From Wikipedia:

The CPU in modern computer hardware performs reads and writes to memory most efficiently when the data is naturally aligned, which generally means that the data address is a multiple of the data size. Data alignment refers to aligning elements according to their natural alignment. To ensure natural alignment, it may be necessary to insert some padding between structure elements or after the last element of a structure.

The #[repr] attribute has a new parameter, align, that sets the alignment of your struct:

# #![allow(unused_variables)]
#fn main() {
struct Number(i32);

assert_eq!(std::mem::align_of::<Number>(), 4);
assert_eq!(std::mem::size_of::<Number>(), 4);

#[repr(align(16))]
struct Align16(i32);

assert_eq!(std::mem::align_of::<Align16>(), 16);
assert_eq!(std::mem::size_of::<Align16>(), 16);
#}

If you’re working with low-level stuff, control of these kinds of things can be very important!

The alignment of a type is normally not worried about as the compiler will "do the right thing" of picking an appropriate alignment for general use cases. There are situations, however, where a nonstandard alignment may be desired when operating with foreign systems. For example these sorts of situations tend to necessitate or be much easier with a custom alignment:

• Hardware can often have obscure requirements such as "this structure is aligned to 32 bytes" when it in fact is only composed of 4-byte values. While this can typically be manually calculated and managed, it's often also useful to express this as a property of a type to get the compiler to do a little extra work instead.
• C compilers like gcc and clang offer the ability to specify a custom alignment for structures, and Rust can much more easily interoperate with these types if Rust can also mirror the request for a custom alignment (e.g. passing a structure to C correctly is much easier).
• Custom alignment can often be used for various tricks here and there and is often convenient as "let's play around with an implementation" tool. For example this can be used to statically allocate page tables in a kernel or create an at-least cache-line-sized structure easily for concurrent programming.

The purpose of this feature is to provide a lightweight annotation to alter the compiler-inferred alignment of a structure to enable these situations much more easily.