INCOMPLETE: Directly reference num sub-crates.

Resolves #57.

Cargo.toml

@@ -21,7 +21,9 @@ coveralls = { repository = "iliekturtles/uom" }
 maintenance = { status = "actively-developed" }
 
 [dependencies]
-num = "0.1"
+num-traits = { version = "0.2", default-features = false }
+num-rational = { version = "0.1", optional = true, default-features = false }
+num-bigint = { version = "0.1", optional = true, default-features = false }
 serde = { version = "1.0", optional = true, default-features = false }
 typenum = "1.9.0"
 
@@ -33,31 +35,34 @@ static_assertions = "0.2.5"
 
 [features]
 default = ["f32", "f64", "si", "std"]
-usize = []
-u8 = []
-u16 = []
-u32 = []
-u64 = []
-isize = []
-i8 = []
-i16 = []
-i32 = []
-i64 = []
-bigint = []
-biguint = []
-rational = []
-rational32 = []
-rational64 = []
-bigrational = []
+usize = ["rational-support"]
+u8 = ["rational-support"]
+u16 = ["rational-support"]
+u32 = ["rational-support"]
+u64 = ["rational-support"]
+isize = ["rational-support"]
+i8 = ["rational-support"]
+i16 = ["rational-support"]
+i32 = ["rational-support"]
+i64 = ["rational-support"]
+bigint = ["bigint-support"]
+biguint = ["bigint-support"]
+rational = ["rational-support"]
+rational32 = ["rational-support"]
+rational64 = ["rational-support"]
+bigrational = ["bigint-support"]
 f32 = []
 f64 = []
 si = []
-std = []
+std = ["num-traits/std"]
 # The `use_serde` feature exists so that, in the future, other dependency features like num/serde
 # can be added. However, num/serde is currently left out because it has not yet been updated to
 # Serde 1.0. It is also necessary to name the feature something other than `serde` because of a
 # cargo bug: https://github.com/rust-lang/cargo/issues/1286
 use_serde = ["serde"]
+# Internal features to include appropriate num-* crates.
+rational-support = ["num-rational"]
+bigint-support = ["num-bigint", "num-rational/bigint"]
 
 [[example]]
 name = "base"

src/lib.rs

@@ -168,7 +168,15 @@ compile_error!("A least one underlying storage type must be enabled. See the fea
     uom documentation for available underlying storage type options.");
 
 #[doc(hidden)]
-pub extern crate num;
+pub extern crate num_traits;
+
+#[doc(hidden)]
+#[cfg(feature = "bigint-support")]
+pub extern crate num_bigint;
+
+#[doc(hidden)]
+#[cfg(any(feature = "rational-support", feature = "bigint-support"))]
+pub extern crate num_rational;
 
 #[doc(hidden)]
 #[cfg(feature = "serde")]
@@ -210,6 +218,31 @@ pub mod lib {
     }
 }
 
+// Conditionally import num sub-crate types based on feature selection.
+#[doc(hidden)]
+pub mod num {
+    #[cfg(not(feature = "std"))]
+    pub use num_traits::float::FloatCore as Float;
+    #[cfg(feature = "std")]
+    pub use num_traits::float::Float;
+
+    pub use num_traits::{FromPrimitive, Num, One, pow, Saturating, Signed, Zero};
+
+    #[cfg(feature = "bigint-support")]
+    pub use num_bigint::{BigInt, BigUint};
+
+    #[cfg(feature = "rational-support")]
+    pub use num_rational::Rational;
+
+    #[cfg(feature = "bigint-support")]
+    pub use num_rational::BigRational;
+
+    #[cfg(any(feature = "rational-support", feature = "bigint-support"))]
+    pub mod rational {
+        pub use num_rational::*;
+    }
+}
+
 #[macro_use]
 mod storage_types;
 
@@ -288,7 +321,8 @@ storage_types! {
     impl ::ConversionFactor<V> for V {
         #[inline(always)]
         fn powi(self, e: i32) -> Self {
-            self.powi(e)
+            //<V as ::num::Float>::powi(self, e)
+            V::powi(self, e)
         }
 
         #[inline(always)]

src/si/length.rs

@@ -68,6 +68,7 @@ where
     V: ::num::Float + ::Conversion<V>,
 {
     /// Calculates the length of the hypotenuse of a right-angle triangle given the legs.
+    #[cfg(feature = "std")]
     #[inline(always)]
     pub fn hypot(self, other: Self) -> Self {
         Length {

src/system.rs

@@ -521,6 +521,7 @@ macro_rules! system {
             /// // error[E0271]: type mismatch resolving ...
             /// let r = Area::new::<square_meter>(8.0).cbrt();
             /// ```
+            #[cfg(feature = "std")]
             #[inline(always)]
             pub fn cbrt(
                 self
@@ -595,6 +596,7 @@ macro_rules! system {
             /// Fused multiply-add. Computes `(self * a) + b` with only one rounding error.
             /// This produces a more accurate result with better performance than a separate
             /// multiplication operation followed by an add.
+            #[cfg(feature = "std")]
             #[inline(always)]
             pub fn mul_add<Da, Ua, Ub>(
                 self,
@@ -682,6 +684,7 @@ macro_rules! system {
             /// // error[E0271]: type mismatch resolving ...
             /// let r = Length::new::<meter>(4.0).sqrt();
             /// ```
+            #[cfg(feature = "std")]
             #[inline(always)]
             pub fn sqrt(
                 self