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<ol class="chapter"><li class="affix"><a href="../../introduction.html">Introduction</a></li><li class="spacer"></li><li><a href="../../examples/index.html"><strong aria-hidden="true">1.</strong> Examples</a></li><li><ol class="section"><li><a href="../../examples/hello-world.html"><strong aria-hidden="true">1.1.</strong> Hello, World!</a></li><li><a href="../../examples/console-log.html"><strong aria-hidden="true">1.2.</strong> Using console.log</a></li><li><a href="../../examples/add.html"><strong aria-hidden="true">1.3.</strong> Small wasm files</a></li><li><a href="../../examples/without-a-bundler.html"><strong aria-hidden="true">1.4.</strong> Without a Bundler</a></li><li><a href="../../examples/wasm2js.html"><strong aria-hidden="true">1.5.</strong> Converting WebAssembly to JS</a></li><li><a href="../../examples/import-js.html"><strong aria-hidden="true">1.6.</strong> Importing functions from JS</a></li><li><a href="../../examples/char.html"><strong aria-hidden="true">1.7.</strong> Working with char</a></li><li><a href="../../examples/wasm-in-wasm.html"><strong aria-hidden="true">1.8.</strong> js-sys: WebAssembly in WebAssembly</a></li><li><a href="../../examples/dom.html"><strong aria-hidden="true">1.9.</strong> web-sys: DOM hello world</a></li><li><a href="../../examples/closures.html"><strong aria-hidden="true">1.10.</strong> web-sys: Closures</a></li><li><a href="../../examples/performance.html"><strong aria-hidden="true">1.11.</strong> web-sys: performance.now</a></li><li><a href="../../examples/fetch.html"><strong aria-hidden="true">1.12.</strong> web-sys: using fetch</a></li><li><a href="../../examples/2d-canvas.html"><strong aria-hidden="true">1.13.</strong> web-sys: canvas hello world</a></li><li><a href="../../examples/julia.html"><strong aria-hidden="true">1.14.</strong> web-sys: canvas Julia set</a></li><li><a href="../../examples/web-audio.html"><strong aria-hidden="true">1.15.</strong> web-sys: WebAudio</a></li><li><a href="../../examples/webgl.html"><strong aria-hidden="true">1.16.</strong> web-sys: WebGL</a></li><li><a href="../../examples/websockets.html"><strong aria-hidden="true">1.17.</strong> web-sys: WebSockets</a></li><li><a href="../../examples/webrtc_datachannel.html"><strong aria-hidden="true">1.18.</strong> web-sys: WebRTC DataChannel</a></li><li><a href="../../examples/request-animation-frame.html"><strong aria-hidden="true">1.19.</strong> web-sys: requestAnimationFrame</a></li><li><a href="../../examples/paint.html"><strong aria-hidden="true">1.20.</strong> web-sys: A Simple Paint Program</a></li><li><a href="../../examples/raytrace.html"><strong aria-hidden="true">1.21.</strong> Parallel Raytracing</a></li><li><a href="../../examples/todomvc.html"><strong aria-hidden="true">1.22.</strong> web-sys: A TODO MVC App</a></li></ol></li><li><a href="../../reference/index.html"><strong aria-hidden="true">2.</strong> Reference</a></li><li><ol class="section"><li><a href="../../reference/deployment.html"><strong aria-hidden="true">2.1.</strong> Deployment</a></li><li><a href="../../reference/js-snippets.html"><strong aria-hidden="true">2.2.</strong> JS snippets</a></li><li><a href="../../reference/passing-rust-closures-to-js.html"><strong aria-hidden="true">2.3.</strong> Passing Rust Closures to JS</a></li><li><a href="../../reference/receiving-js-closures-in-rust.html"><strong aria-hidden="true">2.4.</strong> Receiving JS Closures in Rust</a></li><li><a href="../../reference/js-promises-and-rust-futures.html"><strong aria-hidden="true">2.5.</strong> Promises and Futures</a></li><li><a href="../../reference/iterating-over-js-values.html"><strong aria-hidden="true">2.6.</strong> Iterating over JS Values</a></li><li><a href="../../reference/arbitrary-data-with-serde.html"><strong aria-hidden="true">2.7.