Macro system 

macro_rules! system {
    (
        $(#[$quantities_attr:meta])* quantities: $quantities:ident {
            $($(#[$name_attr:meta])* $name:ident: $unit:ident, $symbol:ident;)+
        }
        $(#[$units_attr:meta])* units: $units:ident {
            $($module:ident::$quantity:ident,)+
        }
    ) => { ... };
    (
        $(#[$quantities_attr:meta])* quantities: $quantities:ident {
            $($(#[$name_attr:meta])* $name:ident: $unit:ident, $symbol:ident;)+
        }
        $(#[$units_attr:meta])* units: $units:ident {
            $(mod $module:ident::$quantity:ident,)+
        }
    ) => { ... };
}

Macro to implement a system of quantities.

• $quantities_attr: System of quantities attributes. Generally used to set documentation comments for the system of quantities.
• $quantities: Name of the system of quantities (e.g. ISQ).
• $name_attr: Base quantity attributes. Generally used to set documentation comments for base units.
• $name: Name of the base quantities for the system of quantities (e.g. length, mass, …). Note that this name must match the module name of the quantity.
• $unit: Base unit of the quantity (e.g. meter, kilogram).
• $symbol: Dimension symbol of the quantity.
• $units_attr: System of units attributes. Generally used to set documentation comments for the system of units.
• $units: Name of the system of units (e.g. SI).
• $module: Module name of the quantity. When prefixed by the mod keyword the module must already be defined with the #[macro_use] attribute. A #[macro_use] pub mod $module; statement is generated if this variable is not prefixed by the mod keyword.
• $quantity: Quantity name (e.g. Length, Mass, …).

An example invocation is given below for a meter-kilogram-second system. The #[macro_use] attribute must be used when including the uom crate to make the system! macro available.

#[macro_use]
extern crate uom;

system! {
    /// System of quantities, Q.
    quantities: Q {
        length: meter, L;
        mass: kilogram, M;
        time: second, T;
    }
    /// System of units, U.
    units: U {
        mod length::Length,
        mod mass::Mass,
        mod time::Time,
    }
}