Document that ManuallyDrop's Box interaction has been fixed

[WaffleLapkin]

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Both #150447 and #150446 has been merged in 1.96.0, fixing the unfortunate ManuallyDrop<Box<_>> behavior.

cc @RalfJung

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[jhpratt]

@bors r+ rollup

[rust-bors]

📌 Commit 8520b8e has been approved by jhpratt

It is now in the queue for this repository.

[RalfJung]

@bors r-

Uh no we can't yet make this a stable guarantee I don't think.

[rust-bors]

This pull request was unapproved.

This PR was contained in a rollup (#155807), which was unapproved.

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[RalfJung]

We don't usually document the behavior of past versions of Rust in the current docs, do we? I guess the point is that people should be aware that there's a sort of MSRV effect by relying on this behavior?

And you even added new examples, making the docs more extensive than they were when this affected latest stable Rust.

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[WaffleLapkin]

The idea is indeed that there is an MSRV effect to this behavior (or luck of the footgun...)

I did not add examples, only move existing ones around (GitHub diff is being very unhelpful here)

[WaffleLapkin]

I think people usually just go to doc.rust-lang.org/stable for docs, rather than checking for docs of their MSRV, so keeping the warning seems like a good idea to be.

[RalfJung]

Oh right I misread the diff.

[RalfJung]

If we make this a stable guarantee then even if MaybeDangling ends up getting rejected, we are committing to having something like this in the language permanently. This should hence go through FCP, for both lang and libs-api I assume.

[nia-e]

This definitely requires signoff from lang, but if they are willing to preserve MaybeDangling or similar semantics into the future we're happy making the API guarantee on the libs-api side per today's meeting ^^

[traviscross]

Thanks @WaffleLapkin. Sounds right to me. This is a kind of partial stabilization of RFC 3336.

@rfcbot fcp merge libs-api

[traviscross]

@rfcbot fcp merge lang,libs-api

[rust-rfcbot]

Team member @traviscross has proposed to merge this. The next step is review by the rest of the tagged team members:

• @Amanieu
• @BurntSushi
• @dtolnay
• @joshtriplett
• @nikomatsakis
• @scottmcm
• @the8472
• @tmandry
• @traviscross

No concerns currently listed.

Once a majority of reviewers approve (and at most 2 approvals are outstanding), this will enter its final comment period. If you spot a major issue that hasn't been raised at any point in this process, please speak up!

cc @rust-lang/lang-advisors: FCP proposed for lang, please feel free to register concerns.
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[RalfJung]

This is a kind of partial stabilization of RFC 3336.

Exactly. It stabilizes the ManuallyDrop effects described there, without (for now) making those effects available as their own type.

[tmandry]

Hypothetically, someone might want ManuallyDrop without MaybeDangling for optimization purposes, but that is less important than making more uses of ManuallyDrop sound. We can expose a more fundamental feature in the future that one could use to implement ManuallyDrop; see e.g. the proposals in https://rust-lang.github.io/rust-project-goals/2026/manually-drop-attr.html.

@rfcbot reviewed