From 2c084ad428b1724eac2c1efe422ba62c15c9fff5 Mon Sep 17 00:00:00 2001
From: KaiserY <yueyang.fanqiang@gmail.com>
Date: Tue, 13 Aug 2019 09:50:26 +0800
Subject: [PATCH] update ch04-02 close #325

---
 src/ch04-02-references-and-borrowing.md | 59 +++++++++++++++++--------
 1 file changed, 40 insertions(+), 19 deletions(-)

diff --git a/src/ch04-02-references-and-borrowing.md b/src/ch04-02-references-and-borrowing.md
index d0be48c..f79947c 100644
--- a/src/ch04-02-references-and-borrowing.md
+++ b/src/ch04-02-references-and-borrowing.md
@@ -2,7 +2,7 @@
 
 > [ch04-02-references-and-borrowing.md](https://github.com/rust-lang/book/blob/master/src/ch04-02-references-and-borrowing.md)
 > <br>
-> commit a86c1d315789b3ca13b20d50ad5005c62bdd9e37
+> commit 6c0d83499c8e77e06a71d28c5e1adccec278d4f3
 
 示例 4-5 中的元组代码有这样一个问题：我们必须将 `String` 返回给调用函数，以便在调用 `calculate_length` 后仍能使用 `String`，因为 `String` 被移动到了 `calculate_length` 内。
 
@@ -130,14 +130,15 @@ println!("{}, {}", r1, r2);
 
 ```text
 error[E0499]: cannot borrow `s` as mutable more than once at a time
- --> src/main.rs:5:10
+ --> src/main.rs:5:14
   |
-4 | let r1 = &mut s;
-  |          ------ first mutable borrow occurs here
-5 | let r2 = &mut s;
-  |          ^^^^^^ second mutable borrow occurs here
-6 | println!("{}, {}", r1, r2);
-  |                    -- borrow later used here
+4 |     let r1 = &mut s;
+  |              ------ first mutable borrow occurs here
+5 |     let r2 = &mut s;
+  |              ^^^^^^ second mutable borrow occurs here
+6 |
+7 |     println!("{}, {}", r1, r2);
+  |                        -- first borrow later used here
 ```
 
 这个限制允许可变性，不过是以一种受限制的方式允许。新 Rustacean 们经常与此作斗争，因为大部分语言中变量任何时候都是可变的。
@@ -168,9 +169,9 @@ let r2 = &mut s;
 ```rust,ignore,does_not_compile
 let mut s = String::from("hello");
 
-let r1 = &s; // no problem
-let r2 = &s; // no problem
-let r3 = &mut s; // BIG PROBLEM
+let r1 = &s; // 没问题
+let r2 = &s; // 没问题
+let r3 = &mut s; // 大问题
 
 println!("{}, {}, and {}", r1, r2, r3);
 ```
@@ -179,20 +180,40 @@ println!("{}, {}, and {}", r1, r2, r3);
 
 ```text
 error[E0502]: cannot borrow `s` as mutable because it is also borrowed as immutable
- --> src/main.rs:6:10
+ --> src/main.rs:6:14
   |
-4 | let r1 = &s; // no problem
-  |          -- immutable borrow occurs here
-5 | let r2 = &s; // no problem
-6 | let r3 = &mut s; // BIG PROBLEM
-  |          ^^^^^^ mutable borrow occurs here
+4 |     let r1 = &s; // 没问题
+  |              -- immutable borrow occurs here
+5 |     let r2 = &s; // 没问题
+6 |     let r3 = &mut s; // 大问题
+  |              ^^^^^^ mutable borrow occurs here
 7 |
-8 | println!("{}, {}, and {}", r1, r2, r3);
-  |                            -- borrow later used here
+8 |     println!("{}, {}, and {}", r1, r2, r3);
+  |                                -- immutable borrow later used here
 ```
 
 哇哦！我们 **也** 不能在拥有不可变引用的同时拥有可变引用。不可变引用的用户可不希望在他们的眼皮底下值就被意外的改变了！然而，多个不可变引用是可以的，因为没有哪个只能读取数据的人有能力影响其他人读取到的数据。
 
+注意一个引用的作用域从声明的地方开始一直持续到最后一次使用为止。例如，因为最后一次使用不可变引用在声明可变引用之前，所以如下代码是可以编译的：
+
+<!-- This example is being ignored because there's a bug in rustdoc making the
+edition2018 not work. The bug is currently fixed in nightly, so when we update
+the book to >= 1.35, `ignore` can be removed from this example. -->
+
+```rust,edition2018,ignore
+let mut s = String::from("hello");
+
+let r1 = &s; // 没问题
+let r2 = &s; // 没问题
+println!("{} and {}", r1, r2);
+// 此位置之后 r1 和 r2 不再使用
+
+let r3 = &mut s; // 没问题
+println!("{}", r3);
+```
+
+不可变引用 `r1` 和 `r2` 的作用域在 `println!` 最后一次使用之后结束，这也是创建可变引用 `r3` 的地方。它们的作用域没有重叠，所以代码是可以编译的。
+
 尽管这些错误有时使人沮丧，但请牢记这是 Rust 编译器在提前指出一个潜在的 bug（在编译时而不是在运行时）并精准显示问题所在。这样你就不必去跟踪为何数据并不是你想象中的那样。
 
 ### 悬垂引用（Dangling References）