</strong> Arbitrary Data with Serde</a></li><li><a href="../../reference/accessing-properties-of-untyped-js-values.html"><strong aria-hidden="true">2.8.</strong> Accessing Properties of Untyped JS Values</a></li><li><a href="../../reference/working-with-duck-typed-interfaces.html"><strong aria-hidden="true">2.9.</strong> Working with Duck-Typed Interfaces</a></li><li><a href="../../reference/cli.html"><strong aria-hidden="true">2.10.</strong> Command Line Interface</a></li><li><a href="../../reference/optimize-size.html"><strong aria-hidden="true">2.11.</strong> Optimizing for Size</a></li><li><a href="../../reference/rust-targets.html"><strong aria-hidden="true">2.12.</strong> Supported Rust Targets</a></li><li><a href="../../reference/browser-support.html"><strong aria-hidden="true">2.13.</strong> Supported Browsers</a></li><li><a href="../../reference/types.html"><strong aria-hidden="true">2.14.</strong> Supported Types</a></li><li><ol class="section"><li><a href="../../reference/types/imported-js-types.html"><strong aria-hidden="true">2.14.1.</strong> Imported JavaScript Types</a></li><li><a href="../../reference/types/exported-rust-types.html"><strong aria-hidden="true">2.14.2.</strong> Exported Rust Types</a></li><li><a href="../../reference/types/jsvalue.html"><strong aria-hidden="true">2.14.3.</strong> JsValue</a></li><li><a href="../../reference/types/boxed-jsvalue-slice.html"><strong aria-hidden="true">2.14.4.</strong> Box<[JsValue]></a></li><li><a href="../../reference/types/pointers.html"><strong aria-hidden="true">2.14.5.</strong> *const T and *mut T</a></li><li><a href="../../reference/types/numbers.html"><strong aria-hidden="true">2.14.6.</strong> Numbers</a></li><li><a href="../../reference/types/bool.html"><strong aria-hidden="true">2.14.7.</strong> bool</a></li><li><a href="../../reference/types/char.html"><strong aria-hidden="true">2.14.8.</strong> char</a></li><li><a href="../../reference/types/str.html"><strong aria-hidden="true">2.14.9.</strong> str</a></li><li><a href="../../reference/types/string.html"><strong aria-hidden="true">2.14.10.</strong> String</a></li><li><a href="../../reference/types/number-slices.html"><strong aria-hidden="true">2.14.11.</strong> Number Slices</a></li><li><a href="../../reference/types/boxed-number-slices.html"><strong aria-hidden="true">2.14.12.</strong> Boxed Number Slices</a></li><li><a href="../../reference/types/result.html"><strong aria-hidden="true">2.14.13.</strong> Result<T, JsValue></a></li></ol></li><li><a href="../../reference/attributes/index.html"><strong aria-hidden="true">2.15.</strong> #[wasm_bindgen] Attributes</a></li><li><ol class="section"><li><a href="../../reference/attributes/on-js-imports/index.html"><strong aria-hidden="true">2.15.1.</strong> On JavaScript Imports</a></li><li><ol class="section"><li><a href="../../reference/attributes/on-js-imports/catch.html"><strong aria-hidden="true">2.15.1.1.</strong> catch</a></li><li><a href="../../reference/attributes/on-js-imports/constructor.html"><strong aria-hidden="true">2.15.1.2.</strong> constructor</a></li><li><a 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js_namespace</a></li><li><a href="../../reference/attributes/on-js-imports/method.html"><strong aria-hidden="true">2.15.1.10.</strong> method</a></li><li><a href="../../reference/attributes/on-js-imports/module.html"><strong aria-hidden="true">2.15.1.11.</strong> module = "blah"</a></li><li><a href="../../reference/attributes/on-js-imports/raw_module.html"><strong aria-hidden="true">2.15.1.12.</strong> raw_module = "blah"</a></li><li><a href="../../reference/attributes/on-js-imports/static_method_of.html"><strong aria-hidden="true">2.15.1.13.</strong> static_method_of = Blah</a></li><li><a href="../../reference/attributes/on-js-imports/structural.html"><strong aria-hidden="true">2.15.1.14.