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<main>
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<h1 id="生命周期过大-02"><a class="header" href="#生命周期过大-02">生命周期过大-02</a></h1>
|
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<p>继上篇文章后，我们再来看一段<strong>可能</strong>涉及生命周期过大导致的无法编译问题:</p>
|
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|
<pre class="playground"><code class="language-rust edition2021">fn bar(writer: &mut Writer) {
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baz(writer.indent());
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writer.write("world");
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}
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fn baz(writer: &mut Writer) {
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writer.write("hello");
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}
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pub struct Writer<'a> {
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|
target: &'a mut String,
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indent: usize,
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}
|
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impl<'a> Writer<'a> {
|
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fn indent(&'a mut self) -> &'a mut Self {
|
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|
&mut Self {
|
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|
target: self.target,
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indent: self.indent + 1,
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}
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}
|
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fn write(&mut self, s: &str) {
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for _ in 0..self.indent {
|
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self.target.push(' ');
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|
}
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self.target.push_str(s);
|
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self.target.push('\n');
|
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|
}
|
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|
}
|
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|
fn main() {}</code></pre>
|
|
|
<p>报错如下：</p>
|
|
|
<pre><code class="language-console">error[E0623]: lifetime mismatch
|
|
|
--> src/main.rs:2:16
|
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|
|
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|
1 | fn bar(writer: &mut Writer) {
|
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|
| -----------
|
|
|
| |
|
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|
| these two types are declared with different lifetimes...
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|
2 | baz(writer.indent());
|
|
|
| ^^^^^^ ...but data from `writer` flows into `writer` here
|
|
|
</code></pre>
|
|
|
<p>WTF，这什么报错，之前都没有见过，而且很难理解，什么叫<code>writer</code>滑入了另一个<code>writer</code>？</p>
|
|
|
<p>别急，我们先来仔细看下代码，注意这一段：</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span>impl<'a> Writer<'a> {
|
|
|
fn indent(&'a mut self) -> &'a mut Self {
|
|
|
&mut Self {
|
|
|
target: self.target,
|
|
|
indent: self.indent + 1,
|
|
|
}
|
|
|
}
|
|
|
<span class="boring">}</span></code></pre>
|
|
|
<p>这里的生命周期定义说明<code>indent</code>方法使用的。。。等等！你的代码错了，你怎么能在一个函数中返回一个新创建实例的引用？！！最重要的是，编译器不提示这个错误，竟然提示一个莫名其妙看不懂的东东。</p>
|
|
|
<p>行，那我们先解决这个问题，将该方法修改为:</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span>fn indent(&'a mut self) -> Writer<'a> {
|
|
|
Writer {
|
|
|
target: self.target,
|
|
|
indent: self.indent + 1,
|
|
|
}
|
|
|
}
|
|
|
<span class="boring">}</span></code></pre>
|
|
|
<p>怀着惴惴这心，再一次运行程序，果不其然，编译器又朝我们扔了一坨错误：</p>
|
|
|
<pre><code class="language-console">error[E0308]: mismatched types
|
|
|
--> src/main.rs:2:9
|
|
|
|
|
|
|
2 | baz(writer.indent());
|
|
|
| ^^^^^^^^^^^^^^^
|
|
|
| |
|
|
|
| expected `&mut Writer<'_>`, found struct `Writer`
|
|
|
| help: consider mutably borrowing here: `&mut writer.indent()`
|
|
|
</code></pre>
|
|
|
<p>哦，这次错误很明显，因为<code>baz</code>需要<code>&mut Writer</code>，但是咱们<code>writer.indent</code>返回了一个<code>Writer</code>，因此修改下即可:</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span>fn bar(writer: &mut Writer) {
|
|
|
baz(&mut writer.indent());
|
|
|
writer.write("world");
|
|
|
}
|
|
|
<span class="boring">}</span></code></pre>
|
|
|
<p>这次总该成功了吧？再次心慌慌的运行编译器，哐：</p>
|
|
|
<pre><code class="language-console">error[E0623]: lifetime mismatch
|
|
|
--> src/main.rs:2:21
|
|
|
|
|
|
|
1 | fn bar(writer: &mut Writer) {
|
|
|
| -----------
|
|
|
| |
|
|
|
| these two types are declared with different lifetimes...
