Rollup merge of #140638 - RalfJung:unsafe-pinned-shared-aliased, r=workingjubilee

UnsafePinned: also include the effects of UnsafeCell

This tackles https://github.com/rust-lang/rust/issues/137750 by including an `UnsafeCell` in `UnsafePinned`, thus imbuing it with all the usual properties of interior mutability (no `noalias` nor `dereferenceable` on shared refs, special treatment by Miri's aliasing model). The soundness issue is not fixed yet because coroutine lowering does not use `UnsafePinned`.

The RFC said that `UnsafePinned` would not permit mutability on shared references, but since then, https://github.com/rust-lang/rust/issues/137750 has demonstrated that this is not tenable. In the face of those examples, I propose that we do the "obvious" thing and permit shared mutable state inside `UnsafePinned`. This seems loosely consistent with the fact that we allow going from `Pin<&mut T>` to `&T` (where the former can be aliased with other pointers that perform mutation, and hence the same goes for the latter) -- but the `as_ref` example shows that we in fact would need to add this `UnsafeCell` even if we didn't have a safe conversion to `&T`, since for the compiler and Miri, `&T` and `Pin<&T>` are basically the same type.

To make this possible, I had to remove the `Copy` and `Clone` impls for `UnsafePinned`.

Tracking issue: https://github.com/rust-lang/rust/issues/125735
Cc ``@rust-lang/lang`` ``@rust-lang/opsem``  ``@Sky9x``
I don't think this needs FCP since the type is still unstable -- we'll finally decide whether we like this approach when `UnsafePinned` is moved towards stabilization (IOW, this PR is reversible). However, I'd still like to make sure that the lang team is okay with the direction I am proposing here.

1 files changed, 43 insertions, 0 deletions

diff --git a/src/tools/miri/tests/fail/async-shared-mutable.stack.stderr b/src/tools/miri/tests/fail/async-shared-mutable.stack.stderr
new file mode 100644
index 00000000000..8f53a55cc3e
--- /dev/null
+++ b/src/tools/miri/tests/fail/async-shared-mutable.stack.stderr
@@ -0,0 +1,43 @@
+error: Undefined Behavior: attempting a write access using <TAG> at ALLOC[OFFSET], but that tag does not exist in the borrow stack for this location
+  --> tests/fail/async-shared-mutable.rs:LL:CC
+   |
+LL |             *x = 1;
+   |             ^^^^^^
+   |             |
+   |             attempting a write access using <TAG> at ALLOC[OFFSET], but that tag does not exist in the borrow stack for this location
+   |             this error occurs as part of an access at ALLOC[OFFSET]
+   |
+   = help: this indicates a potential bug in the program: it performed an invalid operation, but the Stacked Borrows rules it violated are still experimental
+   = help: see https://github.com/rust-lang/unsafe-code-guidelines/blob/master/wip/stacked-borrows.md for further information
+help: <TAG> was created by a Unique retag at offsets [OFFSET]
+  --> tests/fail/async-shared-mutable.rs:LL:CC
+   |
+LL | /         core::future::poll_fn(move |_| {
+LL | |             *x = 1;
+LL | |             Poll::<()>::Pending
+LL | |         })
+LL | |         .await
+   | |______________^
+help: <TAG> was later invalidated at offsets [OFFSET] by a SharedReadOnly retag
+  --> tests/fail/async-shared-mutable.rs:LL:CC
+   |
+LL |     let _: Pin<&_> = f.as_ref(); // Or: `f.as_mut().into_ref()`.
+   |                      ^^^^^^^^^^
+   = note: BACKTRACE (of the first span):
+   = note: inside closure at tests/fail/async-shared-mutable.rs:LL:CC
+   = note: inside `<std::future::PollFn<{closure@tests/fail/async-shared-mutable.rs:LL:CC}> as std::future::Future>::poll` at RUSTLIB/core/src/future/poll_fn.rs:LL:CC
+note: inside closure
+  --> tests/fail/async-shared-mutable.rs:LL:CC
+   |
+LL |         .await
+   |          ^^^^^
+note: inside `main`
+  --> tests/fail/async-shared-mutable.rs:LL:CC
+   |
+LL |     assert_eq!(f.as_mut().poll(&mut cx), Poll::Pending);
+   |                ^^^^^^^^^^^^^^^^^^^^^^^^
+
+note: some details are omitted, run with `MIRIFLAGS=-Zmiri-backtrace=full` for a verbose backtrace
+
+error: aborting due to 1 previous error
+