impl DispatchFromDyn for Cell and UnsafeCell

[dimpolo]

After some fruitful discussion on Internals here's my first PR to rust-lang/rust 🎉

Please let me know if there's something I missed.

This adds DispatchFromDyn impls for Cell, UnsafeCell and SyncUnsafeCell.
An existing test is also expanded to test the Cell impl (which requires the UnsafeCell impl)

The different RefCell types can not implement DispatchFromDyn since they have more than one (non ZST) field.

 

Edit:

What:

These changes allow one to make types like MyRc(code below), to be object safe method receivers after implementing DispatchFromDyn and Deref for them.

This allows for code like this:

struct MyRc<T: ?Sized>(Cell<NonNull<RcBox<T>>>);

/* impls for DispatchFromDyn, CoerceUnsized and Deref for MyRc*/

trait Trait {
    fn foo(self: MyRc<Self>);
}

let impls_trait = ...;
let rc = MyRc::new(impls_trait) as MyRc<dyn Trait>;
rc.foo();

Note: Cell and UnsafeCell won't directly become valid method receivers since they don't implement Deref. Making use of these changes requires a wrapper type and nightly features.

Why:

A custom pointer type with interior mutability allows one to store extra information in the pointer itself.
These changes allow for such a type to be a method receiver.

Examples:

My use case is a cycle aware custom Rc implementation that when dropping a cycle marks some references dangling.

On the forum andersk mentioned that they track if a Gc reference is rooted with an extra bit in the reference itself.

[rust-highfive]

Hey! It looks like you've submitted a new PR for the library teams!

If this PR contains changes to any rust-lang/rust public library APIs then please comment with r? rust-lang/libs-api @rustbot label +T-libs-api -T-libs to request review from a libs-api team reviewer. If you're unsure where your change falls no worries, just leave it as is and the reviewer will take a look and make a decision to forward on if necessary.

Examples of T-libs-api changes:

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• Introducing insta-stable changes such as new implementations of existing stable traits on existing stable types
• Introducing new or changing existing unstable library APIs (excluding permanently unstable features / features without a tracking issue)
• Changing public documentation in ways that create new stability guarantees
• Changing observable runtime behavior of library APIs

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @thomcc (or someone else) soon.

Please see the contribution instructions for more information.

[thomcc]

r? rust-lang/libs-api @rustbot label +T-libs-api -T-libs

[thomcc]

After some fruitful discussion on Internals

Is this the right link? There are no comments on it.

[dimpolo]

Sorry, I should learn that my humour doesn't translate well into text..

Nobody commented, so I thought I'll give this one a try myself :)

[dimpolo]

Would it be fair to ping?

The changes should be uncontroversial hopefully.

Implementing DispatchFromDyn enables a type to be used as a self type, if that type also implements Deref. Since Cell and UnsafeCell don't implement Deref actually making use of these changes requires a custom wrapper type and nighty features (as is shown in the test).

[m-ou-se]

@rfcbot merge

[rfcbot]

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

• @Amanieu
• @BurntSushi
• @dtolnay
• @joshtriplett
• @m-ou-se
• @yaahc

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!

See this document for info about what commands tagged team members can give me.

[BurntSushi]

Implementing DispatchFromDyn enables a type to be used as a self type

Does this mean we should rope the @rust-lang/lang team in on this? I know this is technically adding trait impls and that's libs-api, but this bit makes it sound like the trait impls are actually just a mechanism for a language hook. Increasing the set of types that can be Self types seems like a lang change?

[tmandry]

Suggested change

	trait Trait2 {
	fn cell(self: Cell<Self>); //~ ERROR invalid `self` parameter type: Cell<Self>
	}

Would this make sense to add?

[dimpolo]

If you'd like I'll add.

I don't see what it would be testing though.
I chose Cell<&Self> in the test because it could, at least in theory, provide access to &Self.
Cell<Self> is not able to provide &Self.

[rfcbot]

The final comment period, with a disposition to merge, as per the review above, is now complete.

As the automated representative of the governance process, I would like to thank the author for their work and everyone else who contributed.

This will be merged soon.

[dimpolo]

Rebased to:

• move tests from test/ to tests/
• update a .stderr file

[dimpolo]

This will be merged soon.
Dec 16, 2022

Is something holding up the merge?

[dtolnay]

@bors r+

[bors]

📌 Commit af58854 has been approved by dtolnay

It is now in the queue for this repository.