</strong> structural</a></li><li><a href="../../reference/attributes/on-js-imports/variadic.html"><strong aria-hidden="true">2.15.1.15.</strong> variadic</a></li><li><a href="../../reference/attributes/on-js-imports/vendor_prefix.html"><strong aria-hidden="true">2.15.1.16.</strong> vendor_prefix</a></li></ol></li><li><a href="../../reference/attributes/on-rust-exports/index.html"><strong aria-hidden="true">2.15.2.</strong> On Rust Exports</a></li><li><ol class="section"><li><a href="../../reference/attributes/on-rust-exports/constructor.html"><strong aria-hidden="true">2.15.2.1.</strong> constructor</a></li><li><a href="../../reference/attributes/on-rust-exports/js_name.html"><strong aria-hidden="true">2.15.2.2.</strong> js_name = Blah</a></li><li><a href="../../reference/attributes/on-rust-exports/readonly.html"><strong aria-hidden="true">2.15.2.3.</strong> readonly</a></li><li><a href="../../reference/attributes/on-rust-exports/skip.html"><strong aria-hidden="true">2.15.2.4.</strong> skip</a></li><li><a href="../../reference/attributes/on-rust-exports/start.html"><strong aria-hidden="true">2.15.2.5.</strong> start</a></li><li><a href="../../reference/attributes/on-rust-exports/typescript_custom_section.html"><strong aria-hidden="true">2.15.2.6.</strong> typescript_custom_section</a></li><li><a href="../../reference/attributes/on-rust-exports/getter-and-setter.html"><strong aria-hidden="true">2.15.2.7.</strong> getter and setter</a></li><li><a href="../../reference/attributes/on-rust-exports/inspectable.html"><strong aria-hidden="true">2.15.2.8.</strong> inspectable</a></li><li><a href="../../reference/attributes/on-rust-exports/skip_typescript.html"><strong aria-hidden="true">2.15.2.9.</strong> skip_typescript</a></li><li><a href="../../reference/attributes/on-rust-exports/typescript_type.html"><strong aria-hidden="true">2.15.2.10.</strong> typescript_type</a></li></ol></li></ol></li></ol></li><li><a href="../../web-sys/index.html"><strong aria-hidden="true">3.</strong> web-sys</a></li><li><ol class="section"><li><a href="../../web-sys/using-web-sys.html"><strong aria-hidden="true">3.1.</strong> Using web-sys</a></li><li><a href="../../web-sys/cargo-features.html"><strong aria-hidden="true">3.2.</strong> Cargo Features</a></li><li><a href="../../web-sys/function-overloads.html"><strong aria-hidden="true">3.3.</strong> Function Overloads</a></li><li><a href="../../web-sys/type-translations.html"><strong aria-hidden="true">3.4.</strong> Type Translations</a></li><li><a href="../../web-sys/inheritance.html"><strong aria-hidden="true">3.5.</strong> Inheritance</a></li><li><a href="../../web-sys/unstable-apis.html"><strong aria-hidden="true">3.6.</strong> Unstable APIs</a></li></ol></li><li><a href="../../wasm-bindgen-test/index.html"><strong aria-hidden="true">4.</strong> Testing with wasm-bindgen-test</a></li><li><ol class="section"><li><a href="../../wasm-bindgen-test/usage.html"><strong aria-hidden="true">4.1.</strong> Usage</a></li><li><a href="../../wasm-bindgen-test/asynchronous-tests.html"><strong aria-hidden="true">4.2.</strong> Writing Asynchronous Tests</a></li><li><a href="../../wasm-bindgen-test/browsers.html"><strong aria-hidden="true">4.3.</strong> Testing in Headless Browsers</a></li><li><a href="../../wasm-bindgen-test/continuous-integration.html"><strong aria-hidden="true">4.4.</strong> Continuous Integration</a></li></ol></li><li><a href="../../contributing/index.html"><strong aria-hidden="true">5.</strong> Contributing to wasm-bindgen</a></li><li><ol class="section"><li><a href="../../contributing/testing.html"><strong aria-hidden="true">5.1.</strong> Testing</a></li><li><a href="../../contributing/design/index.html"><strong aria-hidden="true">5.2.</strong> Internal Design</a></li><li><ol class="section"><li><a href="../../contributing/design/js-objects-in-rust.html"><strong aria-hidden="true">5.2.1.