UnsafePinned: implement opsem effects of UnsafeUnpin

compiler/rustc_middle/src/queries.rs

@@ -1694,6 +1694,10 @@ rustc_queries! {
     query is_freeze_raw(env: ty::PseudoCanonicalInput<'tcx, Ty<'tcx>>) -> bool {
         desc { "computing whether `{}` is freeze", env.value }
     }
+    /// Query backing `Ty::is_unsafe_unpin`.
+    query is_unsafe_unpin_raw(env: ty::PseudoCanonicalInput<'tcx, Ty<'tcx>>) -> bool {
+        desc { "computing whether `{}` is `UnsafeUnpin`", env.value }
+    }
     /// Query backing `Ty::is_unpin`.
     query is_unpin_raw(env: ty::PseudoCanonicalInput<'tcx, Ty<'tcx>>) -> bool {
         desc { "computing whether `{}` is `Unpin`", env.value }

compiler/rustc_middle/src/ty/layout.rs

@@ -1049,9 +1049,11 @@ where
                     hir::Mutability::Not => {
                         PointerKind::SharedRef { frozen: optimize && ty.is_freeze(tcx, typing_env) }
                     }
-                    hir::Mutability::Mut => {
+                    hir::Mutability::Mut => PointerKind::MutableRef {
-                        PointerKind::MutableRef { unpin: optimize && ty.is_unpin(tcx, typing_env) }
+                        unpin: optimize
-                    }
+                            && ty.is_unpin(tcx, typing_env)
+                            && ty.is_unsafe_unpin(tcx, typing_env),
+                    },
                 };
 
                 tcx.layout_of(typing_env.as_query_input(ty)).ok().map(|layout| PointeeInfo {
@@ -1146,7 +1148,9 @@ where
                         debug_assert!(pointee.safe.is_none());
                         let optimize = tcx.sess.opts.optimize != OptLevel::No;
                         pointee.safe = Some(PointerKind::Box {
-                            unpin: optimize && boxed_ty.is_unpin(tcx, typing_env),
+                            unpin: optimize
+                                && boxed_ty.is_unpin(tcx, typing_env)
+                                && boxed_ty.is_unsafe_unpin(tcx, typing_env),
                             global: this.ty.is_box_global(tcx),
                         });
                     }

compiler/rustc_middle/src/ty/util.rs

@@ -1189,14 +1189,23 @@ impl<'tcx> Ty<'tcx> {
         }
     }
 
+    /// Checks whether values of this type `T` implement the `UnsafeUnpin` trait.
+    pub fn is_unsafe_unpin(self, tcx: TyCtxt<'tcx>, typing_env: ty::TypingEnv<'tcx>) -> bool {
+        self.is_trivially_unpin() || tcx.is_unsafe_unpin_raw(typing_env.as_query_input(self))
+    }
+
     /// Checks whether values of this type `T` implement the `Unpin` trait.
+    ///
+    /// Note that this is a safe trait, so it cannot be very semantically meaningful.
+    /// However, as a hack to mitigate <https://github.com/rust-lang/rust/issues/63818> until a
+    /// proper solution is implemented, we do give special semantics to the `Unpin` trait.
     pub fn is_unpin(self, tcx: TyCtxt<'tcx>, typing_env: ty::TypingEnv<'tcx>) -> bool {
         self.is_trivially_unpin() || tcx.is_unpin_raw(typing_env.as_query_input(self))
     }
 
-    /// Fast path helper for testing if a type is `Unpin`.
+    /// Fast path helper for testing if a type is `Unpin` *and* `UnsafeUnpin`.
     ///
-    /// Returning true means the type is known to be `Unpin`. Returning
+    /// Returning true means the type is known to be `Unpin` and `UnsafeUnpin`. Returning
     /// `false` means nothing -- could be `Unpin`, might not be.
     fn is_trivially_unpin(self) -> bool {
         match self.kind() {

