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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 > < / ol >
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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 {
($condition:expr) => {
#[deny(const_err)]
#[allow(dead_code)]
const ASSERT: usize = 0 - !$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 > ($condition & & true)< / code > to enforce that we get a < code > bool< / code > (thanks to < code > Talchas< / code > for
that particular suggestion).< / p >
< pre > < pre class = "playpen" > < code class = "language-rust" >
# #![allow(unused_variables)]
#fn main() {
macro_rules! const_assert {
($condition:expr) => {
#[deny(const_err)]
#[allow(dead_code)]
const _: usize = 0 - !($condition & & true) as usize;
}
}
#}< / code > < / pre > < / pre >
< a class = "header" href = "#asserting-something" id = "asserting-something" > < h2 > Asserting Something< / h2 > < / a >
< p > As an example of how we might use a < code > const_assert< / code > , we'll do a demo with colors.
There's a red, blue, and green channel. We store colors in a < code > u16< / code > with 5 bits
for each channel.< / p >
< pre > < pre class = "playpen" > < code class = "language-rust" >
# #![allow(unused_variables)]
#fn main() {
newtype! {
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
Color, u16
}
#}< / code > < / pre > < / pre >
< p > And when we're building a color, we're passing in < code > u16< / code > values, but they could
be using more than just 5 bits of space. We want to make sure that each channel
is 31 or less, so we can make a color builder that does a < code > const_assert!< / code > on the
value of each channel.< / p >
< pre > < pre class = "playpen" > < code class = "language-rust" >
# #![allow(unused_variables)]
#fn main() {
macro_rules! rgb {
($r:expr, $g:expr, $b:expr) => {
{
const_assert!($r < = 31);
const_assert!($g < = 31);
const_assert!($b < = 31);
Color($b < < 10 | $g < < 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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