Rust Error E0588: Packed and Align Representation Conflicts

Understanding Rust Error E0588

Rust Error E0588 occurs when a type with a packed representation hint has a field with an align representation hint. Below is an example of erroneous code causing this error:

#![allow(unused)]
fn main() {
  #[repr(align(16))]
  struct Aligned(i32);

  #[repr(packed)] // error!
  struct Packed(Aligned);
}

Representation Hints: Packed and Align

Rust's #[repr()] attribute gives control over how a struct or enum is stored in memory. Two important hints are packed and align.

The packed hint ensures a struct is stored compactly in memory, reducing padding between fields. This can improve data density but may negatively impact performance.

The align hint specifies the memory alignment for a struct or enum. It's particularly important when working with hardware interfaces or SIMD operations requiring specific alignment.

Resolving the Conflict

To fix Rust Error E0588, do not use the align representation hint within the packed representation hint. You may reverse the hierarchy order - a struct with align hint can contain another struct with a packed hint. Here is an example of proper code:

#![allow(unused)]
fn main() {
  #[repr(packed)]
  struct Packed(i32);

  #[repr(align(16))] // ok!
  struct Aligned(Packed);
}

Considerations and Alternatives

If you need a packed representation for data efficiency but also require specific alignment, it may be necessary to communicate with the other aligned fields through another layer of abstraction. One option may be to use separate structs for packed and aligned operations, with methods to convert between them as needed.