Add a Kind associated type to Dimensions.

The new `Kind` associated type, defaulting to `()`, allows for multiple
quantities that have the same dimensions to be defined. Quantities of
different kind are not comparable. Resolves #78.

src/quantity.rs

@@ -11,6 +11,8 @@
 /// * `$system`: System of quantities type (e.g. `ISQ`).
 /// * `$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`, ...).
+/// * `$kind`: [Kind][kind] of the quantity. Optional. This variable should only be specified when
+///   defining a quantity that has the same dimensions as another quantity but isn't comparable.
 /// * `$unit`: Unit name (e.g. `meter`, `foot`).
 /// * `$conversion`: Conversion from the unit to the base unit of the quantity (e.g. `3.048E-1` to
 ///   convert `foot` to `meter`).
@@ -87,6 +89,7 @@
 ///
 /// [quantity]: http://jcgm.bipm.org/vim/en/1.1.html
 /// [measurement]: http://jcgm.bipm.org/vim/en/1.9.html
+/// [kind]: https://jcgm.bipm.org/vim/en/1.2.html
 #[macro_export]
 macro_rules! quantity {
     (
@@ -96,9 +99,27 @@ macro_rules! quantity {
             $($(#[$unit_attr:meta])* @$unit:ident: $conversion:expr;
                 $abbreviation:expr, $singular:expr, $plural:expr;)+
         }
+    ) => {
+        quantity! {
+            $(#[$quantity_attr])* quantity: $quantity; $description;
+            $(#[$dim_attr])* dimension: $system<$($dimension),+>;
+            kind: ();
+            units {
+                $($(#[$unit_attr])* @$unit: $conversion; $abbreviation, $singular, $plural;)+
+            }
+        }
+    };
+    (
+        $(#[$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: $conversion:expr; $abbreviation:expr,
+                $singular:expr, $plural:expr;)+
+        }
     ) => {
         $(#[$dim_attr])*
-        pub type Dimension = super::$system<$($crate::typenum::$dimension),+>;
+        pub type Dimension = super::$system<$($crate::typenum::$dimension),+, $kind>;
 
         $(#[$quantity_attr])*
         pub type $quantity<U, V> = super::Quantity<Dimension, U, V>;

src/si/thermodynamic_temperature.rs

@@ -1,5 +1,9 @@
 //! Thermodynamic temperature (base unit kelvin, K<sup>1</sup>).
 
+/// Kind of thermodynamic temperature.
+#[derive(Clone, Copy, Debug, Hash)]
+pub struct Temperature;
+
 quantity! {
     /// Thermodynamic temperature (base unit kelvin, K<sup>1</sup>).
     quantity: ThermodynamicTemperature; "thermodynamic temperature";
@@ -12,6 +16,7 @@ quantity! {
         P1,     // thermodynamic temperature
         Z0,     // amount of substance
         Z0>;    // luminous intensity
+    kind: Temperature;
     units {
         @yottakelvin: prefix!(yotta); "YK", "yottakelvin", "yottakelvins";
         @zettakelvin: prefix!(zetta); "ZK", "zettakelvin", "zettakelvins";

src/system.rs

@@ -125,6 +125,11 @@ macro_rules! system {
         pub trait Dimension: Send + Sync {
             $(/// Quantity dimension.
             type $symbol: $crate::typenum::Integer;)+
+            /// [Kind][kind] of the quantity. Quantities of the same dimension but differing kinds
+            /// are not comparable.
+            /// 
+            /// [kind]: https://jcgm.bipm.org/vim/en/1.2.html
+            type Kind;
         }
 
         /// Marker trait to identify a [system of units][units] based on a set of [base units][base]
@@ -236,7 +241,7 @@ macro_rules! system {
         }
 
         // Type alias for dimensions where all exponents of the factors are the given value.
-        type DN<N> = Dimension<$($symbol = system!(@replace $symbol N)),+>;
+        type DN<N> = Dimension<$($symbol = system!(@replace $symbol N),)+ Kind = ()>;
 
         /// Type alias for [dimension one][one] for which all the exponents of the factors
         /// corresponding to the [base quantities][base] are zero.
@@ -247,7 +252,7 @@ macro_rules! system {
         pub type DimensionOne = DN<$crate::typenum::Z0>;
 
         $(#[$quantities_attr])*
-        pub type $quantities<$($symbol),+> = Dimension<$($symbol = $symbol),+>;
+        pub type $quantities<$($symbol,)+ _K = ()> = Dimension<$($symbol = $symbol,)+ Kind = _K>;
 
         $(#[$units_attr])*
         #[allow(unused_qualifications)]