Support `AsBytes` for fields with a size set as a generic constant

[saltatory]

Use Case

I am working on a high-throughput IO project reading/writing fixed size blocks to disk. I have a broadly used compilation parameter for the block id size.

I have a struct like the following:

#[derive(FromBytes, FromZeroes, AsBytes, Unaligned)]
#[repr(packed)]
pub struct IndexEntryFlags(u8);

bitflags! {
    impl IndexEntryFlags: u8 {
        const NONE = 0b00;
        const DELETE = 0b01;
    }
}

#[derive(FromBytes, FromZeroes, AsBytes, Unaligned)]
#[repr(packed)]
pub struct IndexEntry<const SIZE_BLOCK_ID: usize> {
    block_number: U64,
    flags: IndexEntryFlags,
    block_id: [u8; SIZE_BLOCK_ID] 
}

Problem

This generates an error:

error[E0793]: reference to packed field is unaligned

... even though the exact same compilation works with:

...
   block_id: [u8; 32]
...

Request

Support generic constants in fixed size arrays for AsBytes.

[joshlf]

Oh fascinating. I copied your code almost verbatim to the playground and didn't get this error. The only thing I had to change was replacing U64 with [u8; 8] (which is what U64 is under the hood) since the playground presumably doesn't compile with feature = "byteorder".

Are you able to repro this on the playground so that we can play around with it?

[saltatory]

Sorry no. I tried several things including this.

• I could not manage to find zerocopy::native_endian::U64.
• I tried several ways to get the compiler to complain as it does on my machine ^^^
• I assume that the U64 is/was causing the compiler to complain but I cannot be certain.
• I had tried shifting to #[repr(C)] but got the error message about not accepting generics

[joshlf]

Hmmm. I'll have chance to look at this more deeply later, but for the time being, maybe try with our latest alpha release? Total shot in the dark, but a lot has changed between 0.7 and 0.8, and maybe we accidentally fixed whatever the issue is.

[saltatory]

Thanks for your quick and thoughtful response! I'll give that a rip. Likely tomorrow-ish.

[joshlf]

One more thing: What toolchain are you using? It's possible that this is a new rustc bug.

[joshlf]

I've added a new test case for this so at a minimum we hopefully won't regress on it: #1186

[saltatory]

I was on rustc 1.72.0 but I upgraded to test and had the same issue

rustup 1.27.0 (bbb9276d2 2024-03-08)
info: This is the version for the rustup toolchain manager, not the rustc compiler.
info: The currently active `rustc` version is `rustc 1.78.0 (9b00956e5 2024-04-29)`

[joshlf]

Can you maybe paste the entire error you got, not just this single line?

error[E0793]: reference to packed field is unaligned

[saltatory]

Sorry for the delay ... I had moved on in my project and now I cannot repro the error! I swear I was not making it up! Today, with the same computer/toolchain, I created the following test and it compiled fine.

use bitflags::bitflags;
use zerocopy::{AsBytes, FromBytes, FromZeroes, Unaligned};
use zerocopy::big_endian::U64;

#[derive(FromBytes, AsBytes, FromZeroes, Unaligned)]
#[repr(packed)]
pub struct Bar(u8);
bitflags! {
    impl Bar: u8 {
        const NONE = 0;
    }
}

#[derive(FromBytes, AsBytes, FromZeroes, Unaligned)]
#[repr(packed)]
pub struct Foo<const C: usize> {
    data: [u8; C],
    bar: Bar,
    baz: U64
}

#[test]
pub fn t1() {
    let foo = Foo::<32>{ data: [0u8; 32], baz: U64::new(32), bar: Bar::NONE};
}

Thanks sincerely for your help. Closing for now.

[saltatory]

Apologies for the long delay. I re-ran into this issue. It's real. As I think you can see,

Sample Code

use zerocopy::{AsBytes, FromBytes, FromZeroes, Unaligned};

#[derive(FromBytes, AsBytes, FromZeroes, Unaligned)]
#[repr(packed)]
struct Foo<const SIZE: usize> {

    data: [u8; SIZE]

}

impl<const SIZE: usize> Foo<SIZE> {

    pub fn set_data(&mut self, data: &[u8]) {
        self.data.copy_from_slice(data);
    }

}

#[derive(FromBytes, AsBytes, FromZeroes)]
#[repr(C)]
struct Bar<const SIZE: usize> {

    data: [u8; SIZE]

}

impl<const SIZE: usize> Bar<SIZE> {

    pub fn set_data(&mut self, data: &[u8]) {
        self.data.copy_from_slice(data);
    }

}

#[derive(FromBytes, AsBytes, FromZeroes, Unaligned)]
#[repr(packed)]
struct Baz {

    data: [u8; 32]

}

impl Baz {

    pub fn set_data(&mut self, data: &[u8]) {
        self.data.copy_from_slice(data);
    }

}

Error

   Compiling zc-error v0.1.0 (/mnt/workspace/zc-error)
error: unsupported on generic structs that are not repr(transparent) or repr(packed)
  --> src/lib.rs:19:21
   |
19 | #[derive(FromBytes, AsBytes, FromZeroes)]
   |                     ^^^^^^^
   |
   = note: this error originates in the derive macro `AsBytes` (in Nightly builds, run with -Z macro-backtrace for more info)

error[E0793]: reference to packed field is unaligned
  --> src/lib.rs:14:9
   |
14 |         self.data.copy_from_slice(data);
   |         ^^^^^^^^^
   |
   = note: packed structs are only aligned by one byte, and many modern architectures penalize unaligned field accesses
   = note: creating a misaligned reference is undefined behavior (even if that reference is never dereferenced)
   = help: copy the field contents to a local variable, or replace the reference with a raw pointer and use `read_unaligned`/`write_unaligned` (loads and stores via `*p` must be properly aligned even when using raw pointers)

For more information about this error, try `rustc --explain E0793`.
error: could not compile `zc-error` (lib) due to 2 previous errors

[joshlf]

Okay, that repro's for me on the Rust Playground. Thanks! Just a heads up, there's a good chance we won't be able to put time into this until at least late June, but we'll make sure to get to it eventually.

[joshlf]

Hey, I discussed this with @jswrenn and realized that this is actually a fundamental limitation in zerocopy-derive. In particular, with just repr(C), the const parameter can affect whether or not the type has padding. Our padding type is unable to handle generics, including const generics. Unfortunately we can't support this without changes from Rust itself.