compiler/rustc_ty_utils/src/abi.rs

@@ -306,8 +306,12 @@ fn arg_attrs_for_rust_scalar<'tcx>(
         let kind = if let Some(kind) = pointee.safe {
             Some(kind)
         } else if let Some(pointee) = drop_target_pointee {
+            assert_eq!(pointee, layout.ty.builtin_deref(true).unwrap());
+            assert_eq!(offset, Size::ZERO);
             // The argument to `drop_in_place` is semantically equivalent to a mutable reference.
-            Some(PointerKind::MutableRef { unpin: pointee.is_unpin(tcx, cx.typing_env) })
+            let mutref = Ty::new_mut_ref(tcx, tcx.lifetimes.re_erased, pointee);
+            let layout = cx.layout_of(mutref).unwrap();
+            layout.pointee_info_at(&cx, offset).and_then(|pi| pi.safe)
         } else {
             None
         };

compiler/rustc_ty_utils/src/common_traits.rs

@@ -8,36 +8,43 @@ use rustc_span::DUMMY_SP;
 use rustc_trait_selection::traits;
 
 fn is_copy_raw<'tcx>(tcx: TyCtxt<'tcx>, query: ty::PseudoCanonicalInput<'tcx, Ty<'tcx>>) -> bool {
-    is_item_raw(tcx, query, LangItem::Copy)
+    is_trait_raw(tcx, query, LangItem::Copy)
 }
 
 fn is_use_cloned_raw<'tcx>(
     tcx: TyCtxt<'tcx>,
     query: ty::PseudoCanonicalInput<'tcx, Ty<'tcx>>,
 ) -> bool {
-    is_item_raw(tcx, query, LangItem::UseCloned)
+    is_trait_raw(tcx, query, LangItem::UseCloned)
 }
 
 fn is_sized_raw<'tcx>(tcx: TyCtxt<'tcx>, query: ty::PseudoCanonicalInput<'tcx, Ty<'tcx>>) -> bool {
-    is_item_raw(tcx, query, LangItem::Sized)
+    is_trait_raw(tcx, query, LangItem::Sized)
 }
 
 fn is_freeze_raw<'tcx>(tcx: TyCtxt<'tcx>, query: ty::PseudoCanonicalInput<'tcx, Ty<'tcx>>) -> bool {
-    is_item_raw(tcx, query, LangItem::Freeze)
+    is_trait_raw(tcx, query, LangItem::Freeze)
+}
+
+fn is_unsafe_unpin_raw<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    query: ty::PseudoCanonicalInput<'tcx, Ty<'tcx>>,
+) -> bool {
+    is_trait_raw(tcx, query, LangItem::UnsafeUnpin)
 }
 
 fn is_unpin_raw<'tcx>(tcx: TyCtxt<'tcx>, query: ty::PseudoCanonicalInput<'tcx, Ty<'tcx>>) -> bool {
-    is_item_raw(tcx, query, LangItem::Unpin)
+    is_trait_raw(tcx, query, LangItem::Unpin)
 }
 
 fn is_async_drop_raw<'tcx>(
     tcx: TyCtxt<'tcx>,
     query: ty::PseudoCanonicalInput<'tcx, Ty<'tcx>>,
 ) -> bool {
-    is_item_raw(tcx, query, LangItem::AsyncDrop)
+    is_trait_raw(tcx, query, LangItem::AsyncDrop)
 }
 
-fn is_item_raw<'tcx>(
+fn is_trait_raw<'tcx>(
     tcx: TyCtxt<'tcx>,
     query: ty::PseudoCanonicalInput<'tcx, Ty<'tcx>>,
     item: LangItem,
@@ -53,6 +60,7 @@ pub(crate) fn provide(providers: &mut Providers) {
         is_use_cloned_raw,
         is_sized_raw,
         is_freeze_raw,
+        is_unsafe_unpin_raw,
         is_unpin_raw,
         is_async_drop_raw,
         ..*providers

library/core/src/marker.rs

@@ -927,14 +927,20 @@ marker_impls! {
 /// This is part of [RFC 3467](https://rust-lang.github.io/rfcs/3467-unsafe-pinned.html), and is
 /// tracked by [#125735](https://github.com/rust-lang/rust/issues/125735).
 #[lang = "unsafe_unpin"]
-pub(crate) unsafe auto trait UnsafeUnpin {}
+#[unstable(feature = "unsafe_unpin", issue = "125735")]
+pub unsafe auto trait UnsafeUnpin {}
 
