Rollup merge of #97373 - dimpolo:cell_dispatch_from_dyn, r=dtolnay

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.

library/core/src/cell.rs

@@ -196,7 +196,7 @@ use crate::cmp::Ordering;
 use crate::fmt::{self, Debug, Display};
 use crate::marker::{PhantomData, Unsize};
 use crate::mem;
-use crate::ops::{CoerceUnsized, Deref, DerefMut};
+use crate::ops::{CoerceUnsized, Deref, DerefMut, DispatchFromDyn};
 use crate::ptr::{self, NonNull};
 
 mod lazy;
@@ -571,6 +571,16 @@ impl<T: Default> Cell<T> {
 #[unstable(feature = "coerce_unsized", issue = "18598")]
 impl<T: CoerceUnsized<U>, U> CoerceUnsized<Cell<U>> for Cell<T> {}
 
+// Allow types that wrap `Cell` to also implement `DispatchFromDyn`
+// and become object safe method receivers.
+// Note that currently `Cell` itself cannot be a method receiver
+// because it does not implement Deref.
+// In other words:
+// `self: Cell<&Self>` won't work
+// `self: CellWrapper<Self>` becomes possible
+#[unstable(feature = "dispatch_from_dyn", issue = "none")]
+impl<T: DispatchFromDyn<U>, U> DispatchFromDyn<Cell<U>> for Cell<T> {}
+
 impl<T> Cell<[T]> {
     /// Returns a `&[Cell<T>]` from a `&Cell<[T]>`
     ///
@@ -2078,6 +2088,16 @@ impl<T> const From<T> for UnsafeCell<T> {
 #[unstable(feature = "coerce_unsized", issue = "18598")]
 impl<T: CoerceUnsized<U>, U> CoerceUnsized<UnsafeCell<U>> for UnsafeCell<T> {}
 
+// Allow types that wrap `UnsafeCell` to also implement `DispatchFromDyn`
+// and become object safe method receivers.
+// Note that currently `UnsafeCell` itself cannot be a method receiver
+// because it does not implement Deref.
+// In other words:
+// `self: UnsafeCell<&Self>` won't work
+// `self: UnsafeCellWrapper<Self>` becomes possible
+#[unstable(feature = "dispatch_from_dyn", issue = "none")]
+impl<T: DispatchFromDyn<U>, U> DispatchFromDyn<UnsafeCell<U>> for UnsafeCell<T> {}
+
 /// [`UnsafeCell`], but [`Sync`].
 ///
 /// This is just an `UnsafeCell`, except it implements `Sync`
@@ -2169,6 +2189,17 @@ impl<T> const From<T> for SyncUnsafeCell<T> {
 //#[unstable(feature = "sync_unsafe_cell", issue = "95439")]
 impl<T: CoerceUnsized<U>, U> CoerceUnsized<SyncUnsafeCell<U>> for SyncUnsafeCell<T> {}
 
+// Allow types that wrap `SyncUnsafeCell` to also implement `DispatchFromDyn`
+// and become object safe method receivers.
+// Note that currently `SyncUnsafeCell` itself cannot be a method receiver
+// because it does not implement Deref.
+// In other words:
+// `self: SyncUnsafeCell<&Self>` won't work
+// `self: SyncUnsafeCellWrapper<Self>` becomes possible
+#[unstable(feature = "dispatch_from_dyn", issue = "none")]
+//#[unstable(feature = "sync_unsafe_cell", issue = "95439")]
+impl<T: DispatchFromDyn<U>, U> DispatchFromDyn<SyncUnsafeCell<U>> for SyncUnsafeCell<T> {}
+
 #[allow(unused)]
 fn assert_coerce_unsized(
     a: UnsafeCell<&i32>,

tests/ui/feature-gates/feature-gate-dispatch-from-dyn-cell.rs

@@ -0,0 +1,9 @@
+// Check that even though Cell: DispatchFromDyn it remains an invalid self parameter type
+
+use std::cell::Cell;
+
+trait Trait{
+    fn cell(self: Cell<&Self>); //~ ERROR invalid `self` parameter type: Cell<&Self>
+}
+
+fn main() {}

