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Matthias Krüger
e0d71f500c
impl DispatchFromDyn for Cell and UnsafeCell After some fruitful discussion on [Internals](https://internals.rust-lang.org/t/impl-dispatchfromdyn-for-cell-2/16520) 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: ```rust 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](https://internals.rust-lang.org/t/impl-dispatchfromdyn-for-cell/14762/8) andersk mentioned that they track if a `Gc` reference is rooted with an extra bit in the reference itself.