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href=https://github.com/bspeice target=_blank rel="noopener noreferrer" class=authorSocialLink_owbf title=GitHub><svg viewBox="0 0 256 250" width=1em height=1em class="authorSocialLink_owbf githubSvg_Uu4N" style=--dark:#000;--light:#fff preserveAspectRatio=xMidYMid><path d="M128.001 0C57.317 0 0 57.307 0 128.001c0 56.554 36.676 104.535 87.535 121.46 6.397 1.185 8.746-2.777 8.746-6.158 0-3.052-.12-13.135-.174-23.83-35.61 7.742-43.124-15.103-43.124-15.103-5.823-14.795-14.213-18.73-14.213-18.73-11.613-7.944.876-7.78.876-7.78 12.853.902 19.621 13.19 19.621 13.19 11.417 19.568 29.945 13.911 37.249 10.64 1.149-8.272 4.466-13.92 8.127-17.116-28.431-3.236-58.318-14.212-58.318-63.258 0-13.975 5-25.394 13.188-34.358-1.329-3.224-5.71-16.242 1.24-33.874 0 0 10.749-3.44 35.21 13.121 10.21-2.836 21.16-4.258 32.038-4.307 10.878.049 21.837 1.47 32.066 4.307 24.431-16.56 35.165-13.12 35.165-13.12 6.967 17.63 2.584 30.65 1.255 33.873 8.207 8.964 13.173 20.383 13.173 34.358 0 49.163-29.944 59.988-58.447 63.157 4.591 3.972 8.682 11.762 8.682 23.704 0 17.126-.148 30.91-.148 35.126 0 3.407 2.304 7.398 8.792 6.14C219.37 232.5 256 184.537 256 128.002 256 57.307 198.691 0 128.001 0Zm-80.06 182.34c-.282.636-1.283.827-2.194.39-.929-.417-1.45-1.284-1.15-1.922.276-.655 1.279-.838 2.205-.399.93.418 1.46 1.293 1.139 1.931Zm6.296 5.618c-.61.566-1.804.303-2.614-.591-.837-.892-.994-2.086-.375-2.66.63-.566 1.787-.301 2.626.591.838.903 1 2.088.363 2.66Zm4.32 7.188c-.785.545-2.067.034-2.86-1.104-.784-1.138-.784-2.503.017-3.05.795-.547 2.058-.055 2.861 1.075.782 1.157.782 2.522-.019 3.08Zm7.304 8.325c-.701.774-2.196.566-3.29-.49-1.119-1.032-1.43-2.496-.726-3.27.71-.776 2.213-.558 3.315.49 1.11 1.03 1.45 2.505.701 3.27Zm9.442 2.81c-.31 1.003-1.75 1.459-3.199 1.033-1.448-.439-2.395-1.613-2.103-2.626.301-1.01 1.747-1.484 3.207-1.028 1.446.436 2.396 1.602 2.095 2.622Zm10.744 1.193c.036 1.055-1.193 1.93-2.715 1.95-1.53.034-2.769-.82-2.786-1.86 0-1.065 1.202-1.932 2.733-1.958 1.522-.03 2.768.818 2.768 1.868Zm10.555-.405c.182 1.03-.875 2.088-2.387 2.37-1.485.271-2.861-.365-3.05-1.386-.184-1.056.893-2.114 2.376-2.387 1.514-.263 2.868.356 3.061 1.403Z"/></svg></a></div></div></div></div></div></header><div id=__blog-post-container class=markdown><p>I think it's part of the human condition to ignore perfectly good advice when it comes our way. A
bit over a month ago, I was dispensing sage wisdom for the ages:</p>
<blockquote>
<p>I had a really great idea: build a custom allocator that allows you to track your own allocations.
