Macro static_assertions::assert_eq_size

macro_rules! assert_eq_size {
    ($($xs:tt)+) => { ... };
}

Asserts that types are equal in size.

When performing operations such as pointer casts or dealing with usize versus u64 versus u32, the size of your types matter. That is where this macro comes into play.

Alternatives

There also exists assert_eq_size_val and assert_eq_size_ptr. Instead of specifying types to compare, values’ sizes can be directly compared against each other.

Examples

On stable Rust, using the macro requires a unique “label” when used in a module scope:

assert_eq_size!(bytes; (u8, u8), u16);

The labeling limitation is not necessary if compiling on nightly Rust with the nightly feature enabled:

#![feature(underscore_const_names)]

assert_eq_size!(u32, [u16; 2]);

fn main() {
    // Supports unlimited arguments without hitting recursion limit
    assert_eq_size!([u8; 4], (u16, u16), u32);
}

The following produces a compilation failure because u32 has 4 times the size of u8:

assert_eq_size!(u32, u8);