool flags, raw integers, and every custom type convertible into integers (structs/enums)
• Ideal for driver/OS/embedded development (defining HW registers/structures)
• Generates minimalistic, pure, safe rust functions
• Compile-time checks for type and field sizes
• Rust-analyzer friendly (carries over documentation to accessor functions)
• Exports field offsets and sizes as constants (useful for const asserts)
• Generation of fmt::Debug and Default

Usage

Add this to your Cargo.toml:

[dependencies]
bitfield-struct = "0.5"

Basics

Let's begin with a simple example. Suppose we want to store multiple data inside a single Byte, as shown below:

7	6	5	4	3	3	1	0
P	Level		S	Kind

This crate generates a nice wrapper type that makes it easy to do this:

use bitfield_struct::bitfield;

/// Define your type like this with the bitfield attribute
#[bitfield(u8)]
struct MyByte {
    /// The first field occupies the least significant bits
    #[bits(4)]
    kind: usize,
    /// Booleans are 1 bit large
    system: bool,
    /// The bits attribute specifies the bit size of this field
    #[bits(2)]
    level: usize,
    /// The last field spans over the most significant bits
    present: bool
}
// The macro creates three accessor functions for each field:
// <name>, with_<name> and set_<name>
let my_byte = MyByte::new()
    .with_kind(15)
    .with_system(false)
    .with_level(3)
    .with_present(true);

assert!(my_byte.present());

Features

Additionally, this crate has a few useful features, which are shown here in more detail.

The example below shows how attributes are carried over and how signed integers, padding, and custom types are handled.

use bitfield_struct::bitfield;

/// A test bitfield with documentation
#[bitfield(u64)]
#[derive(PartialEq, Eq)] // <- Attributes after `bitfield` are carried over
struct MyBitfield {
    /// Defaults to 16 bits for u16
    int: u16,
    /// Interpreted as 1 bit flag, with a custom default value
    #[bits(default = true)]
    flag: bool,
    /// Custom bit size
    #[bits(1)]
    tiny: u8,
    /// Sign extend for signed integers
    #[bits(13)]
    negative: i16,
    /// Supports any type with `into_bits`/`from_bits` functions
    #[bits(16)]
    custom: CustomEnum,
    /// Public field -> public accessor functions
    #[bits(10)]
    pub public: usize,
    /// Also supports read-only fields
    #[bits(1, access = RO)]
    read_only: bool,
    /// And write-only fields
    #[bits(1, access = WO)]
    write_only: bool,
    /// Padding
    #[bits(5)]
    __: u8,
}

/// A custom enum
#[derive(Debug, PartialEq, Eq)]
#[repr(u64)]
enum CustomEnum {
    A = 0,
    B = 1,
    C = 2,
}
impl CustomEnum {
    // This has to be a const fn
    const fn into_bits(self) -> u64 {
        self as _
    }
    const fn from_bits(value: u64) -> Self {
        match value {
            0 => Self::A,
            1 => Self::B,
            _ => Self::C,
        }
    }
}

// Usage:
let mut val = MyBitfield::new()
    .with_int(3 << 15)
    .with_tiny(1)
    .with_negative(-3)
    .with_custom(CustomEnum::B)
    .with_public(2)
    // .with_read_only(true) <- Would not compile
    .with_write_only(false);

println!("{val:?}");
let raw: u64 = val.into();
println!("{raw:b}");

assert_eq!(val.int(), 3 << 15);
assert_eq!(val.flag(), true);
assert_eq!(val.negative(), -3);
assert_eq!(val.tiny(), 1);
assert_eq!(val.custom(), CustomEnum::B);
assert_eq!(val.public(), 2);
assert_eq!(val.read_only(), false);

// const members
assert_eq!(MyBitfield::FLAG_BITS, 1);
assert_eq!(MyBitfield::FLAG_OFFSET, 16);

val.set_negative(1);
assert_eq!(val.negative(), 1);

The macro generates three accessor functions for each field. Each accessor also inherits the documentation of its field.

