init code

4.Variable Bindings/4.3 Declare first/Cargo.lock

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

4.Variable Bindings/4.3 Declare first/Cargo.toml

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

4.Variable Bindings/4.3 Declare first/src/main.rs

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+//!
+//! 变量的定义和初始化通常是同时进行的。
+//! Rust 允许把两个操作分开，先定义，后初始化
+//! 使用变量的前提条件是变量必须被初始化了以后才能使用，否则在编译阶段就会报错
+//!
+//! 因为使用未初始化的变量通常会导致无法预知的行为，所以不允许使用未初始化的变量
+//!
+fn main() {
+    // 声明一个变量 但不对其进行初始化
+    let a_binding;
+
+    {
+        let x = 2;
+
+        // 初始化变量值
+        a_binding = x * x;
+    }
+
+    println!("a binding: {}", a_binding);
+
+    let another_binding;
+
+    // 报错! 使用了未初始化的变量
+    println!("another binding: {}", another_binding);
+    // FIXME ^ 注释这行代码
+
+    another_binding = 1;
+
+    println!("another binding: {}", another_binding);
+}