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# Miri
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看到这里，大家是不是暗中松了口气？unsafe 不过如此嘛，不知道为何其它人都谈之色变。
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怎么说呢？你以为的编译器已经不是以前的编译器了，它不报错不代表没有错误。包括测试用例也是，正常地运行不能意味着代码没有任何错误。
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在周星驰电影功夫中，还有一个奇怪大叔 10 元一本主动上门卖如来神掌，那么有没有 10 元一本的 Rust 秘笈呢？( 喂，Rust语言圣经都免费让你读了，有了摩托车，还要什么拖拉机... 哈哈，开个玩笑 )
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有的，奇怪大叔正在赶来，他告诉我们先来安装一个命令:
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```shell
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rustup +nightly-2022-01-21 component add miri
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info: syncing channel updates for 'nightly-2022-01-21-x86_64-pc-windows-msvc'
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info: latest update on 2022-01-21, rust version 1.60.0-nightly (777bb86bc 2022-01-20)
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info: downloading component 'cargo'
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info: downloading component 'clippy'
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info: downloading component 'rust-docs'
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info: downloading component 'rust-std'
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info: downloading component 'rustc'
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info: downloading component 'rustfmt'
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info: installing component 'cargo'
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info: installing component 'clippy'
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info: installing component 'rust-docs'
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info: installing component 'rust-std'
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info: installing component 'rustc'
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info: installing component 'rustfmt'
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info: downloading component 'miri'
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info: installing component 'miri'
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```
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等等，你在我电脑上装了什么奇怪的东西？！ "好东西"
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> miri 目前只能在 nightly Rust 上安装，`+nightly-2022-01-21` 告诉 `rustup` 我们想要安装的 `nightly` 版本，事实上，你可以直接通过 `rustup +nightly component add miri` 安装，这里指定版本主要因为 `miri` 有时候会因为某些版本而出错。
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>
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> 2022-01-21 是我所知的 miri 可以成功运行的版本，你可以检查[这个网址](https://rust-lang.github.io/rustup-components-history/)获取更多信息
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>
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> + 是一种临时性的规则运用，如果你不想每次都使用 `+nightly-2022-01-21`，可以使用 [`rustup override set`](https://course.rs/appendix/rust-version.html#rustup-和-rust-nightly-的职责) 命令对当前项目的 Rust 版本进行覆盖
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```shell
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> cargo +nightly-2022-01-21 miri test
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I will run `"cargo.exe" "install" "xargo"` to install
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a recent enough xargo. Proceed? [Y/n]
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```
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额，`xargo` 是什么东东？"不要担心，选择 y 就行，我像是会坑你的人吗？"
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```shell
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> y
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Updating crates.io index
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Installing xargo v0.3.24
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...
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Finished release [optimized] target(s) in 10.65s
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Installing C:\Users\ninte\.cargo\bin\xargo-check.exe
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Installing C:\Users\ninte\.cargo\bin\xargo.exe
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Installed package `xargo v0.3.24` (executables `xargo-check.exe`, `xargo.exe`)
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I will run `"rustup" "component" "add" "rust-src"` to install
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the `rust-src` component for the selected toolchain. Proceed? [Y/n]
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```
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额？ "不要怕，多给你一份 Rust 源代码，不开心嘛？"
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```shell
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> y
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info: downloading component 'rust-src'
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info: installing component 'rust-src'
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```
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"看吧，我就说我不会骗你的，不相信我，等着错误砸脸吧!" 真是一个奇怪的大叔...
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```shell
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Compiling lists v0.1.0 (C:\Users\ninte\dev\tmp\lists)
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Finished test [unoptimized + debuginfo] target(s) in 0.25s
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Running unittests (lists-5cc11d9ee5c3e924.exe)
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error: Undefined Behavior: trying to reborrow for Unique at alloc84055,
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but parent tag <209678> does not have an appropriate item in
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the borrow stack
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--> \lib\rustlib\src\rust\library\core\src\option.rs:846:18
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846 | Some(x) => Some(f(x)),
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| ^ trying to reborrow for Unique at alloc84055,
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| but parent tag <209678> does not have an
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| appropriate item in the borrow stack
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= help: this indicates a potential bug in the program:
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it performed an invalid operation, but the rules it
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violated are still experimental
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= help: see https://github.com/rust-lang/unsafe-code-guidelines/blob/master/wip/stacked-borrows.md
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for further information
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= note: inside `std::option::Option::<std::boxed::Box<fifth::Node<i32>>>::map::<i32, [closure@src\fifth.rs:31:30: 40:10]>` at \lib\rustlib\src\rust\library\core\src\option.rs:846:18
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note: inside `fifth::List::<i32>::pop` at src\fifth.rs:31:9
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--> src\fifth.rs:31:9
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31 | / self.head.take().map(|head| {
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32 | | let head = *head;
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33 | | self.head = head.next;
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34 | |
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... |
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39 | | head.elem
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40 | | })
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| |__________^
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note: inside `fifth::test::basics` at src\fifth.rs:74:20
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--> src\fifth.rs:74:20
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74 | assert_eq!(list.pop(), Some(1));
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| ^^^^^^^^^^
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note: inside closure at src\fifth.rs:62:5
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--> src\fifth.rs:62:5
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61 | #[test]
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| ------- in this procedural macro expansion
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62 | / fn basics() {
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63 | | let mut list = List::new();
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64 | |
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65 | | // Check empty list behaves right
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... |
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96 | | assert_eq!(list.pop(), None);
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97 | | }
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| |_____^
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...
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error: aborting due to previous error
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```
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咦还真有错误，大叔，这是什么错误？大叔？...奇怪的大叔默默离开了，留下我在风中凌乱。
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果然不靠谱...还是得靠自己，首先得了解下何为 `miri`。
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[`miri`](https://github.com/rust-lang/miri) 可以生成 Rust 的中间层表示 MIR，对于编译器来说，我们的 Rust 代码首先会被编译为 MIR ，然后再提交给 LLVM 进行处理。
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可以通过 `rustup component add miri` 来安装它，并通过 `cargo miri` 来使用，同时还可以使用 `cargo miri test` 来运行测试代码。
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`miri` 可以帮助我们检查常见的未定义行为(UB = Undefined Behavior)，以下列出了一部分:
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- 内存越界检查和内存释放后再使用(use-after-free)
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- 使用未初始化的数据
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- 数据竞争
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- 内存对齐问题
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UB 检测是必须的，因为它发生在运行时，因此很难发现，如果 `miri` 能在编译期检测出来，那自然是最好不过的。
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总之，`miri` 的使用很简单:
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```shell
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> cargo +nightly-2022-01-21 miri test
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```
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下面来看看具体的错误：
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```shell
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error: Undefined Behavior: trying to reborrow for Unique at alloc84055, but parent tag <209678> does not have an appropriate item in the borrow stack
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--> \lib\rustlib\src\rust\library\core\src\option.rs:846:18
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846 | Some(x) => Some(f(x)),
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| ^ trying to reborrow for Unique at alloc84055,
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| but parent tag <209678> does not have an
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| appropriate item in the borrow stack
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= help: this indicates a potential bug in the program: it
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performed an invalid operation, but the rules it
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violated are still experimental
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= help: see
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https://github.com/rust-lang/unsafe-code-guidelines/blob/master/wip/stacked-borrows.md
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for further information
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```
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嗯，只能看出是一个错误，其它完全看不懂了，例如什么是 `borrow stack`？
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sunface
3 years ago