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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><li><a href="../01-quirks/05-const_asserts.html"><strong aria-hidden="true">2.5.</strong> Const Asserts</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-link_cable.html"><strong aria-hidden="true">5.6.</strong> Link Cable</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></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>TODO: we've already used newtype twice by now (fixed point values and volatile
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addresses), so we need to adjust how we start this section.</p>
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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>TODO: we've already talked about repr(transparent) by now</p>
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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 the RFC too, of course), if we have a single field struct that's
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sometimes different from having just the bare value, so we should be using
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<code>#[repr(transparent)]</code> 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>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>If you're going to do much with macros you should definitely read through <a href="https://danielkeep.github.io/tlborm/book/index.html">The
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Little Book of Rust
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Macros</a>, but we won't be
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doing too much so you can just follow along here a bit if you like.</p>
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<p>The most basic version of a newtype macro starts 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>The <code>#[macro_export]</code> makes it exported by the current module (like <code>pub</code>
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kinda), and then we have one expansion option that takes an identifier, a <code>,</code>,
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and then a second identifier. The new name is the outer type we'll be using, and
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the old name is the inner type that's being wrapped. You'd use our new macro
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something 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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newtype! {PixelColorCurly, u16}
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newtype!(PixelColorParens, u16);
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newtype![PixelColorBrackets, u16];
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#}</code></pre></pre>
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<p>Note that you can invoke the macro with the outermost grouping as any of <code>()</code>,
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<code>[]</code>, or <code>{}</code>. It makes no particular difference to the macro. Also, that space
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in the first version is kinda to show off that you can put white space in
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between the macro name and the grouping if you want. The difference is mostly
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style, but there are some rules and considerations here:</p>
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<ul>
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<li>If you use curly braces then you <em>must not</em> put a <code>;</code> after the invocation.</li>
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<li>If you use parentheses or brackets then you <em>must</em> put the <code>;</code> at the end.</li>
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<li>Rustfmt cares which you use and formats accordingly:
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<ul>
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<li>Curly brace macro use mostly gets treated like a code block.</li>
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<li>Parentheses macro use mostly gets treated like a function call.</li>
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<li>Bracket macro use mostly gets treated like an array declaration.</li>
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</ul>
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</li>
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</ul>
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<a class="header" href="#upgrade-that-macro" id="upgrade-that-macro"><h2>Upgrade That Macro!</h2></a>
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<p>We also want to be able to add <code>derive</code> stuff and doc comments to our newtype.
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Within the context of <code>macro_rules!</code> definitions these are called "meta". Since
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we can have any number of them we wrap it all up in a "zero or more" matcher.
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Then our macro looks 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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($(#[$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>So now we can write</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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newtype! {
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/// Color on the GBA gives 5 bits for each channel, the highest bit is ignored.
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#[derive(Debug, Clone, Copy)]
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PixelColor, u16
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}
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#}</code></pre></pre>
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<p>And that's about all we'll need for the examples.</p>
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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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<a class="header" href="#potential-homework" id="potential-homework"><h2>Potential Homework</h2></a>
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<p>If you wanted to keep going and get really fancy with it, you could potentially
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add a lot more:</p>
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<ul>
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<li>Make a <code>pub const fn new() -> Self</code> method that outputs the base value in a
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const way. Combine this with builder style "setter" methods that are also
|
|
const and you can get the compiler to do quite a bit of the value building
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work at compile time.</li>
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<li>Making the macro optionally emit a <code>From</code> impl to unwrap it back into the base
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type.</li>
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<li>Allow for visibility modifiers to be applied to the inner field and the newly
|
|
generated type.</li>
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<li>Allowing for generic newtypes. You already saw the need for this once in the
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|
volatile section. Unfortunately, this particular part gets really tricky if
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|
you're using <code>macro_rules!</code>, so you might need to move up to a full
|
|
<code>proc_macro</code>. Having a <code>proc_macro</code> isn't bad except that they have to be
|
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defined in a crate of their own and they're compiled before use. You can't
|
|
ever use them in the crate that defines them, so we won't be using them in any
|
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of our single file examples.</li>
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<li>Allowing for optional <code>Deref</code> and <code>DerefMut</code> of the inner value. This takes
|
|
away most all the safety aspect of doing the newtype, but there may be times
|
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for it. As an example, you could make a newtype with a different form of
|
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Display impl that you want to otherwise treat as the base type in all places.</li>
|
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</ul>
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