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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>
<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>
<h2 id="例子-1"><a class="header" href="#例子-1">例子 1</a></h2>
<pre><pre class="playground"><code class="language-rust edition2021">struct Interface&lt;'a&gt; {
manager: &amp;'a mut Manager&lt;'a&gt;
}
impl&lt;'a&gt; Interface&lt;'a&gt; {
pub fn noop(self) {
println!("interface consumed");
}
}
struct Manager&lt;'a&gt; {
text: &amp;'a str
}
struct List&lt;'a&gt; {
manager: Manager&lt;'a&gt;,
}
impl&lt;'a&gt; List&lt;'a&gt; {
pub fn get_interface(&amp;'a mut self) -&gt; Interface {
Interface {
manager: &amp;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(&amp;list);
}
fn use_list(list: &amp;List) {
println!("{}", list.manager.text);
}</code></pre></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
--&gt; 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(&amp;list);
| ^^^^^
| |
| 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(&amp;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&lt;'a&gt;</code>的生命周期<code>'b</code>，最终代码如下：</p>
<pre><pre class="playground"><code class="language-rust edition2021">struct Interface&lt;'b, 'a: 'b&gt; {
manager: &amp;'b mut Manager&lt;'a&gt;
}
impl&lt;'b, 'a: 'b&gt; Interface&lt;'b, 'a&gt; {
pub fn noop(self) {
println!("interface consumed");
}
}
struct Manager&lt;'a&gt; {
text: &amp;'a str
}
struct List&lt;'a&gt; {
manager: Manager&lt;'a&gt;,
}
impl&lt;'a&gt; List&lt;'a&gt; {
pub fn get_interface&lt;'b&gt;(&amp;'b mut self) -&gt; Interface&lt;'b, 'a&gt;
where 'a: 'b {
Interface {
manager: &amp;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(&amp;list);
}
fn use_list(list: &amp;List) {
println!("{}", list.manager.text);
}</code></pre></pre>
<p>当然，咱还可以给生命周期给予更有意义的名称：</p>
<pre><pre class="playground"><code class="language-rust edition2021">struct Interface&lt;'text, 'manager&gt; {
manager: &amp;'manager mut Manager&lt;'text&gt;
}
impl&lt;'text, 'manager&gt; Interface&lt;'text, 'manager&gt; {
pub fn noop(self) {
println!("interface consumed");
}
}
struct Manager&lt;'text&gt; {
text: &amp;'text str
}
struct List&lt;'text&gt; {
manager: Manager&lt;'text&gt;,
}
impl&lt;'text&gt; List&lt;'text&gt; {
pub fn get_interface&lt;'manager&gt;(&amp;'manager mut self) -&gt; Interface&lt;'text, 'manager&gt;
where 'text: 'manager {
Interface {
manager: &amp;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(&amp;list);
}
fn use_list(list: &amp;List) {
println!("{}", list.manager.text);
}</code></pre></pre>
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