Attribute Macro bin 

#[bin]

#[bin] — the unified whole-message bit codec. One attribute that folds the read codec (BitDecode), the write codec (BitEncode), and a required-by-default builder (BitsBuilder) over a struct of bnb::Bits fields, read and written at arbitrary bit offsets.

#[bin(big, validate = Frame::check)]
#[derive(Debug, PartialEq)]
struct Frame {
    version: u4,
    #[builder(default)] flags: u4,
    #[br(temp)] #[bw(calc = self.payload.len() as u16)] len: u16,
    #[br(count = len)] payload: Vec<u8>,
}
// -> Frame::{decode, peek, decode_exact, decode_from},
//    Frame::{encode, to_bytes, encode_into}, and Frame::builder()

Struct-level options

big / little (byte order), bit_order = msb|lsb, magic = <expr> (a leading constant verified on read / emitted on write), read_only / write_only (directional), no_builder, forward_only (bound decoding to a forward Source), ctx(name: Ty, …) (context from the parent), and validate = <path> (a soundness check run by build() — the parser stays permissive).

Field directives

#[br]/#[bw]: count, ctx { … }, temp + calc, if(…), map/try_map (+ the inverse bw(map)), parse_with/write_with, pad_before/after, align_before/after, seek = <bits>, restore_position, dbg (trace a field as it decodes); #[brw(ignore)] (neither read nor written); plus #[reserved] / #[reserved_with(…)].

On an enum — tagged-union dispatch

#[bin] also applies to an enum, dispatching on two orthogonal concepts: a magic wire constant (a byte string or width-suffixed unsigned int, read and written — the discriminant, or a verified signature on a tag-variant) and a tag selector taken from ctx (read-only, never on the wire). With per-variant magics the enum reads the discriminant and matches (a single == for uniform widths, or a peek-and-starts_with for variable-width byte strings). With #[bin(tag = <ctx-param>)] and variant #[bin(tag = V)] it dispatches on the selector and writes no discriminant; the two compose (and may even be mixed in one enum — tag priority, then magic). The “nothing matched” tail is a #[catch_all] (preserving the unknown discriminant) or a typed no-tag/no-magic fallback variant, not both; without either a magic enum is a closed set. An optional enum-level magic is a leading prefix. Variants may be unit, tuple, named, or #[nested]. The codec also generates decode_as_<variant>, peek_variant/<Name>Kind, decode_tagged, and a magic()/tag() accessor where the discriminant is single-valued. See the bnb::guide::dispatch page.

On a struct, #[bin] lowers to #[derive(BitDecode, BitEncode, BitsBuilder)], which stay usable directly. See the bnb::guide::bin_codec page for a full walkthrough and bnb::guide::directives for one runnable example per directive.