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<main>
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<h1 id="rust-新版解读--178--诊断属性宏"><a class="header" href="#rust-新版解读--178--诊断属性宏">Rust 新版解读 | 1.78 | 诊断属性宏</a></h1>
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<blockquote>
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<p>Rust 1.78 官方 release doc: <a href="https://blog.rust-lang.org/2024/05/02/Rust-1.78.0.html">Announcing Rust 1.78.0 | Rust Blog</a></p>
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</blockquote>
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<p>通过 <a href="https://www.rust-lang.org/tools/install">rustup</a> 安装的同学可以使用以下命令升级到 1.78 版本:</p>
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<pre><code class="language-shell">$ rustup update stable
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</code></pre>
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<h2 id="诊断属性宏"><a class="header" href="#诊断属性宏">诊断属性宏</a></h2>
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<p>Rust 现在支持 <code>#[diagnostic]</code> 属性命名空间，用于影响编译器错误消息。这些被视为提示，编译器不需要使用它们，也不会因为提供了编译器不认识的诊断而报错。这种灵活性允许源代码提供诊断，即使不是所有编译器都支持，无论是不同版本还是完全不同的实现。</p>
|
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<p>随着这个命名空间的出现，第一个支持的属性 <code>#[diagnostic::on_unimplemented]</code> 也随之而来，可以放在一个 trait 上，用于自定义当需要但未在类型上实现该 trait 时的消息。考虑下面来自<a href="https://github.com/rust-lang/rust/pull/119888/">稳定 PR</a> 里的示例：</p>
|
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<pre class="playground"><code class="language-rust edition2021">#[diagnostic::on_unimplemented(
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message = "My Message for `ImportantTrait<{A}>` is not implemented for `{Self}`",
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label = "My Label",
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note = "Note 1",
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note = "Note 2"
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)]
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trait ImportantTrait<A> {}
|
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|
|
|
|
fn use_my_trait(_: impl ImportantTrait<i32>) {}
|
|
|
|
|
|
fn main() {
|
|
|
use_my_trait(String::new());
|
|
|
}</code></pre>
|
|
|
<p>此前，编译器会给出一个内置错误，如下：</p>
|
|
|
<pre><code class="language-shell">error[E0277]: the trait bound `String: ImportantTrait<i32>` is not satisfied
|
|
|
--> src/main.rs:12:18
|
|
|
|
|
|
|
12 | use_my_trait(String::new());
|
|
|
| ------------ ^^^^^^^^^^^^^ the trait `ImportantTrait<i32>` is not implemented for `String`
|
|
|
| |
|
|
|
| required by a bound introduced by this call
|
|
|
|
|
|
|
</code></pre>
|
|
|
<p>现在，使用 <code>#[diagnostic::on_unimplemented]</code>，自定义消息填充主要错误行，自定义标签放在源输出上。原始标签仍然写在帮助输出中，任何自定义注释也会被写入。 (这些细节未来可能会发生变化。)</p>
|
|
|
<pre><code class="language-shell">error[E0277]: My Message for `ImportantTrait<i32>` is not implemented for `String`
|
|
|
--> src/main.rs:12:18
|
|
|
|
|
|
|
12 | use_my_trait(String::new());
|
|
|
| ------------ ^^^^^^^^^^^^^ My Label
|
|
|
| |
|
|
|
| required by a bound introduced by this call