The signatures for int are:

use std::fmt::{Debug, Formatter, Result};

// generated struct
struct MyBitfield(u64);
impl MyBitfield {
    const fn new() -> Self { Self(0) }

    const INT_BITS: usize = 16;
    const INT_OFFSET: usize = 0;

    const fn with_int(self, value: u16) -> Self { todo!() }
    const fn int(&self) -> u16 { todo!() }
    fn set_int(&mut self, value: u16) { todo!() }

    // other field ...
}
// Also generates From<u64>, Into<u64>, Default, and Debug implementations...

Hint: You can use the rust-analyzer "Expand macro recursively" action to view the generated code.

Custom Types

The macro supports any types that are convertible into the underlying bitfield type. This can be enums like in the following example or any other struct.

The conversion and default values can be specified with the following #[bits] parameters:

• from: Function converting from raw bits into the custom type, defaults to <ty>::from_bits
• into: Function converting from the custom type into raw bits, defaults to <ty>::into_bits
• default: Custom expression, defaults to calling <ty>::from_bits(0)

use bitfield_struct::bitfield;

#[bitfield(u16)]
#[derive(PartialEq, Eq)]
struct Bits {
    /// Supports any convertible type
    #[bits(16, default = CustomEnum::B, from = CustomEnum::my_from_bits)]
    custom: CustomEnum,
}

#[derive(Debug, PartialEq, Eq)]
#[repr(u16)]
enum CustomEnum {
    A = 0,
    B = 1,
    C = 2,
}
impl CustomEnum {
    // This has to be a const fn
    const fn into_bits(self) -> u16 {
        self as _
    }
    const fn my_from_bits(value: u16) -> Self {
        match value {
            0 => Self::A,
            1 => Self::B,
            _ => Self::C,
        }
    }
}

Bit Order

The optional order macro argument determines the layout of the bits, with the default being Lsb (least significant bit) first:

use bitfield_struct::bitfield;

#[bitfield(u8, order = Lsb)]
struct MyLsbByte {
    /// The first field occupies the least significant bits
    #[bits(4)]
    kind: usize,
    system: bool,
    #[bits(2)]
    level: usize,
    present: bool
}

let my_byte_lsb = MyLsbByte::new()
    .with_kind(10)
    .with_system(false)
    .with_level(2)
    .with_present(true);

//                         .- present
//                         | .- level
//                         | |  .- system
//                         | |  | .- kind
assert!(my_byte_lsb.0 == 0b1_10_0_1010);

The macro generates the reverse order when Msb (most significant bit) is specified:

use bitfield_struct::bitfield;

#[bitfield(u8, order = Msb)]
struct MyMsbByte {
    /// The first field occupies the most significant bits
    #[bits(4)]
    kind: usize,
    system: bool,
    #[bits(2)]
    level: usize,
    present: bool
}

let my_byte_msb = MyMsbByte::new()
    .with_kind(10)
    .with_system(false)
    .with_level(2)
    .with_present(true);

//                         .- kind
//                         |    .- system
//                         |    | .- level
//                         |    | |  .- present
assert!(my_byte_msb.0 == 0b1010_0_10_1);

fmt::Debug and Default

This macro automatically creates a suitable fmt::Debug and Default implementations similar to the ones created for normal structs by #[derive(Debug, Default)]. You can disable this with the extra debug and default arguments.

use std::fmt::{Debug, Formatter, Result};
use bitfield_struct::bitfield;

#[bitfield(u64, debug = false, default = false)]
struct CustomDebug {
    data: u64
}
impl Debug for CustomDebug {
    fn fmt(&self, f: &mut Formatter<'_>) -> Result {
        write!(f, "0x{:x}", self.data())
    }
}
impl Default for CustomDebug {
    fn default() -> Self {
        Self(123)
    }
}

let val = CustomDebug::default();
println!("{val:?}")