Add num-traits to Cargo.toml

The `num-derive` requires `num-traits` to be explicitly included in
Cargo.toml.  (cf. rust-num/num-derive#34)
This issue may be fixed in
rust-num/num-derive#35.

Author: statiolake

Cargo.lock

@@ -19,6 +19,8 @@ default = ["jemalloc"]
 # 多バイト長整数、分数、複素数など
 num = "=0.2.0"
 num-derive = "=0.3.0"
+# num-derive が依存するため必要
+num-traits = "=0.2.9"
 
 # 多次元配列
 ndarray = "=0.13.0"

Cargo.toml

@@ -0,0 +1,23 @@
+use num_derive::{FromPrimitive, Num, NumCast, NumOps, One, ToPrimitive, Zero};
+
+#[derive(
+    Debug,
+    Eq,
+    PartialEq,
+    PartialOrd,
+    Ord,
+    FromPrimitive,
+    ToPrimitive,
+    One,
+    Zero,
+    Num,
+    NumCast,
+    NumOps,
+)]
+struct Weight(i32);
+
+fn main() {
+    let w1 = Weight(3);
+    let w2 = Weight(4);
+    println!("{:?}", w1 + w2); // => "Weight(7)"
+}