Ch. 20: Address soundness issues and introduce Miri #4062
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Note: this requires Rust 1.82.0, and will be easiest to merge after that version is stabilized in two weeks. Since it is blocked on that anyway, I am also basing it on top of the listing changes.
- Add `SAFETY` documentation on the unsafe function and comments on the unsafe invocation in the code samples. - Discuss the soundness issues in more depth and explain the idiomatic use of those `SAFETY` comments.
We no longer get the raw pointers from references, although we *could*, because we can now use the raw pointer operator rather than an `as` cast and thus can get them directly from a variable in scope.
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Fixes rust-lang#4338 Wrong concept in subsection "Using Miri to check unsafe code" of section "20.1. Unsafe Rust" This is a followup to PR rust-lang#4062 This PR adds a working example of using Miri where the code will compile and run without warnings or errors, but with undefined behavior while it runs. After running it through Miri though it correctly identifies the data race. Feedback welcome on matching the text to the books style. Or if anyone wants to collaborate on other examples of Miri output.
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This makes three major changes to the unsafe section:
It uses the newly-stabilized1
&rawborrow operator to more safely get raw pointers, with&raw constand&raw mutrespectively. These provide a safe way of getting raw pointers. These are part of the Rust effort to handle provenance correctly and thereby make unsafe safer and easier to work with—and while we’re not going to get into those details, this is definitely a better way to work than the castas *const i32andas *mut i32.It updates the
static mut COUNTERexample to use anunsafe fninstead of a safe function around anunsafeblock, since it is necessary for the caller to guarantee that the function is not called from multiple threads. To make the existing safe function actually safe, it would need to introduce some kind of locking mechanism, I think. Leaving it as an unsafe function gives us a nice opportunity to include// SAFETY: …comments, though, and thus to teach a bit more about idiomatic authoring and usage of unsafe code.It introduces Miri at the end of the section! I used Miri to investigate some of the issues folks had flagged up, and credit to the Miri team: it is very easy to use. The main thing I think we should think about here is whether we need more prose or explanation around installing nightly Rust.
Footnotes
as of Rust 1.82.0, which landed on 2024/10/16 ↩