C2Rust-Bitfields enables you to write structs containing bitfields. It has three primary goals:

• Byte compatibility with equivalent C bitfield structs
• The ability to take references/pointers to non bitfield fields
• Provide methods to read from and write to bitfields

We currently provides a single custom derive, BitfieldStruct, as well as a dependent field attribute bitfield.

• Rust 1.30+
• Rust Stable, Beta, or Nightly
• Little Endian Architecture

Suppose you want to write a super compact date struct which only takes up three bytes. In C this would look like this:

struct date {
    unsigned char day: 5;
    unsigned char month: 4;
    unsigned short year: 15;
} __attribute__((packed));

Clang helpfully provides us with this information:

*** Dumping AST Record Layout
         0 | struct date
     0:0-4 |   unsigned char day
     0:5-8 |   unsigned char month
    1:1-15 |   unsigned short year
           | [sizeof=3, align=1]

And this is enough to build our rust struct:

extern crate libc;

#[repr(C, align(1))]
#[derive(BitfieldStruct)]
struct Date {
    #[bitfield(name = "day", ty = "libc::c_uchar", bits = "0..=4")]
    #[bitfield(name = "month", ty = "libc::c_uchar", bits = "5..=8")]
    #[bitfield(name = "year", ty = "libc::c_ushort", bits = "9..=23")]
    day_month_year: [u8; 3]
}

fn main() {
    let mut date = Date {
        day_month_year: [0; 3]
    };

    date.set_day(18);
    date.set_month(7);
    date.set_year(2000);

    assert_eq!(date.day(), 18);
    assert_eq!(date.month(), 7);
    assert_eq!(date.year(), 2000);
}

Furthermore, C bitfield rules for overflow and signed integers are taken into account.

This crate can generate no_std compatible code when the no_std feature flag is provided.

Since rust doesn't support a build.rs exclusively for tests, you must manually compile the c test code and link it in.

$ clang tests/bitfields.c -c -fPIC -o tests/bitfields.o
$ ar -rc tests/libtest.a tests/bitfields.o
$ RUSTFLAGS="-L `pwd`/tests" cargo test

This crate is inspired by the rust-bitfield, packed_struct, and bindgen crates.