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<ol class="chapter"><li><a href="../00-introduction/00-index.html"><strong aria-hidden="true">1.</strong> Introduction</a></li><li><ol class="section"><li><a href="../00-introduction/01-requirements.html"><strong aria-hidden="true">1.1.</strong> Reader Requirements</a></li><li><a href="../00-introduction/02-goals_and_style.html"><strong aria-hidden="true">1.2.</strong> Book Goals and Style</a></li><li><a href="../00-introduction/03-development-setup.html"><strong aria-hidden="true">1.3.</strong> Development Setup</a></li><li><a href="../00-introduction/04-hello-magic.html"><strong aria-hidden="true">1.4.</strong> Hello, Magic</a></li><li><a href="../00-introduction/05-help_and_resources.html"><strong aria-hidden="true">1.5.</strong> Help and Resources</a></li></ol></li><li><a href="../01-quirks/00-index.html"><strong aria-hidden="true">2.</strong> Quirks</a></li><li><ol class="section"><li><a href="../01-quirks/01-no_std.html"><strong aria-hidden="true">2.1.</strong> No Std</a></li><li><a href="../01-quirks/02-fixed_only.html"><strong aria-hidden="true">2.2.</strong> Fixed Only</a></li><li><a href="../01-quirks/03-volatile_destination.html"><strong aria-hidden="true">2.3.</strong> Volatile Destination</a></li><li><a href="../01-quirks/04-newtype.html" class="active"><strong aria-hidden="true">2.4.</strong> Newtype</a></li></ol></li><li><a href="../02-concepts/00-index.html"><strong aria-hidden="true">3.</strong> Concepts</a></li><li><ol class="section"><li><a href="../02-concepts/01-cpu.html"><strong aria-hidden="true">3.1.</strong> CPU</a></li><li><a href="../02-concepts/02-bios.html"><strong aria-hidden="true">3.2.</strong> BIOS</a></li><li><a href="../02-concepts/03-wram.html"><strong aria-hidden="true">3.3.</strong> Work RAM</a></li><li><a href="../02-concepts/04-io-registers.html"><strong aria-hidden="true">3.4.</strong> IO Registers</a></li><li><a href="../02-concepts/05-palram.html"><strong aria-hidden="true">3.5.</strong> Palette RAM</a></li><li><a href="../02-concepts/06-vram.html"><strong aria-hidden="true">3.6.</strong> Video RAM</a></li><li><a href="../02-concepts/07-oam.html"><strong aria-hidden="true">3.7.</strong> Object Attribute Memory</a></li><li><a href="../02-concepts/08-rom.html"><strong aria-hidden="true">3.8.</strong> Game Pak ROM / Flash ROM</a></li><li><a href="../02-concepts/09-sram.html"><strong aria-hidden="true">3.9.</strong> Save RAM</a></li></ol></li><li><a href="../03-video/00-index.html"><strong aria-hidden="true">4.</strong> Video</a></li><li><ol class="section"><li><a href="../03-video/01-rgb15.html"><strong aria-hidden="true">4.1.</strong> RBG15 Color</a></li><li><a href="../03-video/TODO.html"><strong aria-hidden="true">4.2.</strong> TODO</a></li></ol></li><li><a href="../04-non-video/00-index.html"><strong aria-hidden="true">5.</strong> Non-Video</a></li><li><ol class="section"><li><a href="../04-non-video/01-buttons.html"><strong aria-hidden="true">5.1.</strong> Buttons</a></li><li><a href="../04-non-video/02-timers.html"><strong aria-hidden="true">5.2.</strong> Timers</a></li><li><a href="../04-non-video/03-dma.html"><strong aria-hidden="true">5.3.</strong> Direct Memory Access</a></li><li><a href="../04-non-video/04-sound.html"><strong aria-hidden="true">5.4.</strong> Sound</a></li><li><a href="../04-non-video/05-interrupts.html"><strong aria-hidden="true">5.5.</strong> Interrupts</a></li><li><a href="../04-non-video/06-network.html"><strong aria-hidden="true">5.6.</strong> Network</a></li><li><a href="../04-non-video/07-game_pak.html"><strong aria-hidden="true">5.7.</strong> Game Pak</a></li></ol></li><li><a href="../05-examples/00-index.html"><strong aria-hidden="true">6.</strong> Examples</a></li><li><ol class="section"><li><a href="../05-examples/01-hello_magic.html"><strong aria-hidden="true">6.1.</strong> hello_magic</a></li><li><a href="../05-examples/02-hello_world.html"><strong aria-hidden="true">6.2.</strong> hello_world</a></li><li><a href="../05-examples/03-light_cycle.html"><strong aria-hidden="true">6.3.</strong> light_cycle</a></li><li><a href="../05-examples/04-bg_demo.html"><strong aria-hidden="true">6.4.</strong> bg_demo</a></li></ol></li></ol>
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<h1 class="menu-title">Rust GBA Guide</h1>
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<main>
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<a class="header" href="#newtype" id="newtype"><h1>Newtype</h1></a>
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<p>There's a great Zero Cost abstraction that we'll be using a lot that you might
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not already be familiar with: we're talking about the "Newtype Pattern"!</p>
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<p>Now, I told you to read the Rust Book before you read this book, and I'm sure
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you're all good students who wouldn't sneak into this book without doing the
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required reading, so I'm sure you all remember exactly what I'm talking about,
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because they touch on the newtype concept in the book twice, in two <em>very</em> long
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named sections:</p>
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<ul>
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<li><a href="https://doc.rust-lang.org/book/ch19-03-advanced-traits.html#using-the-newtype-pattern-to-implement-external-traits-on-external-types">Using the Newtype Pattern to Implement External Traits on External
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Types</a></li>
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<li><a href="https://doc.rust-lang.org/book/ch19-04-advanced-types.html#using-the-newtype-pattern-for-type-safety-and-abstraction">Using the Newtype Pattern for Type Safety and
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Abstraction</a></li>
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</ul>
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<p>...Yeah... The Rust Book doesn't know how to make a short sub-section name to
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save its life. Shame.</p>
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<a class="header" href="#newtype-basics" id="newtype-basics"><h2>Newtype Basics</h2></a>
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<p>So, we have all these pieces of data, and we want to keep them separated, and we
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don't wanna pay the cost for it at runtime. Well, we're in luck, we can pay the
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cost at compile time.</p>
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<pre><pre class="playpen"><code class="language-rust">
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# #![allow(unused_variables)]
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#fn main() {
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pub struct PixelColor(u16);
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#}</code></pre></pre>
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<p>Ah, except that, as I'm sure you remember from <a href="https://doc.rust-lang.org/nomicon/other-reprs.html#reprtransparent">The
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Rustonomicon</a>
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(and from <a href="https://github.com/rust-lang/rfcs/blob/master/text/1758-repr-transparent.md">the
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RFC</a>
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too, of course), if we have a single field struct that's sometimes different
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from having just the bare value, so we should be using <code>#[repr(transparent)]</code>
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with our newtypes.</p>
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<pre><pre class="playpen"><code class="language-rust">
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# #![allow(unused_variables)]
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#fn main() {
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#[repr(transparent)]
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pub struct PixelColor(u16);
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#}</code></pre></pre>
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<p>Ah, and of course we'll need to make it so you can unwrap the value:</p>
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<pre><pre class="playpen"><code class="language-rust">
