时间戳

首页 > 解决方案 > 改变不可变的局部变量是未定义的行为吗？

rust - 改变不可变的局部变量是未定义的行为吗？

问题描述

Rust Reference似乎说改变不可变的本地数据（不在 a 内UnsafeCell）是未定义的行为：

被认为未定义的行为

  • 改变不可变的数据。const 项内的所有数据都是不可变的。此外，通过共享引用获得的所有数据或不可变绑定拥有的数据都是不可变的，除非该数据包含在UnsafeCell<U>.

以下代码通过将不可变的局部变量重新解释为AtomicU32. 目前代码运行得很好并打印了预期的结果，但它的行为实际上是未定义的吗？

use std::sync::atomic::{AtomicU32, Ordering};

#[repr(C, align(4))]
struct Bytes([u8; 4]);

fn main() {
    let bytes = Bytes([11; 4]);
    let x = unsafe { &*(&bytes as *const Bytes as *const AtomicU32) };
    x.store(12345, Ordering::SeqCst);
    println!("{:?}", bytes.0); // [57, 48, 0, 0]
}

Miri 没有抱怨下面的代码示例，其中字节是可变的。由于这些&AtomicU32字节是通过共享引用（“改变不可变数据[被认为是未定义的行为]”。

use std::sync::atomic::{AtomicU32, Ordering};

#[repr(C, align(4))]
struct Bytes([u8; 4]);

fn main() {
    let mut bytes = Bytes([11; 4]);
    let x = unsafe { &*(&mut bytes as *mut Bytes as *const AtomicU32) };
    x.store(12345, Ordering::SeqCst);
    println!("{:?}", bytes.0); // [57, 48, 0, 0]
}

标签： rustimmutabilityundefined-behavior

解决方案

是的，根据 Miri 的说法：

error: Undefined Behavior: trying to reborrow for SharedReadWrite at alloc1377, but parent tag <untagged> does not have an appropriate item in the borrow stack
 --> src/main.rs:8:22
  |
8 |     let x = unsafe { &*(&bytes as *const Bytes as *const AtomicU32) };
  |                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ trying to reborrow for SharedReadWrite at alloc1377, but parent tag <untagged> 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 `main` at src/main.rs:8:22
  = note: inside `<fn() as std::ops::FnOnce<()>>::call_once - shim(fn())` at /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/ops/function.rs:227:5
  = note: inside `std::sys_common::backtrace::__rust_begin_short_backtrace::<fn(), ()>` at /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/sys_common/backtrace.rs:125:18
  = note: inside closure at /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/rt.rs:66:18
  = note: inside `std::ops::function::impls::<impl std::ops::FnOnce<()> for &dyn std::ops::Fn() -> i32 + std::marker::Sync + std::panic::RefUnwindSafe>::call_once` at /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/ops/function.rs:259:13
  = note: inside `std::panicking::r#try::do_call::<&dyn std::ops::Fn() -> i32 + std::marker::Sync + std::panic::RefUnwindSafe, i32>` at /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/panicking.rs:379:40
  = note: inside `std::panicking::r#try::<i32, &dyn std::ops::Fn() -> i32 + std::marker::Sync + std::panic::RefUnwindSafe>` at /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/panicking.rs:343:19
  = note: inside `std::panic::catch_unwind::<&dyn std::ops::Fn() -> i32 + std::marker::Sync + std::panic::RefUnwindSafe, i32>` at /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/panic.rs:396:14
  = note: inside `std::rt::lang_start_internal` at /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/rt.rs:51:25
  = note: inside `std::rt::lang_start::<()>` at /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/rt.rs:65:5

也可以看看：

因为这些字节正在通过共享引用 ( &AtomicU32)

AtomicU32包含一个UnsafeCell，因此它符合您引用的豁免标准：

        pub struct $atomic_type {
            v: UnsafeCell<$int_type>,
        }

这是 API 的一部分AtomicU32::from_mut，不需要unsafe.

推荐阅读