tests/ui/feature-gates/feature-gate-dispatch-from-dyn-cell.stderr

@@ -0,0 +1,12 @@
+error[E0307]: invalid `self` parameter type: Cell<&Self>
+  --> $DIR/feature-gate-dispatch-from-dyn-cell.rs:6:19
+   |
+LL |     fn cell(self: Cell<&Self>);
+   |                   ^^^^^^^^^^^
+   |
+   = note: type of `self` must be `Self` or a type that dereferences to it
+   = help: consider changing to `self`, `&self`, `&mut self`, `self: Box<Self>`, `self: Rc<Self>`, `self: Arc<Self>`, or `self: Pin<P>` (where P is one of the previous types except `Self`)
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0307`.

tests/ui/feature-gates/feature-gate-dispatch-from-dyn-missing-impl.rs

@@ -0,0 +1,35 @@
+// Check that a self parameter type requires a DispatchFromDyn impl to be object safe
+
+#![feature(arbitrary_self_types, unsize, coerce_unsized)]
+
+use std::{
+    marker::Unsize,
+    ops::{CoerceUnsized, Deref},
+};
+
+struct Ptr<T: ?Sized>(Box<T>);
+
+impl<T: ?Sized> Deref for Ptr<T> {
+    type Target = T;
+
+    fn deref(&self) -> &T {
+        &*self.0
+    }
+}
+
+impl<T: Unsize<U> + ?Sized, U: ?Sized> CoerceUnsized<Ptr<U>> for Ptr<T> {}
+// Because this impl is missing the coercion below fails.
+// impl<T: Unsize<U> + ?Sized, U: ?Sized> DispatchFromDyn<Ptr<U>> for Ptr<T> {}
+
+trait Trait {
+    fn ptr(self: Ptr<Self>);
+}
+impl Trait for i32 {
+    fn ptr(self: Ptr<Self>) {}
+}
+
+fn main() {
+    Ptr(Box::new(4)) as Ptr<dyn Trait>;
+    //~^ ERROR the trait `Trait` cannot be made into an object
+    //~^^ ERROR the trait `Trait` cannot be made into an object
+}

tests/ui/feature-gates/feature-gate-dispatch-from-dyn-missing-impl.stderr

@@ -0,0 +1,45 @@
+error[E0038]: the trait `Trait` cannot be made into an object
+  --> $DIR/feature-gate-dispatch-from-dyn-missing-impl.rs:32:25
+   |
+LL |     fn ptr(self: Ptr<Self>);
+   |                  --------- help: consider changing method `ptr`'s `self` parameter to be `&self`: `&Self`
+...
+LL |     Ptr(Box::new(4)) as Ptr<dyn Trait>;
+   |                         ^^^^^^^^^^^^^^ `Trait` cannot be made into an object
+   |
+note: for a trait to be "object safe" it needs to allow building a vtable to allow the call to be resolvable dynamically; for more information visit <https://doc.rust-lang.org/reference/items/traits.html#object-safety>
+  --> $DIR/feature-gate-dispatch-from-dyn-missing-impl.rs:25:18
+   |
+LL | trait Trait {
+   |       ----- this trait cannot be made into an object...
+LL |     fn ptr(self: Ptr<Self>);
+   |                  ^^^^^^^^^ ...because method `ptr`'s `self` parameter cannot be dispatched on
+
+error[E0038]: the trait `Trait` cannot be made into an object
+  --> $DIR/feature-gate-dispatch-from-dyn-missing-impl.rs:32:5
+   |
+LL |     fn ptr(self: Ptr<Self>);
+   |                  --------- help: consider changing method `ptr`'s `self` parameter to be `&self`: `&Self`
+...
+LL |     Ptr(Box::new(4)) as Ptr<dyn Trait>;
+   |     ^^^^^^^^^^^^^^^^ `Trait` cannot be made into an object
+   |
+note: for a trait to be "object safe" it needs to allow building a vtable to allow the call to be resolvable dynamically; for more information visit <https://doc.rust-lang.org/reference/items/traits.html#object-safety>
+  --> $DIR/feature-gate-dispatch-from-dyn-missing-impl.rs:25:18
+   |
+LL | trait Trait {
+   |       ----- this trait cannot be made into an object...
+LL |     fn ptr(self: Ptr<Self>);
+   |                  ^^^^^^^^^ ...because method `ptr`'s `self` parameter cannot be dispatched on
+note: required for `Ptr<{integer}>` to implement `CoerceUnsized<Ptr<dyn Trait>>`
+  --> $DIR/feature-gate-dispatch-from-dyn-missing-impl.rs:20:40
+   |
+LL | impl<T: Unsize<U> + ?Sized, U: ?Sized> CoerceUnsized<Ptr<U>> for Ptr<T> {}
+   |         ---------                      ^^^^^^^^^^^^^^^^^^^^^     ^^^^^^
+   |         |
+   |         unsatisfied trait bound introduced here
+   = note: required by cast to type `Ptr<dyn Trait>`
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0038`.