|
|
|
|
|
|
|
= help: the trait `ImportantTrait<i32>` is not implemented for `String`
|
|
|
= note: Note 1
|
|
|
= note: Note 2
|
|
|
</code></pre>
|
|
|
<p>对于 trait 作者来说，如果你能提供更好的提示，而不是仅仅给出缺失部分，这种诊断就更有用。例如，这是标准库中的一个示例：</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span>#[diagnostic::on_unimplemented(
|
|
|
message = "the size for values of type `{Self}` cannot be known at compilation time",
|
|
|
label = "doesn't have a size known at compile-time"
|
|
|
)]
|
|
|
pub trait Sized {}
|
|
|
<span class="boring">}</span></code></pre>
|
|
|
<p>更多信息，请参考<a href="https://doc.rust-lang.org/stable/reference/attributes/diagnostics.html#the-diagnostic-tool-attribute-namespace">诊断工具属性命名空间</a>的参考部分。</p>
|
|
|
<h2 id="不安全前提断言"><a class="header" href="#不安全前提断言">不安全前提断言</a></h2>
|
|
|
<p>Rust 标准库有许多用于不安全函数前提的断言，但历史上它们只在标准库的 <code>#[cfg(debug_assertions)]</code> 构建中启用，以避免影响发布性能。然而，由于标准库通常以发布模式编译和分发，大多数 Rust 开发者根本没有执行这些检查。</p>
|
|
|
<p>现在，这些断言的条件被延迟到代码生成，因此它们将根据用户自己对调试断言的设置进行检查（在调试和测试构建中默认启用）。这个变化有助于用户捕获他们代码中的未定义行为。</p>
|
|
|
<p>例如，<code>slice::from_raw_parts</code> 需要一个对齐的非空指针。下面故意错位指针的使用有未定义行为，虽然如果你运气不好，它可能在过去看起来“工作”，但调试断言现在可以捕获它：</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021">fn main() {
|
|
|
let slice: &[u8] = &[1, 2, 3, 4, 5];
|
|
|
let ptr = slice.as_ptr();
|
|
|
|
|
|
// 创建一个 `ptr` 的偏移量，它总是比 `u16` 的正确对齐少一个
|
|
|
let i = usize::from(ptr as usize & 1 == 0);
|
|
|
|
|
|
let slice16: &[u16] = unsafe { std::slice::from_raw_parts(ptr.add(i).cast::<u16>(), 2) };
|
|
|
dbg!(slice16);
|
|
|
}</code></pre>
|
|
|
<p>在调试构建中，这将导致一个 panic：</p>
|
|
|
<pre><code class="language-shell">thread 'main' panicked at library/core/src/panicking.rs:220:5:
|
|
|
unsafe precondition(s) violated: slice::from_raw_parts requires the pointer to be aligned and non-null, and the total size of the slice not to exceed `isize::MAX`
|
|
|
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
|
|
|
thread caused non-unwinding panic. aborting.
|
|
|
</code></pre>
|
|
|
<h2 id="确定性重新对齐"><a class="header" href="#确定性重新对齐">确定性重新对齐</a></h2>
|
|
|
<p>标准库有一些函数可以改变指针和切片的对齐方式，但以前它们有一些注意事项，使它们在实践中难以依赖，如果你严格遵循它们的文档。这些注意事项主要是为了保护对 <code>const</code> 的判断，但它们只对非 <code>const</code> 使用稳定。现在它们承诺根据实际输入具有一致的运行时行为。</p>
|
|
|
<ul>
|
|
|
<li>
|
|
|
<p><a href="https://doc.rust-lang.org/std/primitive.pointer.html#method.align_offset"><code>pointer::align_offset</code></a> 计算改变指针到给定对齐方式所需的偏移量。如果不可能，它将返回 <code>usize::MAX</code>，但以前允许它始终返回 <code>usize::MAX</code>，现在这种行为被移除。</p>
|
|
|
</li>
|
|
|
<li>
|
|
|
<p><a href="https://doc.rust-lang.org/std/primitive.slice.html#method.align_to"><code>slice::align_to</code></a> 和 <a href="https://doc.rust-lang.org/std/primitive.slice.html#method.align_to_mut"><code>slice::align_to_mut</code></a> 都将切片转换为对齐的中间切片和剩余的不对齐头和尾切片。这些方法现在承诺返回最大可能的中间部分，而不允许实现返回不那么优化的东西，比如将所有东西作为头切片返回。</p>
|
|
|
</li>
|
|
|
</ul>
|
|
|
<h2 id="others"><a class="header" href="#others">Others</a></h2>
|
|
|
<p>其它更新细节，和稳定的API列表，参考<a href="https://blog.rust-lang.org/2024/05/02/Rust-1.78.0.html#stabilized-apis">原Blog</a></p>
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