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# #![allow(unused_variables)]
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#fn main() {
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#[repr(transparent)]
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pub struct PixelColor(u16);
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impl From<PixelColor> for u16 {
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fn from(color: PixelColor) -> u16 {
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color.0
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}
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}
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#}</code></pre></pre>
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<p>And then we'll need to do that same thing for <em>every other newtype we want</em>.</p>
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<p>Except there's only two tiny parts that actually differ between newtype
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declarations: the new name and the base type. All the rest is just the same rote
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code over and over. Generating piles and piles of boilerplate code? Sounds like
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a job for a macro to me!</p>
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<a class="header" href="#making-it-a-macro" id="making-it-a-macro"><h2>Making It A Macro</h2></a>
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<p>The most basic version of the macro we want goes like this:</p>
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<pre><pre class="playpen"><code class="language-rust">
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# #![allow(unused_variables)]
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#fn main() {
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#[macro_export]
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macro_rules! newtype {
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($new_name:ident, $old_name:ident) => {
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#[repr(transparent)]
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pub struct $new_name($old_name);
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};
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}
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#}</code></pre></pre>
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<p>Except we also want to be able to add attributes (which includes doc comments),
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so we upgrade our macro a bit:</p>
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<pre><pre class="playpen"><code class="language-rust">
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# #![allow(unused_variables)]
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#fn main() {
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#[macro_export]
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macro_rules! newtype {
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($(#[$attr:meta])* $new_name:ident, $old_name:ident) => {
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$(#[$attr])*
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#[repr(transparent)]
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pub struct $new_name($old_name);
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};
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}
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#}</code></pre></pre>
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<p>And we want to automatically add the ability to turn the wrapper type back into
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the wrapped type.</p>
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<pre><pre class="playpen"><code class="language-rust">
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# #![allow(unused_variables)]
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#fn main() {
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#[macro_export]
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macro_rules! newtype {
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($(#[$attr:meta])* $new_name:ident, $old_name:ident) => {
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$(#[$attr])*
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#[repr(transparent)]
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pub struct $new_name($old_name);
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impl From<$new_name> for $old_name {
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fn from(x: $new_name) -> $old_name {
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x.0
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}
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}
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};
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}
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#}</code></pre></pre>
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<p>That seems like enough for all of our examples, so we'll stop there. We could
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add more things:</p>
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<ul>
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<li>Making the <code>From</code> impl being optional. We'd have to make the newtype
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invocation be more complicated somehow, the user puts ", no-unwrap" after the
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inner type declaration or something, or something like that.</li>
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<li>Allowing for more precise visibility controls on the wrapping type and on the
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inner field. This would add a lot of line noise, so we'll just always have our
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newtypes be <code>pub</code>.</li>
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<li>Allowing for generic newtypes, which might sound silly but that we'll actually
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see an example of soon enough. To do this you might <em>think</em> that we can change
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the <code>:ident</code> declarations to <code>:ty</code>, but since we're declaring a fresh type not
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using an existing type we have to accept it as an <code>:ident</code>. The way you get
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around this is with a proc-macro, which is a lot more powerful but which also
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requires that you write the proc-macro in an entirely other crate that gets
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compiled first. We don't need that much power, so for our examples we'll go
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with the macro_rules version and just do it by hand in the few cases where we
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need a generic newtype.</li>
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<li>Allowing for <code>Deref</code> and <code>DerefMut</code>, which usually defeats the point of doing
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the newtype, but maybe sometimes it's the right thing, so if you were going
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for the full industrial strength version with a proc-macro and all you might
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want to make that part of your optional add-ons as well the same way you might
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want optional <code>From</code>. You'd probably want <code>From</code> to be "on by default" and
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<code>Deref</code>/<code>DerefMut</code> to be "off by default", but whatever.</li>
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</ul>
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<p><strong>As a reminder:</strong> remember that <code>macro_rules</code> macros have to appear <em>before</em>
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they're invoked in your source, so the <code>newtype</code> macro will always have to be at
|
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the very top of your file, or if you put it in a module within your project
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you'll need to declare the module before anything that uses it.</p>
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