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<main>
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<h1 id="生命周期声明的范围过大"><a class="header" href="#生命周期声明的范围过大">生命周期声明的范围过大</a></h1>
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<p>在大多时候，Rust 的生命周期你只要标识了，即可以通过编译，但是总是存在一些情况，会导致编译无法通过，本文就讲述这样一种情况：因为生命周期声明的范围过大，导致了编译无法通过，希望大家喜欢</p>
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<h2 id="例子-1"><a class="header" href="#例子-1">例子 1</a></h2>
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<pre class="playground"><code class="language-rust edition2021">struct Interface<'a> {
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manager: &'a mut Manager<'a>
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}
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impl<'a> Interface<'a> {
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pub fn noop(self) {
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println!("interface consumed");
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}
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}
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struct Manager<'a> {
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text: &'a str
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}
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struct List<'a> {
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manager: Manager<'a>,
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}
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impl<'a> List<'a> {
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pub fn get_interface(&'a mut self) -> Interface {
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Interface {
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manager: &mut self.manager
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}
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}
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}
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fn main() {
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let mut list = List {
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manager: Manager {
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text: "hello"
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}
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};
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list.get_interface().noop();
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println!("Interface should be dropped here and the borrow released");
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// this fails because inmutable/mutable borrow
|
|
|
// but Interface should be already dropped here and the borrow released
|
|
|
use_list(&list);
|
|
|
}
|
|
|
|
|
|
fn use_list(list: &List) {
|
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|
println!("{}", list.manager.text);
|
|
|
}</code></pre>
|
|
|
<p>运行后报错：</p>
|
|
|
<pre><code class="language-console">error[E0502]: cannot borrow `list` as immutable because it is also borrowed as mutable // `list`无法被借用，因为已经被可变借用
|
|
|
--> src/main.rs:40:14
|
|
|
|
|
|
|
34 | list.get_interface().noop();
|
|
|
| ---- mutable borrow occurs here // 可变借用发生在这里
|
|
|
...
|
|
|
40 | use_list(&list);
|
|
|
| ^^^^^
|
|
|
| |
|
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|
| immutable borrow occurs here // 新的不可变借用发生在这
|
|
|
| mutable borrow later used here // 可变借用在这里结束
|
|
|
</code></pre>
|
|
|
<p>这段代码看上去并不复杂，实际上难度挺高的，首先在直觉上，<code>list.get_interface()</code>借用的可变引用，按理来说应该在这行代码结束后，就归还了，为何能持续到<code>use_list(&list)</code>后面呢？</p>
|
|
|
<p>这是因为我们在<code>get_interface</code>方法中声明的<code>lifetime</code>有问题，该方法的参数的生明周期是<code>'a</code>，而<code>List</code>的生命周期也是<code>'a</code>，说明该方法至少活得跟<code>List</code>一样久，再回到<code>main</code>函数中，<code>list</code>可以活到<code>main</code>函数的结束，因此<code>list.get_interface()</code>借用的可变引用也会活到<code>main</code>函数的结束，在此期间，自然无法再进行借用了。</p>
|
|
|
<p>要解决这个问题，我们需要为<code>get_interface</code>方法的参数给予一个不同于<code>List<'a></code>的生命周期<code>'b</code>，最终代码如下：</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021">struct Interface<'b, 'a: 'b> {
|
|
|
manager: &'b mut Manager<'a>
|
|
|
}
|
|
|
|
|
|
impl<'b, 'a: 'b> Interface<'b, 'a> {
|
|
|
pub fn noop(self) {
|
|
|
println!("interface consumed");
|
|
|
}
|
|
|
}
|
|
|
|
|
|
struct Manager<'a> {
|
|
|
text: &'a str
|
|
|
}
|
|
|
|
|
|
struct List<'a> {
|
|
|
manager: Manager<'a>,
|
|
|
}
|
|
|
|
|
|
impl<'a> List<'a> {
|
|
|
pub fn get_interface<'b>(&'b mut self) -> Interface<'b, 'a>
|
|
|
where 'a: 'b {
|
|
|
Interface {
|
|
|
manager: &mut self.manager
|
|
|
}
|
|
|
}
|
|
|
}
|
|
|
|
|
|
fn main() {
|
|
|
|
|
|
let mut list = List {
|
|
|
manager: Manager {
|
|
|
text: "hello"
|
|
|
}
|
|
|
};
|
|
|
|
|
|
list.get_interface().noop();
|
|
|
|
|
|
println!("Interface should be dropped here and the borrow released");
|
|
|
|
|
|
// this fails because inmutable/mutable borrow
|
|
|
// but Interface should be already dropped here and the borrow released
|
|
|
use_list(&list);
|
|
|
}
|
|
|
|
|
|
fn use_list(list: &List) {
|
|
|
println!("{}", list.manager.text);
|
|
|
}</code></pre>
|
|
|
<p>当然，咱还可以给生命周期给予更有意义的名称：</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021">struct Interface<'text, 'manager> {
|
|
|
manager: &'manager mut Manager<'text>
|
|
|
}
|
|
|
|
|
|
impl<'text, 'manager> Interface<'text, 'manager> {
|
|
|
pub fn noop(self) {
|
|
|
println!("interface consumed");
|
|
|
}
|
|
|
}
|
|
|
|
|
|
struct Manager<'text> {
|
|
|
text: &'text str
|
|
|
}
|
|
|
|
|
|
struct List<'text> {
|
|
|
manager: Manager<'text>,
|
|
|
}
|
|
|
|
|
|
impl<'text> List<'text> {
|
|
|
pub fn get_interface<'manager>(&'manager mut self) -> Interface<'text, 'manager>
|
|
|
where 'text: 'manager {
|
|
|
Interface {
|
|
|
manager: &mut self.manager
|
|
|
}
|
|
|
}
|
|
|
}
|
|
|
|
|
|
fn main() {
|
|
|
let mut list = List {
|
|
|
manager: Manager {
|
|
|
text: "hello"
|
|
|
}
|
|
|
};
|
|
|
|
|
|
list.get_interface().noop();
|
|
|
|
|
|
println!("Interface should be dropped here and the borrow released");
|
|
|
|
|
|
// this fails because inmutable/mutable borrow
|
|
|
// but Interface should be already dropped here and the borrow released
|
|
|
use_list(&list);
|
|
|
}
|
|
|
|
|
|
fn use_list(list: &List) {
|
|
|
println!("{}", list.manager.text);
|
|
|
}</code></pre>
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