</strong> JS Objects in Rust</a></li><li><a href="../../contributing/design/exporting-rust.html"><strong aria-hidden="true">5.2.2.</strong> Exporting a function to JS</a></li><li><a href="../../contributing/design/exporting-rust-struct.html"><strong aria-hidden="true">5.2.3.</strong> Exporting a struct to JS</a></li><li><a href="../../contributing/design/importing-js.html"><strong aria-hidden="true">5.2.4.</strong> Importing a function from JS</a></li><li><a href="../../contributing/design/importing-js-struct.html"><strong aria-hidden="true">5.2.5.</strong> Importing a class from JS</a></li><li><a href="../../contributing/design/rust-type-conversions.html" class="active"><strong aria-hidden="true">5.2.6.</strong> Rust Type conversions</a></li><li><a href="../../contributing/design/describe.html"><strong aria-hidden="true">5.2.7.</strong> Types in wasm-bindgen</a></li></ol></li><li><a href="../../contributing/js-sys/index.html"><strong aria-hidden="true">5.3.</strong> js-sys</a></li><li><ol class="section"><li><a href="../../contributing/js-sys/testing.html"><strong aria-hidden="true">5.3.1.</strong> Testing</a></li><li><a href="../../contributing/js-sys/adding-more-apis.html"><strong aria-hidden="true">5.3.2.</strong> Adding More APIs</a></li></ol></li><li><a 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<div id="content" class="content">
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<main>
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<h1><a class="header" href="#rust-type-conversions" id="rust-type-conversions">Rust Type conversions</a></h1>
|
|
<p>Previously we've been seeing mostly abridged versions of type conversions when
|
|
values enter Rust. Here we'll go into some more depth about how this is
|
|
implemented. There are two categories of traits for converting values, traits
|
|
for converting values from Rust to JS and traits for the other way around.</p>
|
|
<h2><a class="header" href="#from-rust-to-js" id="from-rust-to-js">From Rust to JS</a></h2>
|
|
<p>First up let's take a look at going from Rust to JS:</p>
|
|
<pre><pre class="playpen"><code class="language-rust">
|
|
# #![allow(unused_variables)]
|
|
#fn main() {
|
|
pub trait IntoWasmAbi: WasmDescribe {
|
|
type Abi: WasmAbi;
|
|
fn into_abi(self, extra: &mut Stack) -> Self::Abi;
|
|
}
|
|
#}</code></pre></pre>
|
|
<p>And that's it! This is actually the only trait needed currently for translating
|
|
a Rust value to a JS one. There's a few points here:</p>
|
|
<ul>
|
|
<li>We'll get to <code>WasmDescribe</code> later in this section</li>
|
|
<li>The associated type <code>Abi</code> is what will actually be generated as an argument to
|
|
the wasm export. The bound <code>WasmAbi</code> is only implemented for types like <code>u32</code>
|
|
and <code>f64</code>, those which can be placed on the boundary and transmitted
|
|
losslessly.</li>
|
|
<li>And finally we have the <code>into_abi</code> function, returning the <code>Abi</code> associated
|
|
type which will be actually passed to JS. There's also this <code>Stack</code> parameter,
|
|
however. Not all Rust values can be communicated in 32 bits to the <code>Stack</code>
|
|
parameter allows transmitting more data, explained in a moment.</li>
|
|
</ul>
|
|
<p>This trait is implemented for all types that can be converted to JS and is
|
|
unconditionally used during codegen. For example you'll often see <code>IntoWasmAbi for Foo</code> but also <code>IntoWasmAbi for &'a Foo</code>.</p>
|
|
<p>The <code>IntoWasmAbi</code> trait is used in two locations. First it's used to convert
|
|
return values of Rust exported functions to JS. Second it's used to convert the
|
|
Rust arguments of JS functions imported to Rust.</p>
|
|
<h2><a class="header" href="#from-js-to-rust" id="from-js-to-rust">From JS to Rust</a></h2>
|
|
<p>Unfortunately the opposite direction from above, going from JS to Rust, is a bit
