chapa

Bitfield structs, batteries included!

chapa exposes a single attribute macro, #[bitfield], that turns an ordinary struct into newtype backed by a single primitive. Every field maps to an exact range of bits and gets a generated getter, setter, and with_* builder.

Features

• MSB0 and LSB0 support: Naturally write bit orders as per datasheet
• Enum fields: Use enums as bitfield fields with #[derive(BitEnum)]
• Nested bitfields: Embed one bitfield struct inside another
• Readonly fields: Suppress setter generation with readonly or a leading _ prefix
• Aliases: Expose extra accessor names with alias = "name" or alias = ["a", "b"]
• Overlays: Allow multiple logically distinct field groups to share the same bit range
• Bitwise operators: &, |, ^, !, &=, |=, ^= with the backing storage type work directly on the struct
• Bit extraction: extract_bits! masks a value to keep only the specified bit ranges

Installation

[dependencies]
chapa = { git = "http://github.com/ioncodes/chapa" }

Quick start

use chapa::bitfield;

// An 8-bit status register, bit 0 is the LSB
#[bitfield(u8, order = lsb0)]
#[derive(Copy, Clone, Debug, PartialEq)]
pub struct StatusReg {
    #[bits(0)] enabled: bool,
    #[bits(1..=3)] mode: u8,
    #[bits(4..=7)] _reserved: u8 // Can be ommited; "_" makes it readonly
}

let r = StatusReg::new()
    .with_enabled(true)
    .with_mode(5);

assert_eq!(r.enabled(), true);
assert_eq!(r.mode(), 5);
assert_eq!(r.reserved(), 0);    // accessible as `reserved`, not `_reserved`

#[bitfield(...)] options

#[bits(...)] options

MSB-0 example

use chapa::bitfield;

// A 32-bit value where bit 0 is the most-significant bit
#[bitfield(u32, order = msb0)]
#[derive(Copy, Clone, Debug, PartialEq)]
pub struct ControlWord {
    #[bits(0..=3)] opcode: u8,
    #[bits(4..=7)] dst: u8,
    #[bits(8..=31, readonly)] payload: u32,
}

let cw = ControlWord::new()
  .with_opcode(0xA)
  .with_dst(0x3);
assert_eq!(cw.raw(), 0xA300_0000);

Enum fields

Use #[derive(BitEnum)] on an enum to automatically implement BitField, allowing it to be used as a bitfield field type. Copy and Clone are derived automatically.

use chapa::{bitfield, BitEnum};

#[derive(Debug, PartialEq, BitEnum)]
pub enum VideoFormat {
    Ntsc = 0,
    Pal = 1,
    Mpal = 2,
    Debug = 3,
}

#[bitfield(u16, order = lsb0)]
#[derive(Copy, Clone, Debug, PartialEq)]
pub struct DisplayConfig {
    #[bits(0)] enable: bool,
    #[bits(1..=2)] fmt: VideoFormat,
}

let dc = DisplayConfig::new()
    .with_enable(true)
    .with_fmt(VideoFormat::Pal);
assert_eq!(dc.fmt(), VideoFormat::Pal);

Note: Invalid raw values map to the last variant!

Nested bitfields

A field whose type implements chapa::BitField (i.e. any type annotated with #[bitfield]) can be used as a nested field.

use chapa::bitfield;

#[bitfield(u8, order = msb0, width = 4)]
#[derive(Copy, Clone, Debug, PartialEq)]
pub struct Nibble {
    #[bits(0..=1)] high: u8,
    #[bits(2..=3)] low: u8,
}

#[bitfield(u32, order = msb0)]
#[derive(Copy, Clone, Debug, PartialEq)]
pub struct Word {
    #[bits(0..=3)] top: Nibble,
    #[bits(28..=31)] bot: u8,
}

Overlay groups

Fields in different overlay groups may share bit ranges. This is useful for instruction formats where the same bits are interpreted differently depending on other bits. This is useful for instruction decoding, but also to handle specific MMIO registers that change their meaning depending on certain encoded bits.

use chapa::bitfield;

#[bitfield(u32, order = msb0)]
#[derive(Copy, Clone, Debug, PartialEq)]
pub struct Instr {
    #[bits(0..=5)] opcode: u8,

    #[bits(6..=10,  overlay = "r_form")] rs: u8,
    #[bits(11..=15, overlay = "r_form")] ra: u8,
    #[bits(16..=20, overlay = "r_form")] rb: u8,

    #[bits(6..=10,  overlay = "i_form")] dst: u8,
    #[bits(11..=31, overlay = "i_form")] imm: u32,
}

Bitwise operations

Every bitfield struct implements BitAnd, BitOr, BitXor, Not, BitAndAssign, BitOrAssign, and BitXorAssign against its backing storage type.

use chapa::bitfield;

#[bitfield(u32, order = msb0)]
#[derive(Copy, Clone)]
pub struct Msr {
    #[bits(16, alias = "rnd1")] random1: bool,
    #[bits(17, alias = "rnd2")] random2: bool,
}

const RESTORE_MASK: u32 = 0x0000_FF73;

let srr1: u32 = 0x0000_8000;
let msr = Msr::new();

// No .raw() / from_raw() needed:
let updated = (msr & !RESTORE_MASK) | (srr1 & RESTORE_MASK);

Bit extraction

extract_bits! keeps only the specified bit positions from a value, zeroing all others. Bits can be single indices or inclusive start..=end ranges.

For raw integers, specify the ordering and type explicitly:

use chapa::extract_bits;

let val: u32 = 0xFFFF_FFFF;
// MSB0: keep bits 0, 5–9, 16–31
let masked = extract_bits!(msb0 u32; val; 0, 5..=9, 16..=31);
assert_eq!(masked, 0x87C0_FFFF);

// LSB0: keep bits 0–3 and 12–15
let masked = extract_bits!(lsb0 u16; val as u16; 0..=3, 12..=15);
assert_eq!(masked, 0xF00F);

For chapa bitfield structs, omit the ordering, it is deduced from the struct's #[bitfield] definition and the result is returned as the same struct type:

use chapa::{bitfield, extract_bits};

#[bitfield(u32, order = msb0)]
#[derive(Copy, Clone)]
pub struct Msr { /* ... */ }

let msr = Msr::from_raw(0xFFFF_FFFF);
let masked: Msr = extract_bits!(msr; 0..=0, 5..=9, 16..=31);
let srr1: u32 = masked.raw();

The explicit form (msb0 u32) emits const MASK: T = ..., so the mask is guaranteed to be computed at compile time. The struct form calls an #[inline] helper; LLVM should constant-fold the mask in practice, but there is no language-level guarantee.

Generated API

For a field foo: u8 spanning bits 4..=7 the macro generates:

Additionally, every struct implements the following traits: