Macro products 

macro_rules! products {
    ($d:tt $( $(#[$doc:meta])* $variant:ident($ty:ty) => $name:literal, )+) => { ... };
}

Declares the built-in products once. From one list it derives the enum, the runtime name AnyInstrument::product_name returns, the AnyInstrument::PRODUCT_NAMES a caller resolves against, and dispatch!, which writes the forwarding match every per-product method delegates through.

Hand-kept statements of the same list are how the list drifts: five products once reached master priceable but unnameable, and every new product touched four pricer matches that said nothing but “ask the product”. Generated from one list, a new built-in product is a line here, its own impls, and its binding. The leading $ is passed in so the generated dispatch! can have metavariables of its own on stable Rust.