|
|
more complicated. Here we've got three traits:</p>
|
|
<pre><pre class="playpen"><code class="language-rust">
|
|
# #![allow(unused_variables)]
|
|
#fn main() {
|
|
pub trait FromWasmAbi: WasmDescribe {
|
|
type Abi: WasmAbi;
|
|
unsafe fn from_abi(js: Self::Abi, extra: &mut Stack) -> Self;
|
|
}
|
|
|
|
pub trait RefFromWasmAbi: WasmDescribe {
|
|
type Abi: WasmAbi;
|
|
type Anchor: Deref<Target=Self>;
|
|
unsafe fn ref_from_abi(js: Self::Abi, extra: &mut Stack) -> Self::Anchor;
|
|
}
|
|
|
|
pub trait RefMutFromWasmAbi: WasmDescribe {
|
|
type Abi: WasmAbi;
|
|
type Anchor: DerefMut<Target=Self>;
|
|
unsafe fn ref_mut_from_abi(js: Self::Abi, extra: &mut Stack) -> Self::Anchor;
|
|
}
|
|
#}</code></pre></pre>
|
|
<p>The <code>FromWasmAbi</code> is relatively straightforward, basically the opposite of
|
|
<code>IntoWasmAbi</code>. It takes the ABI argument (typically the same as
|
|
<code>IntoWasmAbi::Abi</code>) and then the auxiliary stack to produce an instance of
|
|
<code>Self</code>. This trait is implemented primarily for types that <em>don't</em> have internal
|
|
lifetimes or are references.</p>
|
|
<p>The latter two traits here are mostly the same, and are intended for generating
|
|
references (both shared and mutable references). They look almost the same as
|
|
<code>FromWasmAbi</code> except that they return an <code>Anchor</code> type which implements a
|
|
<code>Deref</code> trait rather than <code>Self</code>.</p>
|
|
<p>The <code>Ref*</code> traits allow having arguments in functions that are references rather
|
|
than bare types, for example <code>&str</code>, <code>&JsValue</code>, or <code>&[u8]</code>. The <code>Anchor</code> here
|
|
is required to ensure that the lifetimes don't persist beyond one function call
|
|
and remain anonymous.</p>
|
|
<p>The <code>From*</code> family of traits are used for converting the Rust arguments in Rust
|
|
exported functions to JS. They are also used for the return value in JS
|
|
functions imported into Rust.</p>
|
|
<h2><a class="header" href="#global-stack" id="global-stack">Global stack</a></h2>
|
|
<p>Mentioned above not all Rust types will fit within 32 bits. While we can
|
|
communicate an <code>f64</code> we don't necessarily have the ability to use all the bits.
|
|
Types like <code>&str</code> need to communicate two items, a pointer and a length (64
|
|
bits). Other types like <code>&Closure<Fn()></code> have even more information to
|
|
transmit.</p>
|
|
<p>As a result we need a method of communicating more data through the signatures
|
|
of functions. While we could add more arguments this is somewhat difficult to do
|
|
in the world of closures where code generation isn't quite as dynamic as a
|
|
procedural macro. Consequently a "global stack" is used to transmit extra
|
|
data for a function call.</p>
|
|
<p>The global stack is a fixed-sized static allocation in the wasm module. This
|
|
stack is temporary scratch space for any one function call from either JS to
|
|
Rust or Rust to JS. Both Rust and the JS shim generated have pointers to this
|
|
global stack and will read/write information from it.</p>
|
|
<p>Using this scheme whenever we want to pass <code>&str</code> from JS to Rust we can pass
|
|
the pointer as the actual ABI argument and the length is then placed in the next
|
|
spot on the global stack.</p>
|
|
<p>The <code>Stack</code> argument to the conversion traits above looks like:</p>
|
|
<pre><pre class="playpen"><code class="language-rust">
|
|
# #![allow(unused_variables)]
|
|
#fn main() {
|
|
pub trait Stack {
|
|
fn push(&mut self, bits: u32);
|
|
fn pop(&mut self) -> u32;
|
|
}
|
|
#}</code></pre></pre>
|
|
<p>A trait is used here to facilitate testing but typically the calls don't end up
|
|
being virtually dispatched at runtime.</p>
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