PinCoerceUnsized trait into core

compiler/rustc_span/src/symbol.rs

@@ -276,6 +276,7 @@ symbols! {
         Path,
         PathBuf,
         Pending,
+        PinCoerceUnsized,
         Pointer,
         Poll,
         ProcMacro,

library/alloc/src/boxed.rs

@@ -203,7 +203,7 @@ use core::ops::{AsyncFn, AsyncFnMut, AsyncFnOnce};
 use core::ops::{
     CoerceUnsized, Coroutine, CoroutineState, Deref, DerefMut, DerefPure, DispatchFromDyn, Receiver,
 };
-use core::pin::Pin;
+use core::pin::{Pin, PinCoerceUnsized};
 use core::ptr::{self, addr_of_mut, NonNull, Unique};
 use core::slice;
 use core::task::{Context, Poll};
@@ -2646,3 +2646,6 @@ impl<T: core::error::Error> core::error::Error for Box<T> {
         core::error::Error::provide(&**self, request);
     }
 }
+
+#[unstable(feature = "pin_coerce_unsized_trait", issue = "123430")]
+unsafe impl<T: ?Sized, A: Allocator> PinCoerceUnsized for Box<T, A> {}

library/alloc/src/lib.rs

@@ -138,6 +138,7 @@
 #![feature(maybe_uninit_uninit_array_transpose)]
 #![feature(panic_internals)]
 #![feature(pattern)]
+#![feature(pin_coerce_unsized_trait)]
 #![feature(ptr_internals)]
 #![feature(ptr_metadata)]
 #![feature(ptr_sub_ptr)]

library/alloc/src/rc.rs

@@ -264,6 +264,7 @@ use core::ops::{CoerceUnsized, Deref, DerefMut, DerefPure, DispatchFromDyn, Rece
 use core::panic::{RefUnwindSafe, UnwindSafe};
 #[cfg(not(no_global_oom_handling))]
 use core::pin::Pin;
+use core::pin::PinCoerceUnsized;
 use core::ptr::{self, drop_in_place, NonNull};
 #[cfg(not(no_global_oom_handling))]
 use core::slice::from_raw_parts_mut;
@@ -2181,6 +2182,9 @@ impl<T: ?Sized, A: Allocator> Deref for Rc<T, A> {
     }
 }
 
+#[unstable(feature = "pin_coerce_unsized_trait", issue = "123430")]
+unsafe impl<T: ?Sized, A: Allocator> PinCoerceUnsized for Rc<T, A> {}
+
 #[unstable(feature = "deref_pure_trait", issue = "87121")]
 unsafe impl<T: ?Sized, A: Allocator> DerefPure for Rc<T, A> {}
 
@@ -3696,6 +3700,9 @@ impl<T: ?Sized, A: Allocator> Deref for UniqueRc<T, A> {
     }
 }
 
+#[unstable(feature = "pin_coerce_unsized_trait", issue = "123430")]
+unsafe impl<T: ?Sized> PinCoerceUnsized for UniqueRc<T> {}
+
 #[unstable(feature = "unique_rc_arc", issue = "112566")]
 impl<T: ?Sized, A: Allocator> DerefMut for UniqueRc<T, A> {
     fn deref_mut(&mut self) -> &mut T {

library/alloc/src/sync.rs

@@ -23,7 +23,7 @@ use core::marker::{PhantomData, Unsize};
 use core::mem::{self, align_of_val_raw};
 use core::ops::{CoerceUnsized, Deref, DerefPure, DispatchFromDyn, Receiver};
 use core::panic::{RefUnwindSafe, UnwindSafe};
-use core::pin::Pin;
+use core::pin::{Pin, PinCoerceUnsized};
 use core::ptr::{self, NonNull};
 #[cfg(not(no_global_oom_handling))]
 use core::slice::from_raw_parts_mut;
@@ -2147,6 +2147,9 @@ impl<T: ?Sized, A: Allocator> Deref for Arc<T, A> {
     }
 }
 
