Rollup merge of #125048 - dingxiangfei2009:stable-deref, r=amanieu

PinCoerceUnsized trait into core

cc ``@Darksonn`` ``@wedsonaf`` ``@ojeda``

This is a PR to introduce a `PinCoerceUnsized` trait in order to make trait impls generated by the proc-macro `#[derive(SmartPointer)]`, proposed by [RFC](e17e19ac7a/text/3621-derive-smart-pointer.md (pincoerceunsized-1)), sound. There you may find explanation, justification and discussion about the alternatives.

Note that we do not seek stabilization of this `PinCoerceUnsized` trait in the near future. The stabilisation of this trait does not block the eventual stabilization process of the `#[derive(SmartPointer)]` macro. Ideally, use of `DerefPure` is more preferrable except this will actually constitute a breaking change. `PinCoerceUnsized` emerges as a solution to the said soundness hole while avoiding the breaking change. More details on the `DerefPure` option have been described in this [section](e17e19ac7a/text/3621-derive-smart-pointer.md (derefpure)) of the RFC linked above.

Earlier discussion can be found in this [Zulip stream](https://rust-lang.zulipchat.com/#narrow/stream/136281-t-opsem/topic/Pin.20and.20soundness.20of.20unsizing.20coercions) and [rust-for-linux thread](https://rust-lang.zulipchat.com/#narrow/stream/425075-rust-for-linux/topic/.23.5Bderive.28SmartPointer.29.5D.20and.20pin.20unsoundness.20rfc.233621).

try-job: dist-various-2

library/core/tests/pin.rs

@@ -29,3 +29,49 @@ fn pin_const() {
 
     pin_mut_const();
 }
+
+#[allow(unused)]
+mod pin_coerce_unsized {
+    use core::cell::{Cell, RefCell, UnsafeCell};
+    use core::pin::Pin;
+    use core::ptr::NonNull;
+
+    pub trait MyTrait {}
+    impl MyTrait for String {}
+
+    // 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 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
+    }
+    pub fn pin_ptr(arg: Pin<*const String>) -> Pin<*const dyn MyTrait> {
+        arg
+    }
+    pub fn pin_ptr_mut(arg: Pin<*mut String>) -> Pin<*mut dyn MyTrait> {
+        arg
+    }
+    pub fn pin_non_null(arg: Pin<NonNull<String>>) -> Pin<NonNull<dyn MyTrait>> {
+        arg
+    }
+    pub fn nesting_pins(arg: Pin<Pin<&String>>) -> Pin<Pin<&dyn MyTrait>> {
+        arg
+    }
+}