Miri

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。

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。

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 )

有的,奇怪大叔正在赶来,他告诉我们先来安装一个命令:

rustup +nightly-2022-01-21 component add miri
info: syncing channel updates for 'nightly-2022-01-21-x86_64-pc-windows-msvc'
info: latest update on 2022-01-21, rust version 1.60.0-nightly (777bb86bc 2022-01-20)
info: downloading component 'cargo'
info: downloading component 'clippy'
info: downloading component 'rust-docs'
info: downloading component 'rust-std'
info: downloading component 'rustc'
info: downloading component 'rustfmt'
info: installing component 'cargo'
info: installing component 'clippy'
info: installing component 'rust-docs'
info: installing component 'rust-std'
info: installing component 'rustc'
info: installing component 'rustfmt'
info: downloading component 'miri'
info: installing component 'miri'

等等,你在我电脑上装了什么奇怪的东西?! "好东西"

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。

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获取更多信息

  • 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 命令对当前项目的 Rust 版本进行覆盖
$ cargo +nightly-2022-01-21 miri test

I will run `"cargo.exe" "install" "xargo"` to install
a recent enough xargo. Proceed? [Y/n]

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?"

> y

    Updating crates.io index
  Installing xargo v0.3.24
...
    Finished release [optimized] target(s) in 10.65s
  Installing C:\Users\ninte\.cargo\bin\xargo-check.exe
  Installing C:\Users\ninte\.cargo\bin\xargo.exe
   Installed package `xargo v0.3.24` (executables `xargo-check.exe`, `xargo.exe`)

I will run `"rustup" "component" "add" "rust-src"` to install 
the `rust-src` component for the selected toolchain. Proceed? [Y/n]

额? "不要怕,多给你一份 Rust 源代码,不开心嘛?"

> y

info: downloading component 'rust-src'
info: installing component 'rust-src'

"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...

 Compiling lists v0.1.0 (C:\Users\ninte\dev\tmp\lists)
    Finished test [unoptimized + debuginfo] target(s) in 0.25s
     Running unittests (lists-5cc11d9ee5c3e924.exe)

error: Undefined Behavior: trying to reborrow for Unique at alloc84055, 
       but parent tag <209678> does not have an appropriate item in 
       the borrow stack

   --> \lib\rustlib\src\rust\library\core\src\option.rs:846:18
    |
846 |             Some(x) => Some(f(x)),
    |                  ^ trying to reborrow for Unique at alloc84055, 
    |                    but parent tag <209678> does not have an 
    |                    appropriate item in the borrow stack
    |
    = help: this indicates a potential bug in the program: 
      it performed an invalid operation, but the rules it 
      violated are still experimental
    = help: see https://github.com/rust-lang/unsafe-code-guidelines/blob/master/wip/stacked-borrows.md 
      for further information

    = note: inside `std::option::Option::<std::boxed::Box<fifth::Node<i32>>>::map::<i32, [[email protected]\fifth.rs:31:30: 40:10]>` at \lib\rustlib\src\rust\library\core\src\option.rs:846:18

note: inside `fifth::List::<i32>::pop` at src\fifth.rs:31:9
   --> src\fifth.rs:31:9
    |
31  | /         self.head.take().map(|head| {
32  | |             let head = *head;
33  | |             self.head = head.next;
34  | |
...   |
39  | |             head.elem
40  | |         })
    | |__________^
note: inside `fifth::test::basics` at src\fifth.rs:74:20
   --> src\fifth.rs:74:20
    |
74  |         assert_eq!(list.pop(), Some(1));
    |                    ^^^^^^^^^^
note: inside closure at src\fifth.rs:62:5
   --> src\fifth.rs:62:5
    |
61  |       #[test]
    |       ------- in this procedural macro expansion
62  | /     fn basics() {
63  | |         let mut list = List::new();
64  | |
65  | |         // Check empty list behaves right
...   |
96  | |         assert_eq!(list.pop(), None);
97  | |     }
    | |_____^
 ...
error: aborting due to previous error

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。

果然不靠谱...还是得靠自己,首先得了解下何为 miri

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。

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。

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:

  • 内存越界检查和内存释放后再使用(use-after-free)
  • 使用未初始化的数据
  • 数据竞争
  • 内存对齐问题

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。

总之,miri 的使用很简单:

$ cargo +nightly-2022-01-21 miri test

下面来看看具体的错误:

error: Undefined Behavior: trying to reborrow for Unique at alloc84055, but parent tag <209678> does not have an appropriate item in the borrow stack

   --> \lib\rustlib\src\rust\library\core\src\option.rs:846:18
    |
846 |             Some(x) => Some(f(x)),
    |                  ^ trying to reborrow for Unique at alloc84055, 
    |                    but parent tag <209678> does not have an 
    |                    appropriate item in the borrow stack
    |

    = help: this indicates a potential bug in the program: it 
      performed an invalid operation, but the rules it 
      violated are still experimental
    
    = help: see 
      https://github.com/rust-lang/unsafe-code-guidelines/blob/master/wip/stacked-borrows.md 
      for further information

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