+#[unstable(feature = "pin_coerce_unsized_trait", issue = "123430")]
+unsafe impl<T: ?Sized, A: Allocator> PinCoerceUnsized for Arc<T, A> {}
+
 #[unstable(feature = "deref_pure_trait", issue = "87121")]
 unsafe impl<T: ?Sized, A: Allocator> DerefPure for Arc<T, A> {}
 

library/alloc/tests/arc.rs

@@ -227,3 +227,17 @@ fn make_mut_unsized() {
     assert_eq!(*data, [11, 21, 31]);
     assert_eq!(*other_data, [110, 20, 30]);
 }
+
+#[allow(unused)]
+mod pin_coerce_unsized {
+    use alloc::sync::Arc;
+    use core::pin::Pin;
+
+    pub trait MyTrait {}
+    impl MyTrait for String {}
+
+    // Pin coercion should work for Arc
+    pub fn pin_arc(arg: Pin<Arc<String>>) -> Pin<Arc<dyn MyTrait>> {
+        arg
+    }
+}

library/alloc/tests/boxed.rs

@@ -179,3 +179,40 @@ unsafe impl Allocator for ConstAllocator {
         self
     }
 }
+
+#[allow(unused)]
+mod pin_coerce_unsized {
+    use alloc::boxed::Box;
+    use core::pin::Pin;
+
+    trait MyTrait {
+        fn action(&self) -> &str;
+    }
+    impl MyTrait for String {
+        fn action(&self) -> &str {
+            &*self
+        }
+    }
+    struct MyStruct;
+    impl MyTrait for MyStruct {
+        fn action(&self) -> &str {
+            "MyStruct"
+        }
+    }
+
+    // Pin coercion should work for Box
+    fn pin_box<T: MyTrait + 'static>(arg: Pin<Box<T>>) -> Pin<Box<dyn MyTrait>> {
+        arg
+    }
+
+    #[test]
+    fn pin_coerce_unsized_box() {
+        let my_string = "my string";
+        let a_string = Box::pin(String::from(my_string));
+        let pin_box_str = pin_box(a_string);
+        assert_eq!(pin_box_str.as_ref().action(), my_string);
+        let a_struct = Box::pin(MyStruct);
+        let pin_box_struct = pin_box(a_struct);
+        assert_eq!(pin_box_struct.as_ref().action(), "MyStruct");
+    }
+}

library/alloc/tests/lib.rs

@@ -40,6 +40,7 @@
 #![feature(drain_keep_rest)]
 #![feature(local_waker)]
 #![feature(vec_pop_if)]
+#![feature(unique_rc_arc)]
 #![allow(internal_features)]
 #![deny(fuzzy_provenance_casts)]
 #![deny(unsafe_op_in_unsafe_fn)]

library/alloc/tests/rc.rs

@@ -205,3 +205,20 @@ fn weak_may_dangle() {
     // `val` dropped here while still borrowed
     // borrow might be used here, when `val` is dropped and runs the `Drop` code for type `std::rc::Weak`
 }
+
+#[allow(unused)]
+mod pin_coerce_unsized {
+    use alloc::rc::{Rc, UniqueRc};
+    use core::pin::Pin;
+
+    pub trait MyTrait {}
+    impl MyTrait for String {}
+
+    // Pin coercion should work for Rc
+    pub fn pin_rc(arg: Pin<Rc<String>>) -> Pin<Rc<dyn MyTrait>> {
+        arg
+    }
+    pub fn pin_unique_rc(arg: Pin<UniqueRc<String>>) -> Pin<UniqueRc<dyn MyTrait>> {
+        arg
+    }
+}

library/core/src/cell.rs

@@ -255,6 +255,7 @@ use crate::fmt::{self, Debug, Display};
 use crate::marker::{PhantomData, Unsize};
 use crate::mem;
 use crate::ops::{CoerceUnsized, Deref, DerefMut, DerefPure, DispatchFromDyn};
+use crate::pin::PinCoerceUnsized;
 use crate::ptr::{self, NonNull};
 
 mod lazy;
@@ -2394,3 +2395,12 @@ fn assert_coerce_unsized(
     let _: Cell<&dyn Send> = c;
     let _: RefCell<&dyn Send> = d;
 }
+
+#[unstable(feature = "pin_coerce_unsized_trait", issue = "123430")]
+unsafe impl<T: ?Sized> PinCoerceUnsized for UnsafeCell<T> {}
+
+#[unstable(feature = "pin_coerce_unsized_trait", issue = "123430")]
+unsafe impl<T: ?Sized> PinCoerceUnsized for Cell<T> {}
+
+#[unstable(feature = "pin_coerce_unsized_trait", issue = "123430")]
+unsafe impl<T: ?Sized> PinCoerceUnsized for RefCell<T> {}

