bitpiece

A powerful Rust crate for working with bitfields. Define compact, type-safe bitfield structures with automatic bit packing and extraction.

Features

• Const-compatible: All operations work in const contexts
• no_std compatible: Works in embedded and bare-metal environments
• Type-safe: Strong typing prevents mixing up different bitfield types
• Flexible bit widths: Support for arbitrary bit widths from 1 to 64 bits
• Signed and unsigned: Both signed (SB*) and unsigned (B*) arbitrary-width types
• Nested bitfields: Compose complex structures from simpler bitfield types
• Enum support: Use enums as bitfield members with automatic bit width calculation
• Zero-cost abstractions: Compiles down to efficient bit manipulation operations

Quick Start

Add to your Cargo.toml:

[dependencies]
bitpiece = "1.0"

Basic usage:

use bitpiece::*;

// Define a 2-bit enum
#[bitpiece(2, all)]
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
enum Priority {
    Low = 0,
    Medium = 1,
    High = 2,
    Critical = 3,
}

// Define an 8-bit struct containing multiple fields
#[bitpiece(8, all)]
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
struct StatusByte {
    enabled: bool,      // 1 bit
    priority: Priority, // 2 bits
    count: B5,          // 5 bits (unsigned, 0-31)
}

fn main() {
    // Create from raw bits
    let status = StatusByte::from_bits(0b10101_01_1);
    
    assert_eq!(status.enabled(), true);
    assert_eq!(status.priority(), Priority::Medium);
    assert_eq!(status.count(), B5::new(21));
    
    // Modify fields
    let updated = status
        .with_priority(Priority::Critical)
        .with_count(B5::new(7));
    
    assert_eq!(updated.to_bits(), 0b00111_11_1);
}

Table of Contents

• The #[bitpiece] Attribute
• Built-in Types
• Defining Bitfield Structs
• Defining Bitfield Enums
• Generated Methods and Types
• Opt-in Features
• Working with Fields
• Nested Bitfields
• Signed Types
• Const Context Usage
• The BitPiece Trait
• Error Handling

The #[bitpiece] Attribute

The #[bitpiece] attribute macro is the main entry point for defining bitfield types. It can be applied to structs and enums.

Syntax

#[bitpiece]                    // Auto-calculate bit length, basic features
#[bitpiece(all)]               // Auto-calculate bit length, all features
#[bitpiece(32)]                // Explicit 32-bit length, basic features
#[bitpiece(32, all)]           // Explicit 32-bit length, all features
#[bitpiece(get, set)]          // Auto-calculate, specific features only
#[bitpiece(16, get, set, with)] // Explicit length with specific features

Arguments

1. Bit length (optional): An integer specifying the exact bit length. If omitted, the bit length is calculated automatically from the fields (for structs) or variant values (for enums).

2. Feature flags (optional): Control which methods and types are generated. See Opt-in Features for details.

Built-in Types

Unsigned Arbitrary-Width Types (B1 - B64)

Types for unsigned integers of specific bit widths:

use bitpiece::*;

let three_bits: B3 = B3::new(0b101);  // 3-bit value (0-7)
let five_bits: B5 = B5::new(31);       // 5-bit value (0-31)

assert_eq!(three_bits.get(), 5);
assert_eq!(B3::MAX.get(), 7);

// Validation
assert!(B3::try_new(7).is_some());   // Valid: fits in 3 bits
assert!(B3::try_new(8).is_none());   // Invalid: requires 4 bits

Signed Arbitrary-Width Types (SB1 - SB64)

Types for signed integers of specific bit widths using two's complement:

use bitpiece::*;

let signed: SB5 = SB5::new(-10);  // 5-bit signed value (-16 to 15)

assert_eq!(signed.get(), -10);
assert_eq!(SB5::MIN.get(), -16);
assert_eq!(SB5::MAX.get(), 15);

// Validation
assert!(SB3::try_new(3).is_some());   // Valid: fits in 3 bits
assert!(SB3::try_new(-4).is_some());  // Valid: minimum for SB3
assert!(SB3::try_new(4).is_none());   // Invalid: too large
assert!(SB3::try_new(-5).is_none());  // Invalid: too small

Standard Integer Types

All standard Rust integer types implement BitPiece:

• Unsigned: u8, u16, u32, u64
• Signed: i8, i16, i32, i64

use bitpiece::*;

#[bitpiece(48, all)]
struct MixedTypes {
    byte: u8,      // 8 bits
    word: u16,     // 16 bits
    flags: B8,     // 8 bits
    signed: i16,   // 16 bits
}

Boolean Type

bool is a 1-bit type:

use bitpiece::*;

#[bitpiece(3, all)]
struct Flags {
    read: bool,    // 1 bit
    write: bool,   // 1 bit
    execute: bool, // 1 bit
}

Defining Bitfield Structs

Structs are the primary way to define composite bitfields. Fields are packed in order from least significant bit (LSB) to most significant bit (MSB).

use bitpiece::*;

#[bitpiece(16, all)]
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
struct Instruction {
    opcode: B4,    // Bits 0-3 (LSB)
    reg_a: B3,     // Bits 4-6
    reg_b: B3,     // Bits 7-9
    immediate: B6, // Bits 10-15 (MSB)
}

// Bit layout:
// [immediate: 6 bits][reg_b: 3 bits][reg_a: 3 bits][opcode: 4 bits]
// MSB                                                          LSB

Field Ordering

Fields are packed starting from bit 0:

#[bitpiece(8, all)]
struct Example {
    a: B2,  // Bits 0-1
    b: B3,  // Bits 2-4
    c: B3,  // Bits 5-7
}

let val = Example::from_bits(0b111_010_01);
assert_eq!(val.a(), B2::new(0b01));
assert_eq!(val.b(), B3::new(0b010));
assert_eq!(val.c(), B3::new(0b111));

Defining Bitfield Enums

Enums can be used as bitfield types. The bit width is automatically calculated from the variant values, or can be specified explicitly.

Exhaustive Enums

When all possible bit patterns map to valid variants:

use bitpiece::*;

#[bitpiece(2, all)]  // 2 bits = 4 possible values
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
enum Direction {
    North = 0,
    East = 1,
    South = 2,
    West = 3,
}

// All 2-bit values (0-3) are valid
let dir = Direction::from_bits(2);
assert_eq!(dir, Direction::South);

Non-Exhaustive Enums

When not all bit patterns are valid variants:

use bitpiece::*;

#[bitpiece(all)]  // Auto-calculated: 7 bits needed for value 100
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
enum ErrorCode {
    Success = 0,
    NotFound = 10,
    PermissionDenied = 50,
    InternalError = 100,
}

// Valid variant
assert_eq!(ErrorCode::from_bits(10), ErrorCode::NotFound);

// Invalid bit pattern panics in from_bits
// Use try_from_bits for safe conversion
assert!(ErrorCode::try_from_bits(25).is_none());
assert!(ErrorCode::try_from_bits(50).is_some());

Explicit Bit Length for Enums

You can specify a larger bit length than required:

use bitpiece::*;

#[bitpiece(16, all)]  // Use 16 bits even though values fit in fewer
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
enum Command {
    Nop = 0,
    Load = 1,
    Store = 2,
}

// Can accept 16-bit values
assert!(Command::try_from_bits(1000).is_none());

Generated Methods and Types

When you apply #[bitpiece] to a struct, several methods and types are generated.

Generated Constants

#[bitpiece(16, all)]
struct MyStruct { /* ... */ }

// Generated constants:
const MY_STRUCT_BIT_LEN: usize = 16;
type MyStructStorageTy = u16;  // Smallest type that fits

Field Constants

For each field, offset and length constants are generated:

#[bitpiece(8, all)]
struct Example {
    a: B3,
    b: B5,
}

// Generated:
// Example::A_OFFSET = 0
// Example::A_LEN = 3
// Example::B_OFFSET = 3
// Example::B_LEN = 5

Core Methods

impl MyStruct {
    // Create from raw bits (panics if invalid for non-exhaustive types)
    pub const fn from_bits(bits: StorageTy) -> Self;
    
    // Try to create from raw bits (returns None if invalid)
    pub const fn try_from_bits(bits: StorageTy) -> Option<Self>;
    
