impl DispatchFromDyn for Cell and UnsafeCell

library/core/src/cell.rs

@@ -196,7 +196,7 @@
 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 @@ pub fn take(&self) -> 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 @@ fn from(t: T) -> 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 @@ fn from(t: T) -> 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/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 @@ fn deref(&self) -> &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 @@ fn wrapper_ptr(self: Wrapper<Ptr<Self>>) -> 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);
 }