Why does this crate use packed representation in a bunch of places?

[jplatte]

A colleague of mine got this linker error today:

ld: warning: alignment (1) of atom 'l_anon.e8717737f2aafb2fc9f63ff91b30211b.2' is too small and may result in unaligned pointers 
ld: warning: pointer not aligned at address 0x10464FB19 ('l_anon.e8717737f2aafb2fc9f63ff91b30211b.2' + 1 from /Users/[...]/libmatrix_sdk_ffi.a(const_panic-ef6c6c5eecaca335.const_panic.519cbb813ef9124e-cgu.8.rcgu.o))
ld: unaligned pointer(s) for architecture arm64
clang: error: linker command failed with exit code 1 (use -v to see invocation)

First I thought that it seems really unlikely for const_panic to actually be related to the problem, but when looking at the codebase a bit I found a few uses of #[repr(packed)] as well as one Packed<&'a str> in PanicVariant that seems highly suspicious. What is the reason for using packed representation, in particular there? Is it just for efficiency?

[rodrimati1992]

Why would this error happen? Since I'm not using unsafe to pack or access the pointer, this can't cause UB (barring unsoundness bugs in rustc).

[jplatte]

I'm far from an expert on linking, so I don't really know why the linker ever cares about alignment.

But separately, I wouldn't be surprised at all if #[repr(packed)] can (still) cause UB even without unsafe accesses, similar to #[no_mangle].

[rodrimati1992]

Well, #[no_mangle] triggers the unsafe_code lint while #[repr(packed)] doesn't, so it's considered safe by the compiler.

no_mangle example

https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=d9a41beab1c5737a04dc4820f54184c9

#![deny(unsafe_code)]

#[no_mangle]
fn foo(){}

error: declaration of a `no_mangle` function
 --> src/lib.rs:3:1
  |
3 | #[no_mangle]
  | ^^^^^^^^^^^^
  |
  = note: the linker's behavior with multiple libraries exporting duplicate symbol names is undefined and Rust cannot provide guarantees when you manually override them
note: the lint level is defined here
 --> src/lib.rs:1:9
  |
1 | #![deny(unsafe_code)]
  |         ^^^^^^^^^^^

#[repr(packed)] example

This example builds without triggering the lint:
https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=0bd9127194d0d9a1b14e35e976767f07

#![deny(unsafe_code)]

#[repr(packed)]
pub struct Foo(pub u8, pub u32, pub &'static str);

pub static F: Foo = Foo(3, 5, "hello");

[jplatte]

Right.. Well the error from the issue description has now infected our CI and my small PR fixes it there as well.

• CI run with const_panic patched to use this git repo, latest main git rev still reproduces the issue
• CI run with my patch works

If you don't want to merge without actually finding out why it might be wrong, I can understand. Maybe my colleage who has started to implement their own linker has some insights, I'll ask them.

[rodrimati1992]

If you don't want to merge without actually finding out why it might be wrong

Yes, I want to know why it happens, to know whether the PR is a fix for a const_panic bug or a workaround for a compiler bug.