    // Convert to raw bits
    pub const fn to_bits(self) -> StorageTy;
}

Associated Constants

impl BitPiece for MyStruct {
    const BITS: usize;   // Total bit length
    const ZEROES: Self;  // All bits set to 0 (for structs: each field's ZEROES)
    const ONES: Self;    // All bits set to 1 (for structs: each field's ONES)
    const MIN: Self;     // The minimum value (for structs: each field's MIN)
    const MAX: Self;     // The maximum value (for structs: each field's MAX)
}

Important distinction between ONES/ZEROES and MAX/MIN:

• ZEROES: All bits are 0. For unsigned types, this equals MIN. For signed types, this is 0 (not the minimum).
• ONES: All bits are 1. For unsigned types, this equals MAX. For signed types like i8, this represents -1 (not the maximum).
• MIN: The minimum representable value. For i8, this is -128.
• MAX: The maximum representable value. For i8, this is 127.

use bitpiece::*;

// For unsigned types: ZEROES == MIN, ONES == MAX
assert_eq!(B8::ZEROES.get(), 0);
assert_eq!(B8::ONES.get(), 255);
assert_eq!(B8::MIN.get(), 0);
assert_eq!(B8::MAX.get(), 255);

// For signed types: ONES != MAX, ZEROES != MIN
assert_eq!(SB8::ZEROES.get(), 0);    // All bits 0 = 0
assert_eq!(SB8::ONES.get(), -1);     // All bits 1 = -1 in two's complement
assert_eq!(SB8::MIN.get(), -128);    // Minimum value
assert_eq!(SB8::MAX.get(), 127);     // Maximum value

Non-exhaustive enums: For enums where not all bit patterns are valid variants, ZEROES and ONES represent the closest valid variant to the all-zeros or all-ones bit pattern (i.e., MIN and MAX respectively). If an enum has no variant with value 0, ZEROES will be the variant with the smallest value, not a value with all bits set to zero.

#[bitpiece(all)]
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
enum Sparse {
    A = 10,
    B = 50,
    C = 100,
}

// No variant has value 0, so ZEROES is the minimum variant
assert_eq!(Sparse::ZEROES, Sparse::A);  // Value 10, not 0
assert_eq!(Sparse::MIN, Sparse::A);
assert_eq!(Sparse::ONES, Sparse::C);    // Maximum variant
assert_eq!(Sparse::MAX, Sparse::C);

Opt-in Features

Control which methods and types are generated using feature flags.

Feature Flags

Flag	Description
get	Field getter methods: field_name()
set	Field setter methods: set_field_name(value)
with	Builder-style methods: with_field_name(value)
get_noshift	Raw bit access: field_name_noshift()
get_mut	Mutable field references: field_name_mut()
const_eq	Const equality comparison
fields_struct	Generate TypeNameFields struct
mut_struct	Generate TypeNameMutRef type
mut_struct_field_get	Getter methods on MutRef
mut_struct_field_set	Setter methods on MutRef
mut_struct_field_get_noshift	Noshift getters on MutRef
mut_struct_field_mut	Nested mutable references on MutRef

Presets

Preset	Includes
basic	get, set, with (default if no flags specified)
all	All features
mut_struct_all	All mut_struct* features

Examples

// Only getters
#[bitpiece(8, get)]
struct ReadOnly { /* ... */ }

// Getters and setters, no builder pattern
#[bitpiece(8, get, set)]
struct Mutable { /* ... */ }

// Everything
#[bitpiece(8, all)]
struct Full { /* ... */ }

// Custom combination
#[bitpiece(8, get, with, fields_struct)]
struct Custom { /* ... */ }

Working with Fields

Getting Field Values

#[bitpiece(8, all)]
struct Packet {
    version: B2,
    flags: B3,
    length: B3,
}

let packet = Packet::from_bits(0b101_110_01);

// Get individual fields
let version = packet.version();  // B2
let flags = packet.flags();      // B3
let length = packet.length();    // B3

assert_eq!(version.get(), 1);
assert_eq!(flags.get(), 6);
assert_eq!(length.get(), 5);

Setting Field Values (Immutable)

The with_* methods return a new instance with the field modified:

let packet = Packet::ZEROES;

// Chain modifications
let updated = packet
    .with_version(B2::new(2))
    .with_flags(B3::new(7))
    .with_length(B3::new(4));

// Original unchanged
assert_eq!(packet.to_bits(), 0);