tests/ui/self/arbitrary_self_types_pointers_and_wrappers.rs

@@ -3,6 +3,7 @@
 #![feature(rustc_attrs)]
 
 use std::{
+    cell::Cell,
     ops::{Deref, CoerceUnsized, DispatchFromDyn},
     marker::Unsize,
 };
@@ -20,6 +21,20 @@ impl<T: ?Sized> Deref for Ptr<T> {
 impl<T: Unsize<U> + ?Sized, U: ?Sized> CoerceUnsized<Ptr<U>> for Ptr<T> {}
 impl<T: Unsize<U> + ?Sized, U: ?Sized> DispatchFromDyn<Ptr<U>> for Ptr<T> {}
 
+
+struct CellPtr<'a, T: ?Sized>(Cell<&'a T>);
+
+impl<'a, T: ?Sized> Deref for CellPtr<'a, T> {
+    type Target = T;
+
+    fn deref(&self) -> &T {
+        self.0.get()
+    }
+}
+
+impl<'a, T: Unsize<U> + ?Sized, U: ?Sized> CoerceUnsized<CellPtr<'a, U>> for CellPtr<'a, T> {}
+impl<'a, T: Unsize<U> + ?Sized, U: ?Sized> DispatchFromDyn<CellPtr<'a, U>> for CellPtr<'a, T> {}
+
 struct Wrapper<T: ?Sized>(T);
 
 impl<T: ?Sized> Deref for Wrapper<T> {
@@ -42,6 +57,7 @@ trait Trait {
     fn ptr_wrapper(self: Ptr<Wrapper<Self>>) -> i32;
     fn wrapper_ptr(self: Wrapper<Ptr<Self>>) -> i32;
     fn wrapper_ptr_wrapper(self: Wrapper<Ptr<Wrapper<Self>>>) -> i32;
+    fn cell(self: CellPtr<Self>) -> i32;
 }
 
 impl Trait for i32 {
@@ -54,6 +70,9 @@ impl Trait for i32 {
     fn wrapper_ptr_wrapper(self: Wrapper<Ptr<Wrapper<Self>>>) -> i32 {
         ***self
     }
+    fn cell(self: CellPtr<Self>) -> i32 {
+        *self
+    }
 }
 
 fn main() {
@@ -65,4 +84,7 @@ fn main() {
 
     let wpw = Wrapper(Ptr(Box::new(Wrapper(7)))) as Wrapper<Ptr<Wrapper<dyn Trait>>>;
     assert_eq!(wpw.wrapper_ptr_wrapper(), 7);
+
+    let c = CellPtr(Cell::new(&8)) as CellPtr<dyn Trait>;
+    assert_eq!(c.cell(), 8);
 }