Derive Macro BitEnum 

#[derive(BitEnum)]
{
    // Attributes available to this derive:
    #[bit_enum]
    #[catch_all]
}

Derives an enum ⇄ integer mapping of a fixed bit width.

#[derive(BitEnum, Clone, Copy, Debug, PartialEq, Eq)]
#[bit_enum(u4)]
enum RCode {
    NoError = 0,
    FormErr = 1,
    #[catch_all]
    Other(u4),   // preserves unknown values (dual-use)
}

#[bit_enum(uN)] sets the width. Exactly one #[catch_all] tuple variant (holding a uN/integer) may capture unknown discriminants. Without a catch-all the variants must cover the whole width, or the enum must be declared #[bit_enum(uN, closed)] to assert a closed set — otherwise it is a compile error, because from_bits (the infallible codec / #[bitfield]-getter path) would panic on an unknown discriminant. A closed enum still unreachable!s on that path; its checked TryFrom rejects unknowns instead.

Generated API

Always: the bnb::{Bits, BitEnum} impls (so the enum nests in a #[bitfield]). For a primitive width (u8/u16/u32/u64/u128) additionally — for num_enum parity:

• From<Enum> for uN (every variant maps to a value);
• with #[catch_all]: From<uN> for Enum (total — unknowns absorbed);
• without it: TryFrom<uN> for Enum, erroring with bnb::UnknownDiscriminant on an unknown value.

A non-primitive width (u4, or even a byte-aligned u24) gets none of these — such an enum is only meaningful nested in a #[bitfield].

See the bnb::guide::enums page for runnable examples (catch-all, closed, the num_enum parity, and nesting).