add code

14.Generics/14.7 newtype idiom/Cargo.lock

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+# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
+version = 3
+
+[[package]]
+name = "new_type_idiom"
+version = "0.1.0"

14.Generics/14.7 newtype idiom/Cargo.toml

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+[package]
+name = "new_type_idiom"
+version = "0.1.0"
+edition = "2021"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]

14.Generics/14.7 newtype idiom/src/main.rs

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+//!
+//! 新类型 （newtype）
+//!
+//! 新类型是指使用元组结构体包装一个其他的类型，然后基于新包装的类型实现类型的扩展或约束，
+//! 这样的好处是可以让 `api` 更加清晰，不会造成歧义。
+//!
+
+struct Years(i64);
+
+struct Days(i64);
+
+impl Years {
+    pub fn to_days(&self) -> Days {
+        Days(self.0 * 365)
+    }
+}
+
+impl Days {
+    /// 舍弃不足的年份
+    pub fn to_years(&self) -> Years {
+        Years(self.0 / 365)
+    }
+}
+
+fn old_enough(age: &Years) -> bool {
+    age.0 >= 18
+}
+
+fn main() {
+    let age = Years(5);
+    let age_days = age.to_days();
+    println!("Old enough {}", old_enough(&age));
+    println!("Old enough {}", old_enough(&age_days.to_years()));
+    // println!("Old enough {}", old_enough(&age_days));
+
+    {
+        let years = Years(42);
+        let years_as_primitive_1: i64 = years.0; // 访问元组数据
+        let Years(years_as_primitive_2) = years; // 解构元组数据
+    }
+}