Attribute Macro assert_size 

#[assert_size]

A compile-time assertion that verifies a type has the expected size in bytes.

This attribute macro generates a compile-time check using std::mem::size_of to ensure the annotated type has exactly the specified size. If the size doesn’t match, compilation will fail with a clear error message.

Parameters

• A single integer literal representing the expected size in bytes

Use Cases

• Catching unintended size changes from code refactoring
• Ensuring types meet specific memory layout requirements (e.g., for FFI or serialization)
• Documenting expected type sizes for performance-critical code
• Detecting platform-specific size variations

How It Works

The macro generates a const assertion that compares the actual size (via core::mem::size_of) with the expected size. The type definition itself is preserved unchanged. The assertion is evaluated at compile time, so there is zero runtime overhead. Works in both std and no_std environments.

Examples

use assert_size_derive::assert_size;

#[assert_size(2)]
struct MyData {
    foo: u8,
    bar: u8,
}

#[assert_size(16)]
enum MyEnum {
    Variant1(u64),
    Variant2(u32),
}

Compile-Time Failure Example

use assert_size_derive::assert_size;

// This will fail to compile because the actual size is 2 bytes, not 1
#[assert_size(1)]
struct TooSmall {
    a: u8,
    b: u8,
}

Compatibility

Works with any type definition: structs, enums, and unions.