Compile-time assertions to ensure that invariants are met.

Usage

This crate is available on crates.io and can be used by adding the following to your project's Cargo.toml:

[dependencies]
static_assertions = "0.1"

and this to your crate root:

#[macro_use]
extern crate static_assertions;

Assert Equal Size

When performing operations such as pointer casts or dealing with usize versus u64 versus u32, the size of your types matter. This is where assert_eq_size comes into play. Types provided as arguments to assert_eq_size are ensured to be the same size at compile-time. If the types differ in size, the code will fail to compile.

assert_eq_size!([u8; 4], (u16, u16), u32);

// Produces a compilation failure:
// assert_eq_size!(u32, u8);

Similar to assert_eq_size, there is assert_eq_size_val. Instead of specifying types to compare, values' sizes can be directly compared against each other.

let x = 42u8;
let y = true;

assert_eq_size_val!(x, y);

assert_eq_size_val doesn't consume its arguments and thus works for non-Cloneable values.

struct Buffer([u8; 256]);

let buf = Buffer([0; 256]);
let val = [0u64; 32];

assert_eq_size_val!(buf, val);

// `buf` and `val` can be used here

Assert Constant Expression

Constant expressions can be ensured to have certain properties via const_assert. If the expression evaluates to false, the file will fail to compile. This is synonymous to static_assert in C++.

const NUM: usize = 32;

const_assert!(NUM * NUM == 1024);

As a shorthand for const_assert!(a == b), there's const_assert_eq:

const NUM: usize = 32;
const_assert_eq!(NUM + NUM, 64);

const TWO: usize = 2;
const_assert_eq!(TWO * TWO, TWO + TWO, 4);

Limitation: Due to implementation details, assert_eq_size, const_assert, and const_assert_eq can only be used from within the context of a function.