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#[repr(packed(N))] (tracking issue for RFC 1399) #33158
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This should probably use
attr_literalsnow likerepr(align)does, e.g.repr(pack(2)).Reacted by Niko MatsakisIs anyone working on this? If not I think I mostly know how to do it, modulo someone telling me how the attribute parsing and verification needs to be extended higher up.
Reacted by Peter Atashian and Ivan UkhovUnless someone comes in and says they're doing this before then, I'm going to take a crack at it Friday or thereabouts. My intent is to use the attribute literals approach (or support both if that's what people want), plus potentially submitting a PR against the RFC repo to update the RFC to attribute literals.
@eddyb or whoever else might know: is inserting a bunch of
[0 x u8]in packed structs going to degrade performance? The easiest approach here seems to be to multiply everything in the type index to memory index vector by 2, then insert such dummy fields in packed structs. I think that everything else after that may just work except maybe constructing constants for them, but iirc constant construction is in one place and relatively easy for me to fix in this way.@camlorn everything currently has those usually-empty arrays, for
#[repr(align(N))].
I also have a work-in-progress branch where I've simplified a lot of trans (including making constant ADTs boring), but there's more to be done - maybe I should try to polish it and open a PR soon.I'd say so, yes, at least for the trans part. In fact, I'm not sure there is anything left to do once I'm done, other than use the value from the attribute in the
alignfield.Is there anything I can do to help implement and stabilize this feature? We need it badly for
bindgen.Reacted by Ivan Ukhov, Peter Atashian, Tim Ryan and Vsevolod Zubarev64 remaining items
The final comment period, with a disposition to merge, as per the review above, is now complete.
Any takers for a stabilization PR?
I'll do that.
Reacted by MSxDOSReference documentation issue: rust-lang/reference#483.
Noting action item
Currently #[repr(packed())] structs cannot transitively contain #[repr(align())] structs due to differing behavior between msvc and gcc. Do we want to keep this a hard error, pick one behavior over the other, or provide some way to choose which behavior is desired?
seems to have been missed in review (unless I missed it reading through all the comments).
That case still errors currently. So the resolution "keep this a hard error" was picked, maybe implicitly. This should be forward-compatible with the other options.
If you have further questions, I suggest asking on Zulip. Issue necromancy doesn't usually end well. ;)
Reacted by DavidThere are already issues for
- having packed & align on the same type: E0587 error on packed and aligned structures from C #59154
- having packed structs with aligned fields: #[repr(align(N))] fields not allowed in #[repr(packed(M>=N))] structs #100743
Reacted by David
Tracking issue for rust-lang/rfcs#1399:
#[repr(packed(N))]Unresolved issues
#[repr(packed(_))]structs cannot transitively contain#[repr(align(_))]structs due to differing behavior between msvc and gcc. Do we want to keep this a hard error, pick one behavior over the other, or provide some way to choose which behavior is desired?