Setting Field Values (Mutable)

The set_* methods modify the instance in place:

let mut packet = Packet::ZEROES;

packet.set_version(B2::new(2));
packet.set_flags(B3::new(7));
packet.set_length(B3::new(4));

Raw Bit Access (Noshift)

Get field bits at their original position without shifting:

#[bitpiece(8, all)]
struct Example {
    a: B3,  // Bits 0-2
    b: B5,  // Bits 3-7
}

let val = Example::from_bits(0b11111_010);

// Normal getter: shifts to bit 0
assert_eq!(val.b().get(), 0b11111);

// Noshift: keeps original position
assert_eq!(val.b_noshift(), 0b11111_000);

Mutable References

Get a mutable reference to a field within the bitfield:

#[bitpiece(8, all)]
struct Container {
    inner: B4,
    outer: B4,
}

let mut container = Container::ZEROES;
{
    let mut inner_ref = container.inner_mut();
    inner_ref.set(B4::new(15));
}
assert_eq!(container.inner().get(), 15);

Nested Bitfields

Bitfield types can be nested within other bitfields:

use bitpiece::*;

#[bitpiece(4, all)]
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
struct Inner {
    x: B2,
    y: B2,
}

#[bitpiece(12, all)]
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
struct Outer {
    a: Inner,    // 4 bits
    b: Inner,    // 4 bits
    c: B4,       // 4 bits
}

let outer = Outer::from_bits(0b1010_0110_0011);

// Access nested fields
assert_eq!(outer.a().x(), B2::new(3));
assert_eq!(outer.a().y(), B2::new(0));
assert_eq!(outer.b().x(), B2::new(2));
assert_eq!(outer.b().y(), B2::new(1));
assert_eq!(outer.c(), B4::new(10));

Deep Nesting

#[bitpiece(8, all)]
struct Level1 {
    data: B4,
    flags: B4,
}

#[bitpiece(16, all)]
struct Level2 {
    l1_a: Level1,
    l1_b: Level1,
}

#[bitpiece(32, all)]
struct Level3 {
    l2: Level2,
    extra: u16,
}

let l3 = Level3::from_bits(0x12345678);
let nested_data = l3.l2().l1_a().data();

Signed Types

Using Standard Signed Integers

#[bitpiece(24, all)]
struct SignedExample {
    small: i8,   // 8-bit signed
    large: i16,  // 16-bit signed
}

let val = SignedExample::from_fields(SignedExampleFields {
    small: -50,
    large: -1000,
});

assert_eq!(val.small(), -50i8);
assert_eq!(val.large(), -1000i16);

Using Arbitrary-Width Signed Types

#[bitpiece(16, all)]
struct CustomSigned {
    a: SB5,   // 5-bit signed (-16 to 15)
    b: bool,
    c: SB7,   // 7-bit signed (-64 to 63)
    d: B3,
}

let val = CustomSigned::from_bits(0b101_0101010_1_11111);

assert_eq!(val.a(), SB5::new(-1));
assert_eq!(val.b(), true);
assert_eq!(val.c(), SB7::new(42));
assert_eq!(val.d(), B3::new(5));

Const Context Usage

All operations work in const contexts:

use bitpiece::*;

#[bitpiece(8, all)]
struct Config {
    mode: B2,
    speed: B3,
    enabled: bool,
    reserved: B2,
}

// Const construction
const DEFAULT_CONFIG: Config = Config::from_bits(0b00_1_101_01);

// Const field access
const DEFAULT_MODE: B2 = DEFAULT_CONFIG.mode();
const DEFAULT_SPEED: B3 = DEFAULT_CONFIG.speed();
const IS_ENABLED: bool = DEFAULT_CONFIG.enabled();

// Const modification
const DISABLED_CONFIG: Config = DEFAULT_CONFIG.with_enabled(false);

// Const assertions
const _: () = assert!(DEFAULT_MODE.get() == 1);
const _: () = assert!(DEFAULT_SPEED.get() == 5);
const _: () = assert!(IS_ENABLED == true);

Const Functions

const fn create_packet(version: u8, flags: u8) -> Packet {
    Packet::ZEROES
        .with_version(B2::new(version))
        .with_flags(B3::new(flags))
}

const PACKET: Packet = create_packet(2, 5);

