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<!doctype html><html lang=en><head><title>Box Is a Unique Type :: nilstriebs blog</title><meta charset=utf-8><meta name=viewport content="width=device-width,initial-scale=1"><meta name=description content="About better aliasing semantics for `Box`"><meta name=keywords content="box,noalias"><meta name=robots content="noodp"><link rel=canonical href=/posts/box-is-a-unique-type/><link rel=stylesheet href=/assets/style.css><link rel=apple-touch-icon href=/img/apple-touch-icon-192x192.png><link rel="shortcut icon" href=/img/favicon/orange.png><meta name=twitter:card content="summary"><meta property="og:locale" content="en"><meta property="og:type" content="article"><meta property="og:title" content="Box Is a Unique Type"><meta property="og:description" content="About better aliasing semantics for `Box`"><meta property="og:url" content="/posts/box-is-a-unique-type/"><meta property="og:site_name" content="nilstriebs blog"><meta property="og:image" content="/"><meta property="og:image:width" content="2048"><meta property="og:image:height" content="1024"><meta property="article:published_time" content="2022-07-22 00:00:00 +0000 UTC"></head><body class=orange><div class="container center headings--one-size"><header class=header><div class=header__inner><div class=header__logo><a href=/><div class=logo>nilstriebs blog</div></a></div></div></header><div class=content><div class=post><h1 class=post-title><a href=/posts/box-is-a-unique-type/>Box Is a Unique Type</a></h1><div class=post-meta><span class=post-date>2022-07-22</span>
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<span class=post-author>:: Nilstrieb</span>
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<span class=post-reading-time>:: 10 min read (2080 words)</span></div><span class=post-tags>#<a href=/tags/rust/>rust</a>
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<span class=post-reading-time>:: 10 min read (2081 words)</span></div><span class=post-tags>#<a href=/tags/rust/>rust</a>
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#<a href=/tags/unsafe-code/>unsafe code</a> </span><div class=post-content><div><p>We have all used <code>Box<T></code> before in our Rust code. It’s a glorious type, with great ergonomics
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and flexibitility. We can use it to put our values on the heap, but it can do even more
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than that!</p><div class=highlight><pre tabindex=0 style=color:#f8f8f2;background-color:#272822;-moz-tab-size:4;-o-tab-size:4;tab-size:4><code class=language-rust data-lang=rust><span style=display:flex><span><span style=color:#66d9ef>struct</span> <span style=color:#a6e22e>Fields</span> {
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@ -43,14 +43,14 @@ well here. But what allows it to make this optimization? Taking a look at the ge
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reveals the solution: (severely shortened to only show the relevant parts)</p><pre tabindex=0><code class=language-llvmir data-lang=llvmir>define void @takes_box_and_ptr_to_it(i8* noalias %0, i8* %ptr) {
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</code></pre><p>See the little attribute on the first parameter called <code>noalias</code>? That’s what’s doing the magic here.
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<code>noalias</code> is an LLVM attribute on pointers that allows for various optimizations. If there are two pointers,
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and at least one of them is <code>noalias</code>, there are some restrictions around the two:</p><ul><li>If one of them writes, they must not point to the same value (alias each other)</li><li>If neither of them writes, they can alias just fine.
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and at least one of them is <code>noalias</code>, there are some restrictions around the two. Approximately:</p><ul><li>If one of them writes, they must not point to the same value (alias each other)</li><li>If neither of them writes, they can alias just fine.
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Therefore, we also apply <code>noalias</code> to <code>&mut T</code> and <code>&T</code> (if it doesn’t contain interior mutability through
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<code>UnsafeCell<T></code>, since they uphold these rules.</li></ul><p>This might sound familiar to you if you’re a viewer of <a href=https://twitter.com/jonhoo>Jon Gjengset</a>’s content (which I can highly recommend). Jon has made an entire video about this before, since his crate <code>left-right</code>
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was affected by this (<a href=https://youtu.be/EY7Wi9fV5bk)>https://youtu.be/EY7Wi9fV5bk)</a>.</p><p>If you’re looking for <em>any</em> hint that using box emits <code>noalias</code>, you have to look no further than the documentation
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for <a href=https://doc.rust-lang.org/nightly/std/boxed/index.html#considerations-for-unsafe-code><code>std::boxed</code></a>. Well, the nightly or beta docs, because I only added this section very recently. For years, this behaviour was not really documented, and you had to
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belong to the arcane circles of the select few who were aware of it. So lots of code was written thinking that box was “just an
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RAII pointer” (a pointer that allocates the value in the constructor, and deallocates it in the destructor on drop) for all
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pointers are concerned.</p><h1 id=stacked-borrows-and-miri>Stacked Borrows and Miri<a href=#stacked-borrows-and-miri class=hanchor arialabel=Anchor>⌗</a></h1><p><a href=https://github.com/rust-lang/miri>Miri</a> is an interpreter for Rust code with the goal of finding undefinde behaviour.
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pointers are concerned.</p><h1 id=stacked-borrows-and-miri>Stacked Borrows and Miri<a href=#stacked-borrows-and-miri class=hanchor arialabel=Anchor>⌗</a></h1><p><a href=https://github.com/rust-lang/miri>Miri</a> is an interpreter for Rust code with the goal of finding undefined behaviour.
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Undefined behaviour, UB for short, is behaviour of a program upon which no restrictions are imposed. If UB is executed,
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<em>anything</em> can happen, including segmentation faults, silent memory corruption, leakage of private keys or exactly
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what you intended to happen. Examples of UB include use-after-free, out of bounds reads or data races.</p><p>I cannot recommend Miri highly enough for all unsafe code you’re writing (sadly support for some IO functions
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@ -51,7 +51,7 @@ reveals the solution: (severely shortened to only show the relevant parts)</p
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<pre tabindex="0"><code class="language-llvmir" data-lang="llvmir">define void @takes_box_and_ptr_to_it(i8* noalias %0, i8* %ptr) {
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</code></pre><p>See the little attribute on the first parameter called <code>noalias</code>? That&rsquo;s what&rsquo;s doing the magic here.
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<code>noalias</code> is an LLVM attribute on pointers that allows for various optimizations. If there are two pointers,
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and at least one of them is <code>noalias</code>, there are some restrictions around the two:</p>
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and at least one of them is <code>noalias</code>, there are some restrictions around the two. Approximately:</p>
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<ul>
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<li>If one of them writes, they must not point to the same value (alias each other)</li>
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<li>If neither of them writes, they can alias just fine.
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@ -66,7 +66,7 @@ belong to the arcane circles of the select few who were aware of it. So lots of
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RAII pointer&rdquo; (a pointer that allocates the value in the constructor, and deallocates it in the destructor on drop) for all
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pointers are concerned.</p>
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<h1 id="stacked-borrows-and-miri">Stacked Borrows and Miri</h1>
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<p><a href="https://github.com/rust-lang/miri">Miri</a> is an interpreter for Rust code with the goal of finding undefinde behaviour.
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<p><a href="https://github.com/rust-lang/miri">Miri</a> is an interpreter for Rust code with the goal of finding undefined behaviour.
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Undefined behaviour, UB for short, is behaviour of a program upon which no restrictions are imposed. If UB is executed,
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<em>anything</em> can happen, including segmentation faults, silent memory corruption, leakage of private keys or exactly
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what you intended to happen. Examples of UB include use-after-free, out of bounds reads or data races.</p>
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