Safe offset_of for packed structs using addr_of?

[phil-opp]

Hi, I just stumbled over:

bytemuck/src/offset_of.rs

Lines 60 to 65 in c705218

	/// # Usage with `#[repr(packed)]` structs
	///
	/// Attempting to compute the offset of a `#[repr(packed)]` struct with
	/// `bytemuck::offset_of!` requires an `unsafe` block. We hope to relax this in
	/// the future, but currently it is required to work around a soundness hole in
	/// Rust (See [rust-lang/rust#27060]).

Wouldn't it be possible to do this safely using the core::ptr::addr_of macro to calculate the offset?

[notgull]

offset_of support predates addr_of in the standard library.

[Lokathor]

We could probably adjust things but it would have to be opt-in, because the offset_of module isn't feature gated and so it needs to work on 1.34 by default.

[phil-opp]

Ah sorry, I missed the README note about the 1.34 MSRV. In that case, the easiest solution is probably to add a new safe_offset_of macro, gated behind a cargo feature. Would that be ok with you?

[Lokathor]

Ah, well we could but I thought that the standard library offset_of was developing at a reasonable enough pace for people to just be able to use that sooner than later.

Also, double checking the file itself I actually don't see the unsafe block that is being referred to, so is that note old?

[phil-opp]

I wasn't aware that the standard library offset_of is so close to stabilization. I'm fine with waiting a bit and using a manual workaround based on addr_of for packed structs until then.

Also, double checking the file itself I actually don't see the unsafe block that is being referred to, so is that note old?

I think the unsafe block is required by Rust when you try to create a reference to a field of a packed struct, as it might be unaligned.

[Lokathor]

phil why are you using packed structs to begin with? They're very bad for your health, you shouldn't ;3

[phil-opp]

Haha, yeah :D. I'm dealing with a file format that contains some 64-bit offsets that are only aligned to 4 bytes. It's just some prototype code to figure out the details of the format, so I'm ignoring endianness for now do a direct cast to Rust structs.

[Lokathor]

https://docs.rs/pack1/latest/pack1/ may help you

[phil-opp]

Thanks, this helps indeed!