Implement PointerLike for isize, NonNull, Cell, UnsafeCell, and SyncUnsafeCell.

Implementing `PointerLike` for `UnsafeCell` enables the possibility of
interior mutable `dyn*` values. Since this means potentially exercising
new codegen behavior, I added a test for it in `tests/ui/dyn-star/cell.rs`.

Also updated UI tests to account for the `isize` implementation changing
error messages.

Author: kpreid

core/src/cell.rs

@@ -252,7 +252,7 @@
 
 use crate::cmp::Ordering;
 use crate::fmt::{self, Debug, Display};
-use crate::marker::{PhantomData, Unsize};
+use crate::marker::{PhantomData, PointerLike, Unsize};
 use crate::mem;
 use crate::ops::{CoerceUnsized, Deref, DerefMut, DerefPure, DispatchFromDyn};
 use crate::pin::PinCoerceUnsized;
@@ -677,6 +677,9 @@ impl<T: CoerceUnsized<U>, U> CoerceUnsized<Cell<U>> for Cell<T> {}
 #[unstable(feature = "dispatch_from_dyn", issue = "none")]
 impl<T: DispatchFromDyn<U>, U> DispatchFromDyn<Cell<U>> for Cell<T> {}
 
+#[unstable(feature = "pointer_like_trait", issue = "none")]
+impl<T: PointerLike> PointerLike for Cell<T> {}
+
 impl<T> Cell<[T]> {
     /// Returns a `&[Cell<T>]` from a `&Cell<[T]>`
     ///
@@ -2258,6 +2261,9 @@ impl<T: CoerceUnsized<U>, U> CoerceUnsized<UnsafeCell<U>> for UnsafeCell<T> {}
 #[unstable(feature = "dispatch_from_dyn", issue = "none")]
 impl<T: DispatchFromDyn<U>, U> DispatchFromDyn<UnsafeCell<U>> for UnsafeCell<T> {}
 
+#[unstable(feature = "pointer_like_trait", issue = "none")]
+impl<T: PointerLike> PointerLike for UnsafeCell<T> {}
+
 /// [`UnsafeCell`], but [`Sync`].
 ///
 /// This is just an `UnsafeCell`, except it implements `Sync`
@@ -2364,6 +2370,9 @@ impl<T: CoerceUnsized<U>, U> CoerceUnsized<SyncUnsafeCell<U>> for SyncUnsafeCell
 //#[unstable(feature = "sync_unsafe_cell", issue = "95439")]
 impl<T: DispatchFromDyn<U>, U> DispatchFromDyn<SyncUnsafeCell<U>> for SyncUnsafeCell<T> {}
 
+#[unstable(feature = "pointer_like_trait", issue = "none")]
+impl<T: PointerLike> PointerLike for SyncUnsafeCell<T> {}
+
 #[allow(unused)]
 fn assert_coerce_unsized(
     a: UnsafeCell<&i32>,

core/src/marker.rs

@@ -1003,6 +1003,7 @@ pub trait PointerLike {}
 marker_impls! {
     #[unstable(feature = "pointer_like_trait", issue = "none")]
     PointerLike for
+        isize,
         usize,
         {T} &T,
         {T} &mut T,

core/src/ptr/non_null.rs

@@ -1555,6 +1555,10 @@ impl<T: ?Sized, U: ?Sized> DispatchFromDyn<NonNull<U>> for NonNull<T> where T: U
 #[stable(feature = "pin", since = "1.33.0")]
 unsafe impl<T: ?Sized> PinCoerceUnsized for NonNull<T> {}
 
+#[unstable(feature = "pointer_like_trait", issue = "none")]
+#[cfg(not(bootstrap))]
+impl<T> core::marker::PointerLike for NonNull<T> {}
+
 #[stable(feature = "nonnull", since = "1.25.0")]
 impl<T: ?Sized> fmt::Debug for NonNull<T> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {