Attribute Macro bitfield 

#[bitfield]

Packs the annotated struct’s fields into a single backing integer.

#[bitfield(u16, bits = msb, bytes = big)]
#[derive(Clone, Copy, Debug, PartialEq, Eq)]
struct State {
    opcode: u5,    // first field -> high bits (msb)
    flags:  Flags, // a nested bitfield
    rcode:  RCode, // a BitEnum; last field -> low bits
}

Attribute arguments

• backing (first, required): the storage primitive — u8, u16, u32, u64, or u128. Must be at least as wide as the fields.
• bits = msb | lsb (default msb): whether the first declared field lands in the high or low bits.
• bytes = big | little (default big): byte order of the backing integer when serialized.

Field widths

A field’s width is, in order of precedence: an explicit #[bits(N)]; an explicit #[bits(A..=B)] range (which also fixes its absolute offset); or, by default, <FieldType as bnb::Bits>::BITS. Use widths/inference for automatic layout, or ranges on every field for fully manual layout — the two styles cannot be mixed in one struct.

Generated API

new() (all-zero; derive Default yourself if you want it), with_<field>/set_<field>, <field>() getters, to_raw()/from_raw(), to_be_bytes()/to_le_bytes()/ from_be_bytes()/from_le_bytes(), and bnb::{Bits, Bitfield} impls.

Every accessor is a const fn (#[view] accessors too, when their raw type is annotated — the view’s const argument asserts it, dynamic opts out; see bnb::guide::bitfields. A custom field type is implemented with bnb::impl_bits! — see bnb::Bits).

Layout

The emitted struct is #[repr(transparent)] over its backing integer: its size, alignment, and ABI are exactly the backing type’s. (transparent, not repr(C), because the struct wraps a single native integer — that is the honest guarantee; before 0.3.2 the layout was formally unspecified.) Writing your own #[repr(...)] on the struct suppresses the generated one — e.g. repr(C), or repr(align(N)), neither of which can combine with transparent.

See the bnb::guide::bitfields page for runnable examples (bit/byte order, inferred vs. ranged widths, nesting).