I gave it a shot, but learned very quickly: <strong>never write your own allocator.</strong></p>
<p>-- <a href=/2018/10/case-study-optimization>me</a></p>
</blockquote>
<p>I proceeded to ignore it, because we never really learn from our mistakes.</p>
<p>There's another part of the human condition that derives joy from seeing things explode.</p>
<center><p><img decoding=async loading=lazy alt=Explosions src=/assets/images/watch-the-world-burn-630e740c91d090f5790a3f4e103f1142.webp width=400 height=280 class=img_ev3q></center>
<p>And <em>that's</em> the part I'm going to focus on.</p>
<h2 class="anchor anchorWithStickyNavbar_LWe7" id=why-an-allocator>Why an Allocator?<a href=#why-an-allocator class=hash-link aria-label="Direct link to Why an Allocator?" title="Direct link to Why an Allocator?"></a></h2>
<p>So why, after complaining about allocators, would I still want to write one? There are three reasons
for that:</p>
<ol>
<li>Allocation/dropping is slow</li>
<li>It's difficult to know exactly when Rust will allocate or drop, especially when using code that
you did not write</li>
<li>I want automated tools to verify behavior, instead of inspecting by hand</li>
</ol>
<p>When I say "slow," it's important to define the terms. If you're writing web applications, you'll
spend orders of magnitude more time waiting for the database than you will the allocator. However,
there's still plenty of code where micro- or nano-seconds matter; think
<a href="https://www.youtube.com/watch?v=NH1Tta7purM" target=_blank rel="noopener noreferrer">finance</a>,
<a href=https://www.reddit.com/r/rust/comments/9hg7yj/synthesizer_progress_update/e6c291f target=_blank rel="noopener noreferrer">real-time audio</a>,
<a href=https://polysync.io/blog/session-types-for-hearty-codecs/ target=_blank rel="noopener noreferrer">self-driving cars</a>, and
<a href=https://carllerche.github.io/bytes/bytes/index.html target=_blank rel="noopener noreferrer">networking</a>. In these situations it's simply
unacceptable for you to spend time doing things that are not your program, and waiting on the
allocator is not cool.</p>
<p>As I continue to learn Rust, it's difficult for me to predict where exactly allocations will happen.
So, I propose we play a quick trivia game: <strong>Does this code invoke the allocator?</strong></p>
<h3 class="anchor anchorWithStickyNavbar_LWe7" id=example-1>Example 1<a href=#example-1 class=hash-link aria-label="Direct link to Example 1" title="Direct link to Example 1"></a></h3>
<div class="language-rust codeBlockContainer_Ckt0 theme-code-block" style="--prism-background-color:hsl(230, 1%, 98%);--prism-color:hsl(230, 8%, 24%)"><div class=codeBlockContent_biex><pre tabindex=0 class="prism-code language-rust codeBlock_bY9V thin-scrollbar" style="background-color:hsl(230, 1%, 98%);color:hsl(230, 8%, 24%)"><code class=codeBlockLines_e6Vv><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token keyword" style="color:hsl(301, 63%, 40%)">fn</span><span class="token plain"> </span><span class="token function-definition function" style="color:hsl(221, 87%, 60%)">my_function</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token plain"> </span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">{</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain">    </span><span class="token keyword" style="color:hsl(301, 63%, 40%)">let</span><span class="token plain"> v</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">:</span><span class="token plain"> </span><span class="token class-name" style="color:hsl(35, 99%, 36%)">Vec</span><span class="token operator" style="color:hsl(221, 87%, 60%)">&lt;</span><span class="token keyword" style="color:hsl(301, 63%, 40%)">u8</span><span class="token operator" style="color:hsl(221, 87%, 60%)">></span><span class="token plain"> </span><span class="token operator" style="color:hsl(221, 87%, 60%)">=</span><span class="token plain"> </span><span class="token class-name" style="color:hsl(35, 99%, 36%)">Vec</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">::</span><span class="token function" style="color:hsl(221, 87%, 60%)">new</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">;</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">}</span><br></span></code></pre><div class=buttonGroup__atx><button type=button aria-label="Copy code to clipboard" title=Copy class=clean-btn><span class=copyButtonIcons_eSgA aria-hidden=true><svg viewBox="0 0 24 24" class=copyButtonIcon_y97N><path fill=currentColor d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"/></svg><svg viewBox="0 0 24 24" class=copyButtonSuccessIcon_LjdS><path fill=currentColor d=M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z /></svg></span></button></div></div></div>
<p><strong>No</strong>: Rust <a href=https://doc.rust-lang.org/std/mem/fn.size_of.html target=_blank rel="noopener noreferrer">knows how big</a> the <code>Vec</code> type is,
and reserves a fixed amount of memory on the stack for the <code>v</code> vector. However, if we wanted to
reserve extra space (using <code>Vec::with_capacity</code>) the allocator would get invoked.</p>
<h3 class="anchor anchorWithStickyNavbar_LWe7" id=example-2>Example 2<a href=#example-2 class=hash-link aria-label="Direct link to Example 2" title="Direct link to Example 2"></a></h3>
<div class="language-rust codeBlockContainer_Ckt0 theme-code-block" style="--prism-background-color:hsl(230, 1%, 98%);--prism-color:hsl(230, 8%, 24%)"><div class=codeBlockContent_biex><pre tabindex=0 class="prism-code language-rust codeBlock_bY9V thin-scrollbar" style="background-color:hsl(230, 1%, 98%);color:hsl(230, 8%, 24%)"><code class=codeBlockLines_e6Vv><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token keyword" style="color:hsl(301, 63%, 40%)">fn</span><span class="token plain"> </span><span class="token function-definition function" style="color:hsl(221, 87%, 60%)">my_function</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token plain"> </span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">{</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain">    </span><span class="token keyword" style="color:hsl(301, 63%, 40%)">let</span><span class="token plain"> v</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">:</span><span class="token plain"> </span><span class="token class-name" style="color:hsl(35, 99%, 36%)">Box</span><span class="token operator" style="color:hsl(221, 87%, 60%)">&lt;</span><span class="token class-name" style="color:hsl(35, 99%, 36%)">Vec</span><span class="token operator" style="color:hsl(221, 87%, 60%)">&lt;</span><span class="token keyword" style="color:hsl(301, 63%, 40%)">u8</span><span class="token operator" style="color:hsl(221, 87%, 60%)">>></span><span class="token plain"> </span><span class="token operator" style="color:hsl(221, 87%, 60%)">=</span><span class="token plain"> </span><span class="token class-name" style="color:hsl(35, 99%, 36%)">Box</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">::</span><span class="token function" style="color:hsl(221, 87%, 60%)">new</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token class-name" style="color:hsl(35, 99%, 36%)">Vec</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">::</span><span class="token function" style="color:hsl(221, 87%, 60%)">new</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">;</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">}</span><br></span></code></pre><div class=buttonGroup__atx><button type=button aria-label="Copy code to clipboard" title=Copy class=clean-btn><span class=copyButtonIcons_eSgA aria-hidden=true><svg viewBox="0 0 24 24" class=copyButtonIcon_y97N><path fill=currentColor d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"/></svg><svg viewBox="0 0 24 24" class=copyButtonSuccessIcon_LjdS><path fill=currentColor d=M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z /></svg></span></button></div></div></div>
<p><strong>Yes</strong>: Because Boxes allow us to work with things that are of unknown size, it has to allocate on
the heap. While the <code>Box</code> is unnecessary in this snippet (release builds will optimize out the
allocation), reserving heap space more generally is needed to pass a dynamically sized type to
another function.</p>
<h3 class="anchor anchorWithStickyNavbar_LWe7" id=example-3>Example 3<a href=#example-3 class=hash-link aria-label="Direct link to Example 3" title="Direct link to Example 3"></a></h3>
<div class="language-rust codeBlockContainer_Ckt0 theme-code-block" style="--prism-background-color:hsl(230, 1%, 98%);--prism-color:hsl(230, 8%, 24%)"><div class=codeBlockContent_biex><pre tabindex=0 class="prism-code language-rust codeBlock_bY9V thin-scrollbar" style="background-color:hsl(230, 1%, 98%);color:hsl(230, 8%, 24%)"><code class=codeBlockLines_e6Vv><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token keyword" style="color:hsl(301, 63%, 40%)">fn</span><span class="token plain"> </span><span class="token function-definition function" style="color:hsl(221, 87%, 60%)">my_function</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token plain">v</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">:</span><span class="token plain"> </span><span class="token class-name" style="color:hsl(35, 99%, 36%)">Vec</span><span class="token operator" style="color:hsl(221, 87%, 60%)">&lt;</span><span class="token keyword" style="color:hsl(301, 63%, 40%)">u8</span><span class="token operator" style="color:hsl(221, 87%, 60%)">></span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token plain"> </span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">{</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain">    v</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">.</span><span class="token function" style="color:hsl(221, 87%, 60%)">push</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token number" style="color:hsl(35, 99%, 36%)">5</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">;</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">}</span><br></span></code></pre><div class=buttonGroup__atx><button type=button aria-label="Copy code to clipboard" title=Copy class=clean-btn><span class=copyButtonIcons_eSgA aria-hidden=true><svg viewBox="0 0 24 24" class=copyButtonIcon_y97N><path fill=currentColor d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"/></svg><svg viewBox="0 0 24 24" class=copyButtonSuccessIcon_LjdS><path fill=currentColor d=M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z /></svg></span></button></div></div></div>
<p><strong>Maybe</strong>: Depending on whether the Vector we were given has space available, we may or may not
allocate. Especially when dealing with code that you did not author, it's difficult to verify that
things behave as you expect them to.</p>
<h2 class="anchor anchorWithStickyNavbar_LWe7" id=blowing-things-up>Blowing Things Up<a href=#blowing-things-up class=hash-link aria-label="Direct link to Blowing Things Up" title="Direct link to Blowing Things Up"></a></h2>
<p>So, how exactly does QADAPT solve these problems? <strong>Whenever an allocation or drop occurs in code
marked allocation-safe, QADAPT triggers a thread panic.</strong> We don't want to let the program continue
as if nothing strange happened, <em>we want things to explode</em>.</p>
<p>However, you don't want code to panic in production because of circumstances you didn't predict.
