Macro instruction_dispatch 

macro_rules! instruction_dispatch {
    (
        $pid:expr, $accs:expr, $data:expr;
        $( $tag:expr => $handler:expr ),+ $(,)?
    ) => { ... };
}

Route instruction data to handler functions based on a single-byte tag.

Replaces Anchor’s #[program] proc macro. Reads byte 0 as the discriminator and dispatches to the matching handler. Returns InvalidInstructionData for unknown tags.

instruction_dispatch! {
    program_id, accounts, instruction_data;
    0 => process_init(program_id, accounts, ix),
    1 => process_deposit(program_id, accounts, ix),
    2 => process_withdraw(program_id, accounts, ix),
}

Inside each arm, ix is a SliceCursor positioned after the tag byte.