No documentation on why packed types + Drop aren't UB

[strega-nil]

https://play.rust-lang.org/?gist=73b84464abe00fb3564dbe30f10a4da7&version=stable&mode=debug&edition=2015

<Dropper as Drop>::drop is passed an unaligned reference to Dropper, which is UB.

cc @rust-lang/lang

[sfackler]

#27060

[matthewjasper]

It doesn't look unaligned https://play.rust-lang.org/?gist=377fe5fbb00ecd7bf72c94a2b2a38fb8&version=stable&mode=debug&edition=2015

[strega-nil]

@sfackler Putting Drop types in a #[repr(packed)] structure should also be deprecated, then.

[strega-nil]

Ah, nevermind; apparently, we generate a shim that copies the value before calling drop on it. We should document this.

[eddyb]

cc @arielb1

[alercah]

Is it possible to break it if Dropper is immovable?

[cramertj]

@alercah @RalfJung just left a comment about that on the Pin tracking issue.

[RalfJung]

@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.

[alercah]

The reference seems like the right place for it.

[strega-nil]

Or the Rustonomicon?

[alercah]

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.

[steveklabnik]

re-filing against the reference rust-lang/reference#488

if anyone wants to put this in the nomicon, they can, but it's optional, IMHO.