The BitPiece Trait

All bitfield types implement the BitPiece trait:

pub trait BitPiece: Copy {
    /// The length in bits of this type
    const BITS: usize;
    
    /// A value with all bits set to 0 (see note below for enums)
    const ZEROES: Self;
    
    /// A value with all bits set to 1 (see note below for enums)
    const ONES: Self;
    
    /// The minimum representable value
    const MIN: Self;
    
    /// The maximum representable value
    const MAX: Self;
    
    /// The storage type used internally
    type Bits: BitStorage;
    
    /// Try to create from raw bits
    fn try_from_bits(bits: Self::Bits) -> Option<Self>;
    
    /// Create from raw bits (may panic)
    fn from_bits(bits: Self::Bits) -> Self;
    
    /// Convert to raw bits
    fn to_bits(self) -> Self::Bits;
}

Using the Trait Generically

use bitpiece::*;

fn print_bitpiece_info<T: BitPiece + core::fmt::Debug>(value: T) {
    println!("Bits: {}", T::BITS);
    println!("Value: {:?}", value);
    println!("Raw: {:?}", value.to_bits());
}

Error Handling

Safe Conversion with try_from_bits

use bitpiece::*;

#[bitpiece(all)]
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
enum Status {
    Ok = 0,
    Error = 1,
    Pending = 2,
}

// Safe conversion
match Status::try_from_bits(1) {
    Some(status) => println!("Status: {:?}", status),
    None => println!("Invalid status code"),
}

// For exhaustive enums, try_from_bits still validates range
assert!(Status::try_from_bits(3).is_none());

Validation for B* and SB* Types

// B types validate that value fits in bit width
assert!(B4::try_new(15).is_some());  // Max for 4 bits
assert!(B4::try_new(16).is_none());  // Too large

// SB types validate signed range
assert!(SB4::try_new(7).is_some());   // Max for 4-bit signed
assert!(SB4::try_new(-8).is_some());  // Min for 4-bit signed
assert!(SB4::try_new(8).is_none());   // Too large
assert!(SB4::try_new(-9).is_none());  // Too small

Panicking Constructors

The new and from_bits methods panic on invalid input:

// These will panic:
// let _ = B3::new(8);           // Value doesn't fit
// let _ = Status::from_bits(5); // Invalid variant

Fields Struct

When fields_struct is enabled, a companion struct is generated for convenient construction:

#[bitpiece(8, all)]
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
struct Packet {
    version: B2,
    flags: B3,
    length: B3,
}

// Generated: PacketFields struct
let fields = PacketFields {
    version: B2::new(1),
    flags: B3::new(5),
    length: B3::new(7),
};

let packet = Packet::from_fields(fields);

// Convert back to fields
let extracted: PacketFields = packet.to_fields();
assert_eq!(fields, extracted);

// From/Into implementations
let packet2: Packet = fields.into();
let fields2: PacketFields = packet2.into();

Nested Fields

For nested bitfields, the fields struct uses the direct bitfield type (not its *Fields type):

#[bitpiece(4, all)]
struct Inner {
    x: B2,
    y: B2,
}

#[bitpiece(8, all)]
struct Outer {
    a: Inner,
    b: B4,
}

// OuterFields uses Inner directly for field 'a'
let fields = OuterFields {
    a: Inner::from_bits(0b1001),  // or use InnerFields and convert
    b: B4::new(15),
};

let outer = Outer::from_fields(fields);

// You can also construct the inner type from its fields and convert:
let fields2 = OuterFields {
    a: InnerFields {
        x: B2::new(1),
        y: B2::new(2),
    }.into(),  // Convert InnerFields to Inner
    b: B4::new(15),
};

Storage Types

The crate automatically selects the smallest storage type that fits the bit length:

Bit Length	Storage Type
1-8	u8
9-16	u16
17-32	u32
33-64	u64

Access the storage directly:

#[bitpiece(12, all)]
struct Example {
    a: B6,
    b: B6,
}

let val = Example::from_bits(0xABC);

// Direct storage access
assert_eq!(val.storage, 0xABC);

// Storage type is u16 for 12 bits
let storage: u16 = val.storage;

License

MIT License - see LICENSE for details.