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@ -2,7 +2,7 @@
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> [ch07-04-bringing-paths-into-scope-with-the-use-keyword.md](https://github.com/rust-lang/book/blob/main/src/ch07-04-bringing-paths-into-scope-with-the-use-keyword.md)
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> <br>
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> commit 6d3e76820418f2d2bb203233c61d90390b5690f1
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> commit 2d605f66b3d891dea0a2f684ece508aa2282b854
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到目前为止,似乎我们编写的用于调用函数的路径都很冗长且重复,并不方便。例如,示例 7-7 中,无论我们选择 `add_to_waitlist` 函数的绝对路径还是相对路径,每次我们想要调用 `add_to_waitlist` 时,都必须指定`front_of_house` 和 `hosting`。幸运的是,有一种方法可以简化这个过程。我们可以使用 `use` 关键字将路径一次性引入作用域,然后调用该路径中的项,就如同它们是本地项一样。
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@ -10,21 +10,8 @@
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<span class="filename">文件名: src/lib.rs</span>
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```rust
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mod front_of_house {
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pub mod hosting {
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pub fn add_to_waitlist() {}
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}
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}
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use crate::front_of_house::hosting;
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pub fn eat_at_restaurant() {
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hosting::add_to_waitlist();
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hosting::add_to_waitlist();
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hosting::add_to_waitlist();
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}
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# fn main() {}
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```rust,noplayground,test_harness
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{{#rustdoc_include ../listings/ch07-managing-growing-projects/listing-07-11/src/lib.rs}}
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```
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<span class="caption">示例 7-11: 使用 `use` 将模块引入作用域</span>
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@ -35,21 +22,8 @@ pub fn eat_at_restaurant() {
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<span class="filename">文件名: src/lib.rs</span>
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```rust
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mod front_of_house {
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pub mod hosting {
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pub fn add_to_waitlist() {}
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}
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}
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use front_of_house::hosting;
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pub fn eat_at_restaurant() {
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hosting::add_to_waitlist();
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hosting::add_to_waitlist();
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hosting::add_to_waitlist();
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}
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# fn main() {}
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```rust,noplayground,test_harness
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{{#rustdoc_include ../listings/ch07-managing-growing-projects/listing-07-12/src/lib.rs}}
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```
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<span class="caption">示例 7-12: 使用 `use` 和相对路径将模块引入作用域</span>
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@ -60,21 +34,8 @@ pub fn eat_at_restaurant() {
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<span class="filename">文件名: src/lib.rs</span>
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```rust
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mod front_of_house {
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pub mod hosting {
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pub fn add_to_waitlist() {}
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}
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}
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use crate::front_of_house::hosting::add_to_waitlist;
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pub fn eat_at_restaurant() {
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add_to_waitlist();
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add_to_waitlist();
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add_to_waitlist();
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}
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# fn main() {}
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```rust,noplayground,test_harness
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{{#rustdoc_include ../listings/ch07-managing-growing-projects/listing-07-13/src/lib.rs}}
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```
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<span class="caption">示例 7-13: 使用 `use` 将 `add_to_waitlist` 函数引入作用域,这并不符合习惯</span>
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@ -86,12 +47,7 @@ pub fn eat_at_restaurant() {
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<span class="filename">文件名: src/main.rs</span>
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```rust
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use std::collections::HashMap;
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fn main() {
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let mut map = HashMap::new();
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map.insert(1, 2);
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}
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{{#rustdoc_include ../listings/ch07-managing-growing-projects/listing-07-14/src/main.rs}}
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```
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<span class="caption">示例 7-14: 将 `HashMap` 引入作用域的习惯用法</span>
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@ -102,19 +58,8 @@ fn main() {
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<span class="filename">文件名: src/lib.rs</span>
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```rust
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use std::fmt;
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use std::io;
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fn function1() -> fmt::Result {
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// --snip--
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# Ok(())
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}
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fn function2() -> io::Result<()> {
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// --snip--
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# Ok(())
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}
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```rust,noplayground
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{{#rustdoc_include ../listings/ch07-managing-growing-projects/listing-07-15/src/lib.rs:here}}
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```
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<span class="caption">示例 7-15: 使用父模块将两个具有相同名称的类型引入同一作用域</span>
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@ -127,19 +72,8 @@ fn function2() -> io::Result<()> {
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<span class="filename">文件名: src/lib.rs</span>
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```rust
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use std::fmt::Result;
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use std::io::Result as IoResult;
