Rust: No documentation on why packed types + Drop aren't UB
<Dropper as Drop>::drop is passed an unaligned reference to Dropper, which is UB.
cc @rust-lang/lang
strega-nil
All 12 comments
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sfackler
on 12 Sep 2018
It doesn't look unaligned https://play.rust-lang.org/?gist=377fe5fbb00ecd7bf72c94a2b2a38fb8&version=stable&mode=debug&edition=2015
matthewjasper
on 12 Sep 2018
@sfackler Putting Drop types in a #[repr(packed)] structure should also be deprecated, then.
strega-nil
on 12 Sep 2018
Ah, nevermind; apparently, we generate a shim that copies the value before calling drop on it. We should document this.
strega-nil
on 12 Sep 2018
cc @arielb1
eddyb
on 12 Sep 2018
Is it possible to break it if Dropper is immovable?
alercah
on 12 Sep 2018
@alercah @RalfJung just left a comment about that on the Pin tracking issue.
cramertj
on 12 Sep 2018
@ubsan what would be a good place to document this? Do we have attribute docs so we can put it with repr(packed)?
Might be worth reopening (and renaming) this issue to track that missing doc.
RalfJung
on 13 Sep 2018
The reference seems like the right place for it.
alercah
on 13 Sep 2018
Or the Rustonomicon?
strega-nil
on 13 Sep 2018
I think the long-term vision should be the reference holding normative text, and the 'Nomicon being more of a practical guide. So probably the correct answer is both.
alercah
on 13 Sep 2018
re-filing against the reference https://github.com/rust-lang-nursery/reference/issues/488
if anyone wants to put this in the nomicon, they can, but it's optional, IMHO.
steveklabnik
on 27 Dec 2018
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I think the long-term vision should be the reference holding normative text, and the 'Nomicon being more of a practical guide. So probably the correct answer is both.