value of the generated flags type.

## Examples

Generate a flags type using `u8` as the bits type:

```
# use bitflags::bitflags;
bitflags! {
    struct Flags: u8 {
        const A = 1;
        const B = 1 << 1;
        const C = 0b0000_0100;
    }
}
```

Flags types are private by default and accept standard visibility modifiers. Flags themselves
are always public:

```
# use bitflags::bitflags;
bitflags! {
    pub struct Flags: u8 {
        // Constants are always `pub`
        const A = 1;
    }
}
```

Flags may refer to other flags using their [`Flags::bits`] value:

```
# use bitflags::bitflags;
bitflags! {
    struct Flags: u8 {
        const A = 1;
        const B = 1 << 1;
        const AB = Flags::A.bits() | Flags::B.bits();
    }
}
```

A single `bitflags` invocation may include zero or more flags type declarations:

```
# use bitflags::bitflags;
bitflags! {}

bitflags! {
    struct Flags1: u8 {
        const A = 1;
    }

    struct Flags2: u8 {
        const A = 1;
    }
}
```

# `impl` mode

A declaration that begins with `impl` will only generate methods and trait implementations for the
`struct` defined outside of the `bitflags` macro.

The struct itself must be a newtype using the bits type as its field.

The syntax for `impl` mode is identical to `struct` mode besides the starting token.

## Examples

Implement flags methods and traits for a custom flags type using `u8` as its underlying bits type:

```
# use bitflags::bitflags;
struct Flags(u8);

bitflags! {
    impl Flags: u8 {
        const A = 1;
        const B = 1 << 1;
        const C = 0b0000_0100;
    }
}
```

# Named and unnamed flags

Constants in the body of a declaration are flags. The identifier of the constant is the name of
the flag. If the identifier is `_`, then the flag is unnamed. Unnamed flags don't appear in the
generated API, but affect how bits are truncated.

## Examples

Adding an unnamed flag that makes all bits known:

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

        const _ = !0;
    }
}
```

Flags types may define multiple unnamed flags:

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