Just like <a href=https://doc.rust-lang.org/std/macro.debug_assert.html target=_blank rel="noopener noreferrer"><code>debug_assert!</code></a>, <strong>QADAPT will
strip out its own code when building in release mode to guarantee no panics and no performance
impact.</strong></p>
<p>Finally, there are three ways to have QADAPT check that your code will not invoke the allocator:</p>
<h3 class="anchor anchorWithStickyNavbar_LWe7" id=using-a-procedural-macro>Using a procedural macro<a href=#using-a-procedural-macro class=hash-link aria-label="Direct link to Using a procedural macro" title="Direct link to Using a procedural macro"></a></h3>
<p>The easiest method, watch an entire function for allocator invocation:</p>
<div class="language-rust codeBlockContainer_Ckt0 theme-code-block" style="--prism-background-color:hsl(230, 1%, 98%);--prism-color:hsl(230, 8%, 24%)"><div class=codeBlockContent_biex><pre tabindex=0 class="prism-code language-rust codeBlock_bY9V thin-scrollbar" style="background-color:hsl(230, 1%, 98%);color:hsl(230, 8%, 24%)"><code class=codeBlockLines_e6Vv><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token keyword" style="color:hsl(301, 63%, 40%)">use</span><span class="token plain"> </span><span class="token namespace">qadapt</span><span class="token namespace punctuation" style="color:hsl(119, 34%, 47%)">::</span><span class="token plain">no_alloc</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">;</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token keyword" style="color:hsl(301, 63%, 40%)">use</span><span class="token plain"> </span><span class="token namespace">qadapt</span><span class="token namespace punctuation" style="color:hsl(119, 34%, 47%)">::</span><span class="token constant" style="color:hsl(35, 99%, 36%)">QADAPT</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">;</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain" style=display:inline-block></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token attribute attr-name" style="color:hsl(35, 99%, 36%)">#[global_allocator]</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token keyword" style="color:hsl(301, 63%, 40%)">static</span><span class="token plain"> </span><span class="token class-name" style="color:hsl(35, 99%, 36%)">Q</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">:</span><span class="token plain"> </span><span class="token constant" style="color:hsl(35, 99%, 36%)">QADAPT</span><span class="token plain"> </span><span class="token operator" style="color:hsl(221, 87%, 60%)">=</span><span class="token plain"> </span><span class="token constant" style="color:hsl(35, 99%, 36%)">QADAPT</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">;</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain" style=display:inline-block></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token attribute attr-name" style="color:hsl(35, 99%, 36%)">#[no_alloc]</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token keyword" style="color:hsl(301, 63%, 40%)">fn</span><span class="token plain"> </span><span class="token function-definition function" style="color:hsl(221, 87%, 60%)">push_vec</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token plain">v</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">:</span><span class="token plain"> </span><span class="token operator" style="color:hsl(221, 87%, 60%)">&</span><span class="token keyword" style="color:hsl(301, 63%, 40%)">mut</span><span class="token plain"> </span><span class="token class-name" style="color:hsl(35, 99%, 36%)">Vec</span><span class="token operator" style="color:hsl(221, 87%, 60%)">&lt;</span><span class="token keyword" style="color:hsl(301, 63%, 40%)">u8</span><span class="token operator" style="color:hsl(221, 87%, 60%)">></span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token plain"> </span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">{</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain">    </span><span class="token comment" style="color:hsl(230, 4%, 64%)">// This triggers a panic if v.len() == v.capacity()</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain">    v</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">.</span><span class="token function" style="color:hsl(221, 87%, 60%)">push</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token number" style="color:hsl(35, 99%, 36%)">5</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">;</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">}</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain" style=display:inline-block></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token keyword" style="color:hsl(301, 63%, 40%)">fn</span><span class="token plain"> </span><span class="token function-definition function" style="color:hsl(221, 87%, 60%)">main</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token plain"> </span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">{</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain">    </span><span class="token keyword" style="color:hsl(301, 63%, 40%)">let</span><span class="token plain"> v </span><span class="token operator" style="color:hsl(221, 87%, 60%)">=</span><span class="token plain"> </span><span class="token class-name" style="color:hsl(35, 99%, 36%)">Vec</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">::</span><span class="token function" style="color:hsl(221, 87%, 60%)">with_capacity</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token number" style="color:hsl(35, 99%, 36%)">1</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">;</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain" style=display:inline-block></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain">    </span><span class="token comment" style="color:hsl(230, 4%, 64%)">// This will *not* trigger a panic</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain">    </span><span class="token function" style="color:hsl(221, 87%, 60%)">push_vec</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token operator" style="color:hsl(221, 