|
|
|
2 | baz(&mut writer.indent());
|
|
|
| ^^^^^^ ...but data from `writer` flows into `writer` here
|
|
|
</code></pre>
|
|
|
<p>可恶，还是这个看不懂的错误，仔细检查了下代码，这次真的没有其他错误了，只能硬着头皮上。</p>
|
|
|
<p>大概的意思可以分析，生命周期范围不匹配，说明一个大一个小，然后一个<code>writer</code>中流入到另一个<code>writer</code>说明，两个<code>writer</code>的生命周期定义错了，既然这里提到了<code>indent</code>方法调用，那么我们再去仔细看一眼：</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span>impl<'a> Writer<'a> {
|
|
|
fn indent(&'a mut self) -> Writer<'a> {
|
|
|
Writer {
|
|
|
target: self.target,
|
|
|
indent: self.indent + 1,
|
|
|
}
|
|
|
}
|
|
|
...
|
|
|
}
|
|
|
<span class="boring">}</span></code></pre>
|
|
|
<p>好像有点问题，<code>indent</code>返回的<code>Writer</code>的生命周期和外面调用者的<code>Writer</code>的生命周期一模一样，这很不合理，一眼就能看出前者远小于后者。</p>
|
|
|
<p>这里稍微展开以下，为何<code>indent</code>方法返回值的生命周期不能与参数中的<code>self</code>相同。<strong>首先，我们假设它们可以相同，也就是上面的代码可以编译通过</strong>，由于此时在返回值中借用了<code>self</code>的可变引用，意味着<strong>如果你在返回值被使用后，还继续使用<code>self</code> 会导致重复借用的错误，因为返回值的生命周期将持续到 <code>self</code> 结束</strong>。</p>
|
|
|
<p>既然不能相同，那我们尝试着修改下<code>indent</code>:</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span> fn indent<'b>(&'b mut self) -> Writer<'b> {
|
|
|
Writer {
|
|
|
target: self.target,
|
|
|
indent: self.indent + 1,
|
|
|
}
|
|
|
}
|
|
|
<span class="boring">}</span></code></pre>
|
|
|
<p>Bang! 编译成功，不过稍等，回想下生命周期消除的规则，我们还可以实现的更优雅：</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021">fn bar(writer: &mut Writer) {
|
|
|
baz(&mut writer.indent());
|
|
|
writer.write("world");
|
|
|
}
|
|
|
|
|
|
fn baz(writer: &mut Writer) {
|
|
|
writer.write("hello");
|
|
|
}
|
|
|
|
|
|
pub struct Writer<'a> {
|
|
|
target: &'a mut String,
|
|
|
indent: usize,
|
|
|
}
|
|
|
|
|
|
impl<'a> Writer<'a> {
|
|
|
fn indent(&mut self) -> Writer {
|
|
|
Writer {
|
|
|
target: self.target,
|
|
|
indent: self.indent + 1,
|
|
|
}
|
|
|
}
|
|
|
|
|
|
fn write(&mut self, s: &str) {
|
|
|
for _ in 0..self.indent {
|
|
|
self.target.push(' ');
|
|
|
}
|
|
|
self.target.push_str(s);
|
|
|
self.target.push('\n');
|
|
|
}
|
|
|
}
|
|
|
|
|
|
fn main() {}</code></pre>
|
|
|
<p>至此，问题彻底解决，太好了，我感觉我又变强了。可是默默看了眼自己的头发，只能以<code>哎～</code>一声叹息结束本章内容。</p>
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