Skip to content
New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

Sign up for GitHub

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

Jump to bottom

Validity of ManuallyDrop, or: ManuallyDrop<Box<_>> is strange #245

Open
RalfJung opened this issue Aug 12, 2020 · 0 comments
Open

Validity of ManuallyDrop, or: ManuallyDrop<Box<_>> is strange #245

RalfJung opened this issue Aug 12, 2020 · 0 comments
Labels
A-validity Topic: Related to validity invariants C-open-question Category: An open question that we should revisit

Comments

@RalfJung
Copy link
Member

The ManuallyDrop docs say

ManuallyDrop is subject to the same layout optimizations as T. As a consequence, it has no effect on the assumptions that the compiler makes about its contents.

In particular, this means that a ManuallyDrop<Box<T>> must not dangle. But that means that after calling drop on a ManuallDrop<Box<T>>, that value violates the validity invariant and must not be used or passed around any more! I don't think that is the behavior we want.

We want ManuallyDrop to preserve niches, so we cannot really relax what it says about validity. But what we could relax is validity of Box -- we could say that validity is purely about the pointer value itself (it must be non-NULL and properly aligned). We could specify that the "dereferencability" only comes in through the aliasing model, and we could make that model stop looking into ManuallyDrop.
@RalfJung RalfJung mentioned this issue Aug 12, 2020
Closed
@RalfJung RalfJung added C-open-question Category: An open question that we should revisit A-validity Topic: Related to validity invariants labels Aug 12, 2020
@RalfJung RalfJung mentioned this issue Aug 12, 2020
Closed
This was referenced Dec 26, 2020
Open
Closed
@LegionMammal978 LegionMammal978 mentioned this issue Jan 19, 2022
Closed
@SabrinaJewson SabrinaJewson mentioned this issue Mar 13, 2022
Merged
@theemathas theemathas mentioned this issue Sep 9, 2024
Closed
@WaffleLapkin WaffleLapkin mentioned this issue Sep 9, 2024
Merged
@theemathas theemathas mentioned this issue Sep 12, 2024
Merged
GuillaumeGomez added a commit to GuillaumeGomez/rust that referenced this issue Sep 26, 2024
@GuillaumeGomez
Rollup merge of rust-lang#130279 - theemathas:manually-drop-docs, r=t…
8a827c7 
…homcc,traviscross

Document subtleties of `ManuallyDrop`

After seeing rust-lang#130140 and rust-lang#130141, I figured that `ManuallyDrop` needs documentation explaining its subtleties, hence this PR.

See also rust-lang/unsafe-code-guidelines#245
rust-timer added a commit to rust-lang-ci/rust that referenced this issue Sep 27, 2024
@rust-timer
Unrolled build for rust-lang#130279
4a29775 
Rollup merge of rust-lang#130279 - theemathas:manually-drop-docs, r=thomcc,traviscross

Document subtleties of `ManuallyDrop`

After seeing rust-lang#130140 and rust-lang#130141, I figured that `ManuallyDrop` needs documentation explaining its subtleties, hence this PR.

See also rust-lang/unsafe-code-guidelines#245
Sign up for free to join this conversation on GitHub. Already have an account? Sign in to comment
Assignees
No one assigned
Labels
A-validity Topic: Related to validity invariants C-open-question Category: An open question that we should revisit
Projects
None yet
Milestone
No milestone
Development

No branches or pull requests
1 participant
@RalfJung