library/core/src/pin.rs

@@ -1717,10 +1717,50 @@ impl<Ptr: fmt::Pointer> fmt::Pointer for Pin<Ptr> {
 // for other reasons, though, so we just need to take care not to allow such
 // impls to land in std.
 #[stable(feature = "pin", since = "1.33.0")]
-impl<Ptr, U> CoerceUnsized<Pin<U>> for Pin<Ptr> where Ptr: CoerceUnsized<U> {}
+impl<Ptr, U> CoerceUnsized<Pin<U>> for Pin<Ptr>
+where
+    Ptr: CoerceUnsized<U> + PinCoerceUnsized,
+    U: PinCoerceUnsized,
+{
+}
+
+#[stable(feature = "pin", since = "1.33.0")]
+impl<Ptr, U> DispatchFromDyn<Pin<U>> for Pin<Ptr>
+where
+    Ptr: DispatchFromDyn<U> + PinCoerceUnsized,
+    U: PinCoerceUnsized,
+{
+}
+
+#[unstable(feature = "pin_coerce_unsized_trait", issue = "123430")]
+/// # Safety
+///
+/// Implementing this unsafe traits requires the guarantee that any two calls
+/// to `Deref::deref` must return the same value at the same address, **even after moves
+/// or unsize-coercions of `self`**, with exceptions of mutations to `self`.
+///
+/// Here, "same value" means that if `deref` returns a trait object, then the actual
+/// concrete type behind that trait object must not change.
+/// Additionally, when you unsize- coerce from `Self` to `Unsized`,
+/// then if you call `deref` on `Unsized` which returns a trait object reference,
+/// the underlying type of that trait object must be `<Self as Deref>::Target`.
+///
+/// Analogous requirements apply to other unsized types. E.g., if `deref` returns
+/// `[T]`, then the length must not change. In other words, the underlying type
+/// must not change from `[T; N]` to `[T; M]` with an `N` different from `M`.
+///
+/// If this type alos implements `DerefMut`, then the same guarantee must be upheld by
+/// calls to `deref_mut`.
+pub unsafe trait PinCoerceUnsized {}
+
+#[stable(feature = "pin", since = "1.33.0")]
+unsafe impl<'a, T: ?Sized> PinCoerceUnsized for &'a T {}
+
+#[stable(feature = "pin", since = "1.33.0")]
+unsafe impl<'a, T: ?Sized> PinCoerceUnsized for &'a mut T {}
 
 #[stable(feature = "pin", since = "1.33.0")]
-impl<Ptr, U> DispatchFromDyn<Pin<U>> for Pin<Ptr> where Ptr: DispatchFromDyn<U> {}
+unsafe impl<T: PinCoerceUnsized> PinCoerceUnsized for Pin<T> {}
 
 /// Constructs a <code>[Pin]<[&mut] T></code>, by pinning a `value: T` locally.
 ///

library/core/tests/pin.rs

@@ -29,3 +29,35 @@ fn pin_const() {
 
     pin_mut_const();
 }
+
+#[allow(unused)]
+mod pin_coerce_unsized {
+    use core::cell::{Cell, RefCell, UnsafeCell};
+    use core::pin::Pin;
+
+    // These Pins should continue to compile.
+    // Do note that these instances of Pin types cannot be used
+    // meaningfully because all methods require a Deref/DerefMut
+    // bounds on the pointer type and Cell, RefCell and UnsafeCell
+    // do not implement Deref/DerefMut.
+    pub trait MyTrait {}
+    impl MyTrait for String {}
+
+    pub fn cell(arg: Pin<Cell<Box<String>>>) -> Pin<Cell<Box<dyn MyTrait>>> {
+        arg
+    }
+    pub fn ref_cell(arg: Pin<RefCell<Box<String>>>) -> Pin<RefCell<Box<dyn MyTrait>>> {
+        arg
+    }
+    pub fn unsafe_cell(arg: Pin<UnsafeCell<Box<String>>>) -> Pin<UnsafeCell<Box<dyn MyTrait>>> {
+        arg
+    }
+
+    // These sensible Pin coercions are possible.
+    pub fn pin_mut_ref(arg: Pin<&mut String>) -> Pin<&mut dyn MyTrait> {
+        arg
+    }
+    pub fn pin_ref(arg: Pin<&String>) -> Pin<&dyn MyTrait> {
+        arg
+    }
+}