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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"><strong aria-hidden="true">2.4.</strong> Newtype</a></li><li><a href="../01-quirks/05-const_asserts.html" class="active"><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><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="
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<a class="header" href="#constant-assertions" id="constant-assertions"><h1>Constant Assertions</h1></a>
<p>Have you ever wanted to assert things <em>even before runtime</em>? We all have, of
course. Particularly when the runtime machine is a poor little GBA, we'd like to
have the machine doing the compile handle as much checking as possible.</p>
<p>Enter the <a href="https://docs.rs/static_assertions/">static assertions</a> crate, which
provides a way to let you assert on a <code>const</code> expression.</p>
<p>This is an amazing crate that you should definitely use when you can.</p>
<p>It's written by <a href="https://github.com/nvzqz">Nikolai Vazquez</a>, and they kindly
wrote up a <a href="https://nikolaivazquez.com/posts/programming/rust-static-assertions/">blog
post</a> that
explains the thinking behind it.</p>
<p>However, I promised that each example would be single file, and I also promised
to explain what's going on as we go, so we'll briefly touch upon giving an
explanation here.</p>
<a class="header" href="#how-we-const-assert" id="how-we-const-assert"><h2>How We Const Assert</h2></a>
<p>Alright, as it stands (2018-12-15), we can't use <code>if</code> in a <code>const</code> context.</p>
<p>Since we can't use <code>if</code>, we can't use a normal <code>assert!</code>. Some day it will be
possible, and a failed assert at compile time will be a compile error and a
failed assert at run time will be a panic and we'll have a nice unified
programming experience. We can add runtime-only assertions by being a little
tricky with the compiler.</p>
<p>If we write</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
const ASSERT: usize = 0 - 1;
#}</code></pre></pre>
<p>that gives a warning, since the math would underflow. We can upgrade that
warning to a hard error:</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
#[deny(const_err)]
const ASSERT: usize = 0 - 1;
#}</code></pre></pre>
<p>And to make our construction reusable we can enable the
<a href="https://github.com/rust-lang/rust/issues/54912">underscore_const_names</a> feature
in our program (or library) and then give each such const an underscore for a
name.</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#![feature(underscore_const_names)]
#fn main() {
#[deny(const_err)]
const _: usize = 0 - 1;
#}</code></pre></pre>
<p>Now we wrap this in a macro where we give a <code>bool</code> expression as input. We
negate the bool then cast it to a <code>usize</code>, meaning that <code>true</code> negates into
<code>false</code>, which becomes <code>0usize</code>, and then there's no underflow error. Or if the
input was <code>false</code>, it negates into <code>true</code>, then becomes <code>1usize</code>, and then the
underflow error fires.</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
macro_rules! const_assert {
($condition:expr) =&gt; {
#[deny(const_err)]
#[allow(dead_code)]
const ASSERT: usize = 0 - !$condition as usize;
}
}
#}</code></pre></pre>
<p>Technically, written like this, the expression can be anything with a
<code>core::ops::Not</code> implementation that can also be <code>as</code> cast into <code>usize</code>. That's
<code>bool</code>, but also basically all the other number types. Since we want to ensure
that we get proper looking type errors when things go wrong, we can use
<code>($condition &amp;&amp; true)</code> to enforce that we get a <code>bool</code> (thanks to <code>Talchas</code> for
that particular suggestion).</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
macro_rules! const_assert {
($condition:expr) =&gt; {
#[deny(const_err)]
#[allow(dead_code)]
const _: usize = 0 - !($condition &amp;&amp; true) as usize;
}
}
#}</code></pre></pre>
<a class="header" href="#asserting-something" id="asserting-something"><h2>Asserting Something</h2></a>
<p>As an example of how we might use a <code>const_assert</code>, we'll do a demo with colors.
There's a red, blue, and green channel. We store colors in a <code>u16</code> with 5 bits
for each channel.</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
newtype! {
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
Color, u16
}
#}</code></pre></pre>
<p>And when we're building a color, we're passing in <code>u16</code> values, but they could
be using more than just 5 bits of space. We want to make sure that each channel
is 31 or less, so we can make a color builder that does a <code>const_assert!</code> on the
value of each channel.</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
macro_rules! rgb {
($r:expr, $g:expr, $b:expr) =&gt; {
{
const_assert!($r &lt;= 31);
const_assert!($g &lt;= 31);
const_assert!($b &lt;= 31);
Color($b &lt;&lt; 10 | $g &lt;&lt; 5 | $r)
}
}
}
#}</code></pre></pre>
<p>And then we can declare some colors</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
const RED: Color = rgb!(31, 0, 0);
const BLUE: Color = rgb!(31, 500, 0);
#}</code></pre></pre>
<p>The second one is clearly out of bounds and it fires an error just like we
wanted.</p>
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