Derive Macro Compact

#[derive(Compact)]
{
    // Attributes available to this derive:
    #[maybe_zero]
    #[reth_codecs]
}

Derives the Compact trait for custom structs, optimizing serialization with a possible bitflag struct.

Implementation:

The derived Compact implementation leverages a bitflag struct when needed to manage the presence of certain field types, primarily for compacting fields efficiently. This bitflag struct records information about fields that require a small, fixed number of bits for their encoding, such as bool, Option<T>, or other small types.

Bit Sizes for Fields:

The amount of bits used to store a field size is determined by the field’s type. For specific types, a fixed number of bits is allocated (from fn get_bit_size):

• bool, Option<T>, TransactionKind, Signature: 1 bit
• TxType: 2 bits
• u64, BlockNumber, TxNumber, ChainId, NumTransactions: 4 bits
• u128: 5 bits
• U256: 6 bits

Warning: Extending structs, unused bits and backwards compatibility:

When the bitflag only has one bit left (for example, when adding many Option<T> fields), you should introduce a new struct (e.g., TExtension) with additional fields, and use Option<TExtension> in the original struct. This approach allows further field extensions while maintaining backward compatibility.

Limitations:

• Fields not listed above, or types such Vec, or large composite types, should manage their own encoding and do not rely on the bitflag struct.
• Bytes fields and any types containing a Bytes field should be placed last to ensure efficient decoding.