87%, 60%)">&</span><span class="token plain">v</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">;</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain" style=display:inline-block></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain">    </span><span class="token comment" style="color:hsl(230, 4%, 64%)">// This *will* trigger a panic</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain">    </span><span class="token function" style="color:hsl(221, 87%, 60%)">push_vec</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token operator" style="color:hsl(221, 87%, 60%)">&</span><span class="token plain">v</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">;</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">}</span><br></span></code></pre><div class=buttonGroup__atx><button type=button aria-label="Copy code to clipboard" title=Copy class=clean-btn><span class=copyButtonIcons_eSgA aria-hidden=true><svg viewBox="0 0 24 24" class=copyButtonIcon_y97N><path fill=currentColor d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"/></svg><svg viewBox="0 0 24 24" class=copyButtonSuccessIcon_LjdS><path fill=currentColor d=M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z /></svg></span></button></div></div></div>
<h3 class="anchor anchorWithStickyNavbar_LWe7" id=using-a-regular-macro>Using a regular macro<a href=#using-a-regular-macro class=hash-link aria-label="Direct link to Using a regular macro" title="Direct link to Using a regular macro"></a></h3>
<p>For times when you need more precision:</p>
<div class="language-rust codeBlockContainer_Ckt0 theme-code-block" style="--prism-background-color:hsl(230, 1%, 98%);--prism-color:hsl(230, 8%, 24%)"><div class=codeBlockContent_biex><pre tabindex=0 class="prism-code language-rust codeBlock_bY9V thin-scrollbar" style="background-color:hsl(230, 1%, 98%);color:hsl(230, 8%, 24%)"><code class=codeBlockLines_e6Vv><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token keyword" style="color:hsl(301, 63%, 40%)">use</span><span class="token plain"> </span><span class="token namespace">qadapt</span><span class="token namespace punctuation" style="color:hsl(119, 34%, 47%)">::</span><span class="token plain">assert_no_alloc</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">;</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token keyword" style="color:hsl(301, 63%, 40%)">use</span><span class="token plain"> </span><span class="token namespace">qadapt</span><span class="token namespace punctuation" style="color:hsl(119, 34%, 47%)">::</span><span class="token constant" style="color:hsl(35, 99%, 36%)">QADAPT</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">;</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain" style=display:inline-block></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token attribute attr-name" style="color:hsl(35, 99%, 36%)">#[global_allocator]</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token keyword" style="color:hsl(301, 63%, 40%)">static</span><span class="token plain"> </span><span class="token class-name" style="color:hsl(35, 99%, 36%)">Q</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">:</span><span class="token plain"> </span><span class="token constant" style="color:hsl(35, 99%, 36%)">QADAPT</span><span class="token plain"> </span><span class="token operator" style="color:hsl(221, 87%, 60%)">=</span><span class="token plain"> </span><span class="token constant" style="color:hsl(35, 99%, 36%)">QADAPT</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">;</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain" style=display:inline-block></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token keyword" style="color:hsl(301, 63%, 40%)">fn</span><span class="token plain"> </span><span class="token function-definition function" style="color:hsl(221, 87%, 60%)">main</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token plain"> </span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">{</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain">    </span><span class="token keyword" style="color:hsl(301, 63%, 40%)">let</span><span class="token plain"> v </span><span class="token operator" style="color:hsl(221, 87%, 60%)">=</span><span class="token plain"> </span><span class="token class-name" style="color:hsl(35, 99%, 36%)">Vec</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">::</span><span class="token function" style="color:hsl(221, 87%, 60%)">with_capacity</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token number" style="color:hsl(35, 99%, 36%)">1</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">;</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain" style=display:inline-block></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain">    </span><span class="token comment" style="color:hsl(230, 4%, 64%)">// No allocations here, we already have space reserved</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain">    </span><span class="token macro property" style="color:hsl(5, 74%, 59%)">assert_no_alloc!</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token plain">v</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">.</span><span class="token function" style="color:hsl(221, 87%, 60%)">push</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token number" style="color:hsl(35, 99%, 36%)">5</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">;</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain" style=display:inline-block></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain">    </span><span class="token comment" style="color:hsl(230, 4%, 64%)">// Even though we remove an item, it doesn't trigger a drop</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain">    </span><span class="token comment" style="color:hsl(230, 4%, 64%)">// because it's a scalar. If it were a `Box&lt;_>` type,</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain">    </span><span class="token comment" style="color:hsl(230, 4%, 64%)">// a drop would trigger.