bitflags generates flags enums with well-defined semantics and ergonomic end-user APIs.

You can use bitflags to:

• provide more user-friendly bindings to C APIs where flags may or may not be fully known in advance.
• generate efficient options types with string parsing and formatting support.

You can't use bitflags to:

• guarantee only bits corresponding to defined flags will ever be set. bitflags allows access to the underlying bits type so arbitrary bits may be set.

• define bitfields. bitflags only generates types where set bits denote the presence of some combination of flags.

• Documentation

• Specification

• Release notes

Add this to your Cargo.toml:

[dependencies]
bitflags = "2.13.2"

and this to your source code:

use bitflags::bitflags;

Generate a flags structure:

use bitflags::bitflags;

// The `bitflags!` macro generates `struct`s that manage a set of flags.
bitflags! {
    /// Represents a set of flags.
    #[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash)]
    struct Flags: u32 {
        /// The value `A`, at bit position `0`.
        const A = 0b00000001;
        /// The value `B`, at bit position `1`.
        const B = 0b00000010;
        /// The value `C`, at bit position `2`.
        const C = 0b00000100;

        /// The combination of `A`, `B`, and `C`.
        const ABC = Self::A.bits() | Self::B.bits() | Self::C.bits();
    }
}

fn main() {
    let e1 = Flags::A | Flags::C;
    let e2 = Flags::B | Flags::C;
    assert_eq!((e1 | e2), Flags::ABC);   // union
    assert_eq!((e1 & e2), Flags::C);     // intersection
    assert_eq!((e1 - e2), Flags::A);     // set difference
    assert_eq!(!e2, Flags::A);           // set complement
}

The bitflags library defines a few Cargo features that you can opt-in to:

• std: Implement the Error trait on error types used by bitflags.
• serde: Support deriving serde traits on generated flags types.
• arbitrary: Support deriving arbitrary traits on generated flags types.
• bytemuck: Support deriving bytemuck traits on generated flags types.

Also see bitflags_derive for other flags-aware traits.

The minimum supported Rust version is documented in the Cargo.toml file. This may be bumped in minor releases as necessary.