+#[unstable(feature = "unsafe_unpin", issue = "125735")]
 impl<T: ?Sized> !UnsafeUnpin for UnsafePinned<T> {}
-unsafe impl<T: ?Sized> UnsafeUnpin for PhantomData<T> {}
+marker_impls! {
-unsafe impl<T: ?Sized> UnsafeUnpin for *const T {}
+#[unstable(feature = "unsafe_unpin", issue = "125735")]
-unsafe impl<T: ?Sized> UnsafeUnpin for *mut T {}
+    unsafe UnsafeUnpin for
-unsafe impl<T: ?Sized> UnsafeUnpin for &T {}
+        {T: ?Sized} PhantomData<T>,
-unsafe impl<T: ?Sized> UnsafeUnpin for &mut T {}
+        {T: ?Sized} *const T,
+        {T: ?Sized} *mut T,
+        {T: ?Sized} &T,
+        {T: ?Sized} &mut T,
+}
 
 /// Types that do not require any pinning guarantees.
 ///
@@ -1027,6 +1033,7 @@ impl !Unpin for PhantomPinned {}
 // continue working. Ideally PhantomPinned could just wrap an `UnsafePinned<()>` to get the same
 // effect, but we can't add a new field to an already stable unit struct -- that would be a breaking
 // change.
+#[unstable(feature = "unsafe_unpin", issue = "125735")]
 impl !UnsafeUnpin for PhantomPinned {}
 
 marker_impls! {

src/tools/miri/src/borrow_tracker/stacked_borrows/mod.rs

@@ -71,7 +71,9 @@ impl NewPermission {
                         access: None,
                         protector: None,
                     }
-                } else if pointee.is_unpin(*cx.tcx, cx.typing_env()) {
+                } else if pointee.is_unpin(*cx.tcx, cx.typing_env())
+                    && pointee.is_unsafe_unpin(*cx.tcx, cx.typing_env())
+                {
                     // A regular full mutable reference. On `FnEntry` this is `noalias` and `dereferenceable`.
                     NewPermission::Uniform {
                         perm: Permission::Unique,
@@ -129,7 +131,9 @@ impl NewPermission {
     fn from_box_ty<'tcx>(ty: Ty<'tcx>, kind: RetagKind, cx: &crate::MiriInterpCx<'tcx>) -> Self {
         // `ty` is not the `Box` but the field of the Box with this pointer (due to allocator handling).
         let pointee = ty.builtin_deref(true).unwrap();
-        if pointee.is_unpin(*cx.tcx, cx.typing_env()) {
+        if pointee.is_unpin(*cx.tcx, cx.typing_env())
+            && pointee.is_unsafe_unpin(*cx.tcx, cx.typing_env())
+        {
             // A regular box. On `FnEntry` this is `noalias`, but not `dereferenceable` (hence only
             // a weak protector).
             NewPermission::Uniform {

src/tools/miri/src/borrow_tracker/tree_borrows/mod.rs

@@ -133,7 +133,8 @@ impl<'tcx> NewPermission {
         retag_kind: RetagKind,
         cx: &crate::MiriInterpCx<'tcx>,
     ) -> Option<Self> {
-        let ty_is_unpin = pointee.is_unpin(*cx.tcx, cx.typing_env());
+        let ty_is_unpin = pointee.is_unpin(*cx.tcx, cx.typing_env())
+            && pointee.is_unsafe_unpin(*cx.tcx, cx.typing_env());
         let ty_is_freeze = pointee.is_freeze(*cx.tcx, cx.typing_env());
         let is_protected = retag_kind == RetagKind::FnEntry;
 

src/tools/miri/tests/pass/both_borrows/unsafe_pinned.rs

@@ -9,8 +9,36 @@ fn mutate(x: &UnsafePinned<i32>) {
     unsafe { ptr.write(42) };
 }
 