</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain">    </span><span class="token macro property" style="color:hsl(5, 74%, 59%)">assert_no_alloc!</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">{</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain">        v</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">.</span><span class="token function" style="color:hsl(221, 87%, 60%)">pop</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">.</span><span class="token function" style="color:hsl(221, 87%, 60%)">unwrap</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">;</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain">    </span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">}</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">;</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">}</span><br></span></code></pre><div class=buttonGroup__atx><button type=button aria-label="Copy code to clipboard" title=Copy class=clean-btn><span class=copyButtonIcons_eSgA aria-hidden=true><svg viewBox="0 0 24 24" class=copyButtonIcon_y97N><path fill=currentColor d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"/></svg><svg viewBox="0 0 24 24" class=copyButtonSuccessIcon_LjdS><path fill=currentColor d=M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z /></svg></span></button></div></div></div>
<h3 class="anchor anchorWithStickyNavbar_LWe7" id=using-function-calls>Using function calls<a href=#using-function-calls class=hash-link aria-label="Direct link to Using function calls" title="Direct link to Using function calls"></a></h3>
<p>Both the most precise and most tedious:</p>
<div class="language-rust codeBlockContainer_Ckt0 theme-code-block" style="--prism-background-color:hsl(230, 1%, 98%);--prism-color:hsl(230, 8%, 24%)"><div class=codeBlockContent_biex><pre tabindex=0 class="prism-code language-rust codeBlock_bY9V thin-scrollbar" style="background-color:hsl(230, 1%, 98%);color:hsl(230, 8%, 24%)"><code class=codeBlockLines_e6Vv><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token keyword" style="color:hsl(301, 63%, 40%)">use</span><span class="token plain"> </span><span class="token namespace">qadapt</span><span class="token namespace punctuation" style="color:hsl(119, 34%, 47%)">::</span><span class="token plain">enter_protected</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">;</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token keyword" style="color:hsl(301, 63%, 40%)">use</span><span class="token plain"> </span><span class="token namespace">qadapt</span><span class="token namespace punctuation" style="color:hsl(119, 34%, 47%)">::</span><span class="token plain">exit_protected</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">;</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token keyword" style="color:hsl(301, 63%, 40%)">use</span><span class="token plain"> </span><span class="token namespace">qadapt</span><span class="token namespace punctuation" style="color:hsl(119, 34%, 47%)">::</span><span class="token constant" style="color:hsl(35, 99%, 36%)">QADAPT</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">;</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain" style=display:inline-block></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token attribute attr-name" style="color:hsl(35, 99%, 36%)">#[global_allocator]</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token keyword" style="color:hsl(301, 63%, 40%)">static</span><span class="token plain"> </span><span class="token class-name" style="color:hsl(35, 99%, 36%)">Q</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">:</span><span class="token plain"> </span><span class="token constant" style="color:hsl(35, 99%, 36%)">QADAPT</span><span class="token plain"> </span><span class="token operator" style="color:hsl(221, 87%, 60%)">=</span><span class="token plain"> </span><span class="token constant" style="color:hsl(35, 99%, 36%)">QADAPT</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">;</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain" style=display:inline-block></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token keyword" style="color:hsl(301, 63%, 40%)">fn</span><span class="token plain"> </span><span class="token function-definition function" style="color:hsl(221, 87%, 60%)">main</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token plain"> </span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">{</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain">    </span><span class="token comment" style="color:hsl(230, 4%, 64%)">// This triggers an allocation (on non-release builds)</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain">    </span><span class="token keyword" style="color:hsl(301, 63%, 40%)">let</span><span class="token plain"> v </span><span class="token operator" style="color:hsl(221, 87%, 60%)">=</span><span class="token plain"> </span><span class="token class-name" style="color:hsl(35, 99%, 36%)">Vec</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">::</span><span class="token function" style="color:hsl(221, 87%, 60%)">with_capacity</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token number" style="color:hsl(35, 99%, 36%)">1</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">;</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain" style=display:inline-block></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain">    </span><span class="token function" style="color:hsl(221, 87%, 60%)">enter_protected</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">;</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain">    </span><span class="token comment" style="color:hsl(230, 4%, 64%)">// This does not trigger an allocation because we've reserved size</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain">    v</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">.