It helps compatibility when the external source
may start setting additional bits at any time. The [known and unknown bits](#known-and-unknown-bits)
section has more details on this behavior.

### Custom derives

You can derive some traits on generated flags types if you enable Cargo features. The following
libraries are currently supported:

- `serde`: Support `#[derive(Serialize, Deserialize)]`, using text for human-readable formats,
  and a raw number for binary formats.
- `arbitrary`: Support `#[derive(Arbitrary)]`, only generating flags values with known bits.
- `bytemuck`: Support `#[derive(Pod, Zeroable)]`, for casting between flags values and their
  underlying bits values.

You can also define your own flags type outside of the [`bitflags`] macro and then use it to generate methods.
This can be useful if you need a custom `#[derive]` attribute for a library that `bitflags` doesn't
natively support:

```rust
# use std::fmt::Debug as SomeTrait;
# use bitflags::bitflags;
#[derive(SomeTrait)]
pub struct Flags(u32);

bitflags! {
    impl Flags: u32 {
        const A = 0b00000001;
        const B = 0b00000010;
        const C = 0b00000100;
    }
}
```

### Adding custom methods

The [`bitflags`] macro supports attributes on generated flags types within the macro itself, while
`impl` blocks can be added outside of it:

```rust
# use bitflags::bitflags;
bitflags! {
    // Attributes can be applied to flags types
    #[repr(transparent)]
    #[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
    pub struct Flags: u32 {
        const A = 0b00000001;
        const B = 0b00000010;
        const C = 0b00000100;
    }
}

// Impl blocks can be added to flags types
impl Flags {
    pub fn as_u64(&self) -> u64 {
        self.bits() as u64
    }
}
```

## Working with flags values

Use generated constants and standard bitwise operators to interact with flags values:

```rust
# use bitflags::bitflags;
# bitflags! {
#     #[derive(Clone, Copy, Debug, PartialEq, Eq, Hash)]
#     pub struct Flags: u32 {
#         const A = 0b00000001;
#         const B = 0b00000010;
#         const C = 0b00000100;
#     }
# }
// union
let ab = Flags::A | Flags::B;

// intersection
let a = ab & Flags::A;

// difference
let b = ab - Flags::A;

// complement
let c = !ab;
```

See the docs for the [`Flags`] trait for more details on operators and how they behave.

# Formatting and parsing

`bitflags` defines a text format that can be used to convert any flags value to and from strings.

See the [`parser`] module for more details.

# Specification

The terminology and behavior of generated flags types is
[specified in the source repository](https://github.com/bitflags/bitflags/blob/main/spec.md).
Details are repeated in these docs where appropriate, but is exhaustively listed in the spec. Some
things are worth calling out explicitly here.

## Flags types, flags values, flags

The spec and these docs use consistent terminology to refer to things in the bitflags domain:

- **Bits type**: A type that defines a fixed number of bits at specific locations.
- **Flag**: A set of bits in a bits type that may have a unique name.
- **Flags type**: A set of defined flags over a specific bits type.
- **Flags value**: An instance of a flags type using its specific bits value for storage.

```
# use bitflags::bitflags;
bitflags! {
    struct FlagsType: u8 {
//                    -- Bits type
//         --------- Flags type
        const A = 1;
//            ----- Flag
    }
}

let flag = FlagsType::A;
//  ---- Flags value
```

## Known and unknown bits

Any bits in a flag you define are called _known bits_. Any other bits are _unknown bits_.
In the following flags type:

```
# use bitflags::bitflags;
bitflags! {
    struct Flags: u8 {
        const A = 1;
        const B = 1 << 1;
        const C = 1 << 2;
    }
}
```

The known bits are `0b0000_0111` and the unknown bits are `0b1111_1000`.

`bitflags` doesn't guarantee that a flags value will only ever have known bits set, but some operators
will unset any unknown bits they encounter. In a future version of `bitflags`, all operators will
unset unknown bits.

If you're using `bitflags` for flags types defined externally, such as from C, you probably want all
bits to be considered known, in case that external source changes. You can do this using an unnamed
flag, as described in [externally defined flags](#externally-defined-flags).

## Zero-bit flags

Flags with no bits set should be avoided because they interact strangely with [`Flags::contains`]
and [`Flags::intersects`]. A zero-bit flag is always contained, but is never intersected. The
names of zero-bit flags can be parsed, but are never formatted.

## Multi-bit flags

Flags that set multiple bits should be avoided unless each bit is also in a single-bit flag.
Take the following flags type as an example:

```
# use bitflags::bitflags;
bitflags! {
    struct Flags: u8 {
        const A = 1;
        const B = 1 | 1 << 1;
    }
}
```

The result of `Flags::A ^ Flags::B` is `0b0000_0010`, which doesn't correspond to either
`Flags::A` or `Flags::B` even though it's still a known bit.