-fn main() {
+fn mut_alias(x: &mut UnsafePinned<i32>, y: &mut UnsafePinned<i32>) {
-    let x = UnsafePinned::new(0);
+    unsafe {
-    mutate(&x);
+        x.get().write(0);
-    assert_eq!(x.into_inner(), 42);
+        y.get().write(0);
+        x.get().write(0);
+        y.get().write(0);
+    }
+}
+
+// Also try this with a type for which we implement `Unpin`, just to be extra mean.
+struct MyUnsafePinned<T>(UnsafePinned<T>);
+impl<T> Unpin for MyUnsafePinned<T> {}
+
+fn my_mut_alias(x: &mut MyUnsafePinned<i32>, y: &mut MyUnsafePinned<i32>) {
+    unsafe {
+        x.0.get().write(0);
+        y.0.get().write(0);
+        x.0.get().write(0);
+        y.0.get().write(0);
+    }
+}
+
+fn main() {
+    let mut x = UnsafePinned::new(0i32);
+    mutate(&x);
+    assert_eq!(unsafe { x.get().read() }, 42);
+
+    let ptr = &raw mut x;
+    unsafe { mut_alias(&mut *ptr, &mut *ptr) };
+
+    let ptr = ptr.cast::<MyUnsafePinned<i32>>();
+    unsafe { my_mut_alias(&mut *ptr, &mut *ptr) };
 }

tests/auxiliary/minicore.rs

@@ -76,6 +76,9 @@ pub trait BikeshedGuaranteedNoDrop {}
 #[lang = "freeze"]
 pub unsafe auto trait Freeze {}
 
+#[lang = "unsafe_unpin"]
+pub unsafe auto trait UnsafeUnpin {}
+
 #[lang = "unpin"]
 #[diagnostic::on_unimplemented(
     note = "consider using the `pin!` macro\nconsider using `Box::pin` if you need to access the pinned value outside of the current scope",

tests/codegen-llvm/function-arguments.rs

@@ -1,9 +1,9 @@
 //@ compile-flags: -Copt-level=3 -C no-prepopulate-passes
 #![crate_type = "lib"]
 #![feature(rustc_attrs)]
-#![feature(allocator_api)]
+#![feature(allocator_api, unsafe_unpin)]
 
-use std::marker::PhantomPinned;
+use std::marker::{PhantomPinned, UnsafeUnpin};
 use std::mem::MaybeUninit;
 use std::num::NonZero;
 use std::ptr::NonNull;
@@ -259,11 +259,21 @@ pub fn trait_raw(_: *const dyn Drop) {}
 
 // CHECK: @trait_box(ptr noalias noundef nonnull align 1{{( %0)?}}, {{.+}} noalias noundef readonly align {{.*}} dereferenceable({{.*}}){{( %1)?}})
 #[no_mangle]
-pub fn trait_box(_: Box<dyn Drop + Unpin>) {}
+pub fn trait_box(_: Box<dyn Drop + Unpin + UnsafeUnpin>) {}
+
+// Ensure that removing *either* `Unpin` or `UnsafeUnpin` is enough to lose the attribute.
+// CHECK: @trait_box_pin1(ptr noundef nonnull align 1{{( %0)?}}, {{.+}} noalias noundef readonly align {{.*}} dereferenceable({{.*}}){{( %1)?}})
+#[no_mangle]
+pub fn trait_box_pin1(_: Box<dyn Drop + Unpin>) {}
+// CHECK: @trait_box_pin2(ptr noundef nonnull align 1{{( %0)?}}, {{.+}} noalias noundef readonly align {{.*}} dereferenceable({{.*}}){{( %1)?}})
+#[no_mangle]
+pub fn trait_box_pin2(_: Box<dyn Drop + UnsafeUnpin>) {}
 
 // CHECK: { ptr, ptr } @trait_option(ptr noalias noundef align 1 %x.0, ptr %x.1)
 #[no_mangle]
-pub fn trait_option(x: Option<Box<dyn Drop + Unpin>>) -> Option<Box<dyn Drop + Unpin>> {
+pub fn trait_option(
+    x: Option<Box<dyn Drop + Unpin + UnsafeUnpin>>,
+) -> Option<Box<dyn Drop + Unpin + UnsafeUnpin>> {
     x
 }