</span><span class="token function" style="color:hsl(221, 87%, 60%)">push</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token number" style="color:hsl(35, 99%, 36%)">0</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">;</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain">    </span><span class="token function" style="color:hsl(221, 87%, 60%)">exit_protected</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">;</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain" style=display:inline-block></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain">    </span><span class="token comment" style="color:hsl(230, 4%, 64%)">// This triggers an allocation because we ran out of size,</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain">    </span><span class="token comment" style="color:hsl(230, 4%, 64%)">// but doesn't panic because we're no longer protected.</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain">    v</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">.</span><span class="token function" style="color:hsl(221, 87%, 60%)">push</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token number" style="color:hsl(35, 99%, 36%)">1</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">;</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">}</span><br></span></code></pre><div class=buttonGroup__atx><button type=button aria-label="Copy code to clipboard" title=Copy class=clean-btn><span class=copyButtonIcons_eSgA aria-hidden=true><svg viewBox="0 0 24 24" class=copyButtonIcon_y97N><path fill=currentColor d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"/></svg><svg viewBox="0 0 24 24" class=copyButtonSuccessIcon_LjdS><path fill=currentColor d=M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z /></svg></span></button></div></div></div>
<h3 class="anchor anchorWithStickyNavbar_LWe7" id=caveats>Caveats<a href=#caveats class=hash-link aria-label="Direct link to Caveats" title="Direct link to Caveats"></a></h3>
<p>It's important to point out that QADAPT code is synchronous, so please be careful when mixing in
asynchronous functions:</p>
<div class="language-rust codeBlockContainer_Ckt0 theme-code-block" style="--prism-background-color:hsl(230, 1%, 98%);--prism-color:hsl(230, 8%, 24%)"><div class=codeBlockContent_biex><pre tabindex=0 class="prism-code language-rust codeBlock_bY9V thin-scrollbar" style="background-color:hsl(230, 1%, 98%);color:hsl(230, 8%, 24%)"><code class=codeBlockLines_e6Vv><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token keyword" style="color:hsl(301, 63%, 40%)">use</span><span class="token plain"> </span><span class="token namespace">futures</span><span class="token namespace punctuation" style="color:hsl(119, 34%, 47%)">::</span><span class="token namespace">future</span><span class="token namespace punctuation" style="color:hsl(119, 34%, 47%)">::</span><span class="token class-name" style="color:hsl(35, 99%, 36%)">Future</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">;</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token keyword" style="color:hsl(301, 63%, 40%)">use</span><span class="token plain"> </span><span class="token namespace">futures</span><span class="token namespace punctuation" style="color:hsl(119, 34%, 47%)">::</span><span class="token namespace">future</span><span class="token namespace punctuation" style="color:hsl(119, 34%, 47%)">::</span><span class="token plain">ok</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">;</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain" style=display:inline-block></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token attribute attr-name" style="color:hsl(35, 99%, 36%)">#[no_alloc]</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token keyword" style="color:hsl(301, 63%, 40%)">fn</span><span class="token plain"> </span><span class="token function-definition function" style="color:hsl(221, 87%, 60%)">async_capacity</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token plain"> </span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">-></span><span class="token plain"> </span><span class="token keyword" style="color:hsl(301, 63%, 40%)">impl</span><span class="token plain"> </span><span class="token class-name" style="color:hsl(35, 99%, 36%)">Future</span><span class="token operator" style="color:hsl(221, 87%, 60%)">&lt;</span><span class="token class-name" style="color:hsl(35, 99%, 36%)">Item</span><span class="token operator" style="color:hsl(221, 87%, 60%)">=</span><span class="token class-name" style="color:hsl(35, 99%, 36%)">Vec</span><span class="token operator" style="color:hsl(221, 87%, 60%)">&lt;</span><span class="token keyword" style="color:hsl(301, 63%, 40%)">u8</span><span class="token operator" style="color:hsl(221, 87%, 60%)">></span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">,</span><span class="token plain"> </span><span class="token class-name" style="color:hsl(35, 99%, 36%)">Error</span><span class="token operator" style="color:hsl(221, 87%, 60%)">=</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token operator" style="color:hsl(221, 87%, 60%)">></span><span class="token plain"> </span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">{</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain">    </span><span class="token function" style="color:hsl(221, 87%, 60%)">ok</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token number" style="color:hsl(35, 99%, 36%)">12</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">.