Rust Error E0587: Packed and Align Representation Hints Conflict

Understanding Rust Error E0587

Rust error code E0587 occurs when both packed and align representation hints are applied to a type. This is a conflicting use of representation hints, and the Rust compiler will return an error message.

Erroneous code example:

#![allow(unused)]
fn main() {
#[repr(packed, align(8))] // error!
struct Umbrella(i32);
}

In this erroneous code example, both packed and align(8) representation hints are applied to the Umbrella structure. The correct approach is to use only one of these hints at a time.

Using packed representation hint with a given size:

To resolve Rust error E0587, you can provide a size to the packed representation hint instead of using both packed and align hints for the same type. This will pack the type according to the specified size.

Code example with packed size hint:

#![allow(unused)]
fn main() {
#[repr(packed(8))] // ok!
struct Umbrella(i32);
}

In this code example, we have replaced the conflicting representation hints with a single packed(8) hint. The struct Umbrella will now be packed with a size of 8 bytes.

Alternative: Using separate align representation hint

Another way to resolve the Rust error E0587 is by removing the packed hint and using only the align representation hint, if the alignment requirement is the main concern.

Code example with align hint only:

#![allow(unused)]
fn main() {
#[repr(align(8))] // ok!
struct Umbrella(i32);
}

In this alternative code example, we have removed the packed hint entirely and kept only the align(8) hint. The struct Umbrella will now be aligned to 8-byte boundaries.