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fn function1() -> Result {
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// --snip--
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# Ok(())
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}
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fn function2() -> IoResult<()> {
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// --snip--
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# Ok(())
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}
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```rust,noplayground
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{{#rustdoc_include ../listings/ch07-managing-growing-projects/listing-07-16/src/lib.rs:here}}
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```
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<span class="caption">示例 7-16: 使用 `as` 关键字重命名引入作用域的类型</span>
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@ -154,21 +88,8 @@ fn function2() -> IoResult<()> {
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<span class="filename">文件名: src/lib.rs</span>
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```rust
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mod front_of_house {
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pub mod hosting {
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pub fn add_to_waitlist() {}
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}
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}
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pub use crate::front_of_house::hosting;
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pub fn eat_at_restaurant() {
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hosting::add_to_waitlist();
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hosting::add_to_waitlist();
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hosting::add_to_waitlist();
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}
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# fn main() {}
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```rust,noplayground,test_harness
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{{#rustdoc_include ../listings/ch07-managing-growing-projects/listing-07-17/src/lib.rs}}
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```
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<span class="caption">示例 7-17: 通过 `pub use` 使名称可引入任何代码的作用域中</span>
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@ -184,8 +105,7 @@ pub fn eat_at_restaurant() {
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<span class="filename">文件名: Cargo.toml</span>
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```toml
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[dependencies]
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rand = "0.5.5"
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{{#include ../listings/ch02-guessing-game-tutorial/listing-02-02/Cargo.toml:9:}}
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```
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在 *Cargo.toml* 中加入 `rand` 依赖告诉了 Cargo 要从 [crates.io](https://crates.io) 下载 `rand` 和其依赖,并使其可在项目代码中使用。
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@ -193,11 +113,7 @@ rand = "0.5.5"
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接着,为了将 `rand` 定义引入项目包的作用域,我们加入一行 `use` 起始的包名,它以 `rand` 包名开头并列出了需要引入作用域的项。回忆一下第二章的 “生成一个随机数” 部分,我们曾将 `Rng` trait 引入作用域并调用了 `rand::thread_rng` 函数:
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```rust,ignore
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use rand::Rng;
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fn main() {
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let secret_number = rand::thread_rng().gen_range(1, 101);
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}
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{{#rustdoc_include ../listings/ch02-guessing-game-tutorial/listing-02-03/src/main.rs:ch07-04}}
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```
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[crates.io](https://crates.io) 上有很多 Rust 社区成员发布的包,将其引入你自己的项目都需要一道相同的步骤:在 *Cargo.toml* 列出它们并通过 `use` 将其中定义的项引入项目包的作用域中。
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@ -216,19 +132,16 @@ use std::collections::HashMap;
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<span class="filename">文件名: src/main.rs</span>
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```rust
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use std::cmp::Ordering;
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use std::io;
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// ---snip---
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```rust,ignore
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{{#rustdoc_include ../listings/ch07-managing-growing-projects/no-listing-01-use-std-unnested/src/main.rs:here}}
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```
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相反,我们可以使用嵌套路径将相同的项在一行中引入作用域。这么做需要指定路径的相同部分,接着是两个冒号,接着是大括号中的各自不同的路径部分,如示例 7-18 所示。
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<span class="filename">文件名: src/main.rs</span>
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```rust
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use std::{cmp::Ordering, io};
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// ---snip---
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```rust,ignore
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{{#rustdoc_include ../listings/ch07-managing-growing-projects/listing-07-18/src/main.rs:here}}
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```
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<span class="caption">示例 7-18: 指定嵌套的路径在一行中将多个带有相同前缀的项引入作用域</span>
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@ -239,9 +152,8 @@ use std::{cmp::Ordering, io};
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<span class="filename">文件名: src/lib.rs</span>
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```rust
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use std::io;
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use std::io::Write;
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```rust,noplayground
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{{#rustdoc_include ../listings/ch07-managing-growing-projects/listing-07-19/src/lib.rs}}
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```
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<span class="caption">示例 7-19: 通过两行 `use` 语句引入两个路径,其中一个是另一个的子路径</span>
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@ -250,8 +162,8 @@ use std::io::Write;
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<span class="filename">文件名: src/lib.rs</span>
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```rust
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use std::io::{self, Write};
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```rust,noplayground
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{{#rustdoc_include ../listings/ch07-managing-growing-projects/listing-07-20/src/lib.rs}}
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```
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<span class="caption">示例 7-20: 将示例 7-19 中部分重复的路径合并为一个 `use` 语句</span>
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@ -269,3 +181,6 @@ use std::collections::*;
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这个 `use` 语句将 `std::collections` 中定义的所有公有项引入当前作用域。使用 glob 运算符时请多加小心!Glob 会使得我们难以推导作用域中有什么名称和它们是在何处定义的。
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glob 运算符经常用于测试模块 `tests` 中,这时会将所有内容引入作用域;我们将在第十一章 “如何编写测试” 部分讲解。glob 运算符有时也用于 prelude 模式;查看 [标准库中的文档](https://doc.rust-lang.org/std/prelude/index.html#other-preludes) 了解这个模式的更多细节。
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[rand]: ch02-00-guessing-game-tutorial.html#生成一个随机数
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[writing-tests]: ch11-01-writing-tests.html#如何编写测试
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