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<!DOCTYPE html><html lang="en"><head><meta charset="utf-8"><meta name="viewport" content="width=device-width, initial-scale=1.0"><meta name="generator" content="rustdoc"><meta name="description" content="API documentation for the Rust `externref_heap_live_count` fn in crate `wasm_bindgen`."><meta name="keywords" content="rust, rustlang, rust-lang, externref_heap_live_count"><title>wasm_bindgen::externref_heap_live_count - Rust</title><link rel="stylesheet" type="text/css" href="../normalize.css"><link rel="stylesheet" type="text/css" href="../rustdoc.css" id="mainThemeStyle"><link rel="stylesheet" type="text/css" href="../dark.css"><link rel="stylesheet" type="text/css" href="../light.css" id="themeStyle"><script src="../storage.js"></script><noscript><link rel="stylesheet" href="../noscript.css"></noscript><link rel="shortcut icon" href="../favicon.ico"><style type="text/css">#crate-search{background-image:url("../down-arrow.svg");}</style></head><body class="rustdoc fn"><!--[if lte IE 8]><div class="warning">This old browser is unsupported and will most likely display funky things.</div><![endif]--><nav class="sidebar"><div class="sidebar-menu">☰</div><a href='../wasm_bindgen/index.html'><div class='logo-container'><img src='../rust-logo.png' alt='logo'></div></a><div class="sidebar-elems"><p class='location'><a href='index.html'>wasm_bindgen</a></p><script>window.sidebarCurrent = {name: 'externref_heap_live_count', ty: 'fn', relpath: ''};</script><script defer src="sidebar-items.js"></script></div></nav><div class="theme-picker"><button id="theme-picker" aria-label="Pick another theme!"><img src="../brush.svg" width="18" alt="Pick another theme!"></button><div id="theme-choices"></div></div><script src="../theme.js"></script><nav class="sub"><form class="search-form"><div class="search-container"><div><select id="crate-search"><option value="All crates">All crates</option></select><input class="search-input" name="search" disabled autocomplete="off" spellcheck="false" placeholder="Click or press ‘S’ to search, ‘?’ for more options…" type="search"></div><a id="settings-menu" href="../settings.html"><img src="../wheel.svg" width="18" alt="Change settings"></a></div></form></nav><section id="main" class="content"><h1 class='fqn'><span class='out-of-band'><span id='render-detail'><a id="toggle-all-docs" href="javascript:void(0)" title="collapse all docs">[<span class='inner'>−</span>]</a></span><a class='srclink' href='../src/wasm_bindgen/lib.rs.html#706-708' title='goto source code'>[src]</a></span><span class='in-band'>Function <a href='index.html'>wasm_bindgen</a>::<wbr><a class="fn" href=''>externref_heap_live_count</a></span></h1><pre class='rust fn'>pub fn externref_heap_live_count() -> <a class="primitive" href="https://doc.rust-lang.org/nightly/std/primitive.u32.html">u32</a></pre><div class='docblock'><p>Get the count of live <code>externref</code>s / <code>JsValue</code>s in <code>wasm-bindgen</code>'s heap.</p>
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<h2 id="usage" class="section-header"><a href="#usage">Usage</a></h2>
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<p>This is intended for debugging and writing tests.</p>
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<p>To write a test that asserts against unnecessarily keeping <code>anref</code>s /
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<code>JsValue</code>s alive:</p>
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<ul>
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<li>
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<p>get an initial live count,</p>
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</li>
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<li>
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<p>perform some series of operations or function calls that should clean up
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after themselves, and should not keep holding onto <code>externref</code>s / <code>JsValue</code>s
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after completion,</p>
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</li>
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<li>
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<p>get the final live count,</p>
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</li>
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<li>
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<p>and assert that the initial and final counts are the same.</p>
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</li>
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</ul>
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<h2 id="what-is-counted" class="section-header"><a href="#what-is-counted">What is Counted</a></h2>
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<p>Note that this only counts the <em>owned</em> <code>externref</code>s / <code>JsValue</code>s that end up in
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<code>wasm-bindgen</code>'s heap. It does not count borrowed <code>externref</code>s / <code>JsValue</code>s
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that are on its stack.</p>
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<p>For example, these <code>JsValue</code>s are accounted for:</p>
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<div class='information'><div class='tooltip ignore'>ⓘ<span class='tooltiptext'>This example is not tested</span></div></div><div class="example-wrap"><pre class="rust rust-example-rendered ignore">
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<span class="attribute">#[<span class="ident">wasm_bindgen</span>]</span>
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<span class="kw">pub</span> <span class="kw">fn</span> <span class="ident">my_function</span>(<span class="ident">this_is_counted</span>: <span class="ident">JsValue</span>) {
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<span class="kw">let</span> <span class="ident">also_counted</span> <span class="op">=</span> <span class="ident">JsValue</span>::<span class="ident">from_str</span>(<span class="string">"hi"</span>);
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<span class="macro">assert</span><span class="macro">!</span>(<span class="ident">wasm_bindgen</span>::<span class="ident">externref_heap_live_count</span>() <span class="op">></span><span class="op">=</span> <span class="number">2</span>);
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}</pre></div>
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<p>While this borrowed <code>JsValue</code> ends up on the stack, not the heap, and
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therefore is not accounted for:</p>
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<div class='information'><div class='tooltip ignore'>ⓘ<span class='tooltiptext'>This example is not tested</span></div></div><div class="example-wrap"><pre class="rust rust-example-rendered ignore">
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<span class="attribute">#[<span class="ident">wasm_bindgen</span>]</span>
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<span class="kw">pub</span> <span class="kw">fn</span> <span class="ident">my_other_function</span>(<span class="ident">this_is_not_counted</span>: <span class="kw-2">&</span><span class="ident">JsValue</span>) {
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<span class="comment">// ...</span>
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}</pre></div>
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