# Pre-RFC: #\[align\] attribute

**URL:** <https://internals.rust-lang.org/t/pre-rfc-align-attribute/21004>\
**Category:** language design\
**Created:** [June 10, 2024, 7:12pm UTC](https://internals.rust-lang.org/t/pre-rfc-align-attribute/21004 "2024-06-10T19:12:11Z")\
**Posts on this page:** 1\
**Showing post:** 20

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**Author:** ![2e71828](https://avatars.discourse-cdn.com/v4/letter/2/3e96dc/32.png) [@2e71828](https://internals.rust-lang.org/u/2e71828)\
**Post date:** [June 11, 2024, 8:01pm UTC](https://internals.rust-lang.org/t/pre-rfc-align-attribute/21004/20 "2024-06-11T20:01:31Z")

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> [@scottmcm](#):
>
> This is basically [Frequently Requested Changes - The Rust Language Design Team](https://lang-team.rust-lang.org/frequently-requested-changes.html#size--stride)

It’s slightly different because here we’re talking about the alignment of _fields_ in-situ, rather than a general requirement for all instances of the type.

Given these `WGSL` structures:

```rust
struct A {
    v1: vec3<u32>,
    a: f32,
    v2: vec3<u32>,
}

struct B {
    v1: vec3<u32>,
    v2: vec3<u32>,
}

```

The best Rust translations right now are:

```rust
#[repr(C, align(16))]
struct A {
    v1: [u32;3],
    a: f32,
    v2: [u32;3],
}

#[repr(C, align(16))]
struct B {
    v1: [u32;3],
    _padding: MaybeUninit<u32>,
    v2: [u32;3],
}

```

In both cases, the padding at the end of the Rust struct to bring the size to a multiple of 16 is also required by WGSL, and for the same reason.

An alternative approach would be to define a newtype wrapper around `[u32;3]` with an alignment of 16. This would eliminate the manual padding in `B` but make `A` unrepresentable— To match this alternate Rust definition of `A`, manual padding entries would be required in the `WGSL` code, and the whole structure will be significantly larger.

* * *

With a field alignment attribute, however, we could then represent both structures on the Rust side in a way that directly corresponds to the WGSL definition:

```rust
#[repr(C)]
struct A {
    #[align(16)] // optional, because it’s the first field
    v1: [u32;3],

    a: f32,

    #[align(16)]
    v2: [u32;3],
}

#[repr(C)]
struct B {
    #[align(16)] // optional, because it’s the first field
    v1: [u32;3],

    #[align(16)]
    v2: [u32;3],
}

```

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_[View the full topic](https://internals.rust-lang.org/t/pre-rfc-align-attribute/21004)._