How can I align a struct to a specified byte boundary?

huon's answer is good, but it's out of date.

As of Rust 1.25.0, you may now align a type to N bytes using the attribute #[repr(align(N))]. It is documented under the reference's Type Layout section. Note that the alignment must be a power of 2, you may not mix align and packed representations, and aligning a type may add extra padding to the type. Here's an example of how to use the feature:

#[repr(align(64))]
struct S(u8);

fn main() {
    println!("size of S: {}", std::mem::size_of::<S>());
    println!("align of S: {}", std::mem::align_of::<S>());
}

There's no way to specify alignment directly at the moment, but it is definitely desirable and useful. It is covered by issue #33626, and its RFC issue.

A current work-around to force alignment of some struct Foo to be as large as the alignment of some type T is to include a field of type [T; 0] which has size zero and so won't otherwise affect the behaviour of the struct, e.g. struct Foo { data: A, more_data: B, _align: [T; 0] }.

On nightly, this can be combined with SIMD types to get a specific high alignment, since they have alignment equal to their size (well, the next power of two), e.g.

#[repr(simd)]
struct SixteenBytes(u64, u64);

struct Foo {
    data: A,
    more_data: B,
    _align: [SixteenBytes; 0]
}