Macro quantity 

macro_rules! quantity {
    (
        $(#[$quantity_attr:meta])* quantity: $quantity:ident; $description:expr;
        $(#[$dim_attr:meta])* dimension: $system:ident<$($dimension:ident),+>;
        $(kind: $kind:ty;)?
        units {
            $($(#[$unit_attr:meta])* @$unit:ident: $coefficient:expr $(, $constant:expr)?;
                $abbreviation:expr, $singular:expr, $plural:expr;)+
        }
    ) => { ... };
}

Macro to implement a quantity and associated measurement units. Note that this macro must be executed in direct submodules of the module where the system! macro was executed.

• $quantity_attr: Quantity attributes. Generally used to set documentation comments for the quantity.
• $quantity: Quantity name (e.g. Length).
• $description: Quantity description (e.g. "length").
• $dim_attr: Dimension attributes. Generally used to set documentation comments for the quantity’s dimension type alias.
• $system: System of quantities type (e.g. ISQ).
• $dimension: Power of a factor for each base quantity in the system. Power should be represented as a typenum type-level integer (e.g. N1, Z0, P1, P2, …).
• $kind: Kind of the quantity. Optional. This variable should only be specified when defining a quantity that has the same dimensions as another quantity but isn’t comparable. When not specified crate::Kind is used.
• $unit: Unit name (e.g. meter, foot).
• $coefficient: Conversion coefficient from the unit to the base unit of the quantity (e.g. 3.048_E-1 to convert foot to meter, 1.0_E0 to convert celsius to kelvin).
• $constant: Optional conversion constant factor from the unit to the base unit of the quantity (e.g. 273.15_E0 to convert celsius to kelvin). Note that using a unit with a non-zero constant factor is not currently supported as a base unit.
• $abbreviation: Unit abbreviation (e.g. "m").
• $singular: Singular unit description (e.g. "meter").
• $plural: Plural unit description (e.g. "meters").

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

#[macro_use]
extern crate uom;

#[macro_use]
mod length {
    quantity! {
        /// Length (base unit meter, m).
        quantity: Length; "length";
        /// Length dimension, m.
        dimension: Q<P1 /*length*/, Z0 /*mass*/, Z0 /*time*/>;
        units {
            @meter: 1.0E0; "m", "meter", "meters";
            @foot: 3.048E-1; "ft", "foot", "feet";
        }
    }
}