</span><span class="token function" style="color:hsl(221, 87%, 60%)">and_then</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token closure-params closure-punctuation punctuation" style="color:hsl(119, 34%, 47%)">|</span><span class="token closure-params">e</span><span class="token closure-params closure-punctuation punctuation" style="color:hsl(119, 34%, 47%)">|</span><span class="token plain"> </span><span class="token class-name" style="color:hsl(35, 99%, 36%)">Ok</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token class-name" style="color:hsl(35, 99%, 36%)">Vec</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">::</span><span class="token function" style="color:hsl(221, 87%, 60%)">with_capacity</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token plain">e</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">}</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain" style=display:inline-block></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token keyword" style="color:hsl(301, 63%, 40%)">fn</span><span class="token plain"> </span><span class="token function-definition function" style="color:hsl(221, 87%, 60%)">main</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token plain"> </span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">{</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain">    </span><span class="token comment" style="color:hsl(230, 4%, 64%)">// This doesn't trigger a panic because the `and_then` closure</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain">    </span><span class="token comment" style="color:hsl(230, 4%, 64%)">// wasn't run during the function call.</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain">    </span><span class="token function" style="color:hsl(221, 87%, 60%)">async_capacity</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">;</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain" style=display:inline-block></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain">    </span><span class="token comment" style="color:hsl(230, 4%, 64%)">// Still no panic</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain">    </span><span class="token macro property" style="color:hsl(5, 74%, 59%)">assert_no_alloc!</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token function" style="color:hsl(221, 87%, 60%)">async_capacity</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">;</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain" style=display:inline-block></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain">    </span><span class="token comment" style="color:hsl(230, 4%, 64%)">// This will panic because the allocation happens during `unwrap`</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain">    </span><span class="token comment" style="color:hsl(230, 4%, 64%)">// in the `assert_no_alloc!` macro</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain">    </span><span class="token macro property" style="color:hsl(5, 74%, 59%)">assert_no_alloc!</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token function" style="color:hsl(221, 87%, 60%)">async_capacity</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">.</span><span class="token function" style="color:hsl(221, 87%, 60%)">poll</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">.</span><span class="token function" style="color:hsl(221, 87%, 60%)">unwrap</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">;</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">}</span><br></span></code></pre><div class=buttonGroup__atx><button type=button aria-label="Copy code to clipboard" title=Copy class=clean-btn><span class=copyButtonIcons_eSgA aria-hidden=true><svg viewBox="0 0 24 24" class=copyButtonIcon_y97N><path fill=currentColor d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"/></svg><svg viewBox="0 0 24 24" class=copyButtonSuccessIcon_LjdS><path fill=currentColor d=M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z /></svg></span></button></div></div></div>
<h2 class="anchor anchorWithStickyNavbar_LWe7" id=conclusion>Conclusion<a href=#conclusion class=hash-link aria-label="Direct link to Conclusion" title="Direct link to Conclusion"></a></h2>
<p>While there's a lot more to writing high-performance code than managing your usage of the allocator,
it's critical that you do use the allocator correctly. QADAPT will verify that your code is doing
what you expect. It's usable even on stable Rust from version 1.31 onward, which isn't the case for
most allocators. Version 1.0 was released today, and you can check it out over at
<a href=https://crates.io/crates/qadapt target=_blank rel="noopener noreferrer">crates.io</a> or on <a href=https://github.com/bspeice/qadapt target=_blank rel="noopener noreferrer">github</a>.</p>
<p>I'm hoping to write more about high-performance Rust in the future, and I expect that QADAPT will
help guide that. If there are topics you're interested in, let me know in the comments below!</div></article><nav class="pagination-nav docusaurus-mt-lg" aria-label="Blog post page navigation"><a class="pagination-nav__link pagination-nav__link--prev" href=/2018/12/what-small-business-really-means><div class=pagination-nav__sublabel>Older post</div><div class=pagination-nav__label>More "what companies really mean"</div></a><a class="pagination-nav__link pagination-nav__link--next" href=/2019/02/understanding-allocations-in-rust><div class=pagination-nav__sublabel>Newer post</div><div class=pagination-nav__label>Allocations in Rust: Foreword</div></a></nav></main><div class="col col--2"><div class="tableOfContents_bqdL thin-scrollbar"><ul class="table-of-contents table-of-contents__left-border"><li><a href=#why-an-allocator class="table-of-contents__link toc-highlight">Why an Allocator?</a><ul><li><a href=#example-1 class="table-of-contents__link toc-highlight">Example 1</a><li><a href=#example-2 class="table-of-contents__link toc-highlight">Example 2</a><li><a href=#example-3 class="table-of-contents__link toc-highlight">Example 3</a></ul><li><a href=#blowing-things-up class="table-of-contents__link toc-highlight">Blowing Things Up</a><ul><li><a href=#using-a-procedural-macro class="table-of-contents__link toc-highlight">Using a procedural macro</a><li><a href=#using-a-regular-macro class="table-of-contents__link toc-highlight">Using a regular macro</a><li><a href=#using-function-calls class="table-of-contents__link toc-highlight">Using function calls</a><li><a href=#caveats class="table-of-contents__link toc-highlight">Caveats</a></ul><li><a href=#conclusion class="table-of-contents__link toc-highlight">Conclusion</a></ul></div></div></div></div></div><footer class=footer><div class="container container-fluid"><div class="footer__bottom text--center"><div class=footer__copyright>Copyright © 2024 Bradlee Speice</div></div></div></footer></div>