Remove ptr::Unique

rust-lang#71507 (comment) discusses whether `Unique` not actually being unique is UB.

`Unique` has very limited use over `NonNull`, so it doesn’t seem worth
spending a lot of effort defining an abstraction and coming up with
rules about what uses of it are or aren’t sound.

Author: SimonSapin

src/liballoc/alloc.rs

@@ -3,7 +3,7 @@
 #![stable(feature = "alloc_module", since = "1.28.0")]
 
 use core::intrinsics::{self, min_align_of_val, size_of_val};
-use core::ptr::{NonNull, Unique};
+use core::ptr::NonNull;
 
 #[stable(feature = "alloc_module", since = "1.28.0")]
 #[doc(inline)]
@@ -258,7 +258,7 @@ unsafe impl AllocRef for Global {
     }
 }
 
-/// The allocator for unique pointers.
+/// The allocator for Box.
 // This function must not unwind. If it does, MIR codegen will fail.
 #[cfg(not(test))]
 #[lang = "exchange_malloc"]
@@ -276,9 +276,9 @@ unsafe fn exchange_malloc(size: usize, align: usize) -> *mut u8 {
 // This signature has to be the same as `Box`, otherwise an ICE will happen.
 // When an additional parameter to `Box` is added (like `A: AllocRef`), this has to be added here as
 // well.
-// For example if `Box` is changed to  `struct Box<T: ?Sized, A: AllocRef>(Unique<T>, A)`,
-// this function has to be changed to `fn box_free<T: ?Sized, A: AllocRef>(Unique<T>, A)` as well.
-pub(crate) unsafe fn box_free<T: ?Sized>(ptr: Unique<T>) {
+// For example if `Box` is changed to  `struct Box<T: ?Sized, A: AllocRef>(NonNull<T>, A)`,
+// this function has to be changed to `fn box_free<T: ?Sized, A: AllocRef>(NonNull<T>, A)` as well.
+pub(crate) unsafe fn box_free<T: ?Sized>(ptr: NonNull<T>) {
     let size = size_of_val(ptr.as_ref());
     let align = min_align_of_val(ptr.as_ref());
     let layout = Layout::from_size_align_unchecked(size, align);

src/liballoc/boxed.rs

@@ -136,13 +136,13 @@ use core::fmt;
 use core::future::Future;
 use core::hash::{Hash, Hasher};
 use core::iter::{FromIterator, FusedIterator, Iterator};
-use core::marker::{Unpin, Unsize};
+use core::marker::{PhantomData, Unpin, Unsize};
 use core::mem;
 use core::ops::{
     CoerceUnsized, Deref, DerefMut, DispatchFromDyn, Generator, GeneratorState, Receiver,
 };
 use core::pin::Pin;
-use core::ptr::{self, NonNull, Unique};
+use core::ptr::{self, NonNull};
 use core::task::{Context, Poll};
 
 use crate::alloc::{self, AllocInit, AllocRef, Global};
@@ -156,7 +156,26 @@ use crate::vec::Vec;
 #[lang = "owned_box"]
 #[fundamental]
 #[stable(feature = "rust1", since = "1.0.0")]
-pub struct Box<T: ?Sized>(Unique<T>);
+pub struct Box<T: ?Sized>(BoxPtr<T>);
+
+// FIXME: remove this struct and give `Box` two fields.
+// Currently this causes an ICE with 'assertion failed: sig.c_variadic || extra_args.is_empty()'
+struct BoxPtr<T: ?Sized> {
+    ptr: NonNull<T>,
+
+    // NOTE: this marker has no consequences for variance, but is necessary
+    // for dropck to understand that we logically own a `T`.
+    //
+    // For details, see:
+    // https://github.com/rust-lang/rfcs/blob/master/text/0769-sound-generic-drop.md#phantom-data
+    _phantom: PhantomData<T>,
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+unsafe impl<T: Send + ?Sized> Send for Box<T> {}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+unsafe impl<T: Sync + ?Sized> Sync for Box<T> {}
 
 impl<T> Box<T> {
     /// Allocates memory on the heap and then places `x` into it.
@@ -382,7 +401,7 @@ impl<T: ?Sized> Box<T> {
     #[stable(feature = "box_raw", since = "1.4.0")]
     #[inline]
     pub unsafe fn from_raw(raw: *mut T) -> Self {
-        Box(Unique::new_unchecked(raw))
+        Box(BoxPtr { ptr: NonNull::new_unchecked(raw), _phantom: PhantomData })
     }
 
     /// Consumes the `Box`, returning a wrapped raw pointer.
@@ -462,22 +481,14 @@ impl<T: ?Sized> Box<T> {
     #[unstable(feature = "box_into_raw_non_null", issue = "47336")]
     #[inline]
     pub fn into_raw_non_null(b: Box<T>) -> NonNull<T> {
-        Box::into_unique(b).into()
-    }
-
-    #[unstable(feature = "ptr_internals", issue = "none", reason = "use into_raw_non_null instead")]
-    #[inline]
-    #[doc(hidden)]
-    pub fn into_unique(b: Box<T>) -> Unique<T> {
-        let b = mem::ManuallyDrop::new(b);
-        let mut unique = b.0;
+        let mut b = mem::ManuallyDrop::new(b);
         // Box is kind-of a library type, but recognized as a "unique pointer" by
         // Stacked Borrows.  This function here corresponds to "reborrowing to
         // a raw pointer", but there is no actual reborrow here -- so
         // without some care, the pointer we are returning here still carries
         // the tag of `b`, with `Unique` permission.
         // We round-trip through a mutable reference to avoid that.
-        unsafe { Unique::new_unchecked(unique.as_mut() as *mut T) }
+        unsafe { NonNull::new_unchecked(b.0.ptr.as_mut() as *mut T) }
     }
 
     /// Consumes and leaks the `Box`, returning a mutable reference,
@@ -910,7 +921,7 @@ impl<T: fmt::Debug + ?Sized> fmt::Debug for Box<T> {
 impl<T: ?Sized> fmt::Pointer for Box<T> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         // It's not possible to extract the inner Uniq directly from the Box,
-        // instead we cast it to a *const which aliases the Unique
+        // instead we cast it to a *const which aliases the NonNull
         let ptr: *const T = &**self;
         fmt::Pointer::fmt(&ptr, f)
     }
@@ -1025,9 +1036,11 @@ impl<A, F: Fn<A> + ?Sized> Fn<A> for Box<F> {
 
 #[unstable(feature = "coerce_unsized", issue = "27732")]
 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Box<U>> for Box<T> {}
+impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<BoxPtr<U>> for BoxPtr<T> {}
 
 #[unstable(feature = "dispatch_from_dyn", issue = "none")]
 impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Box<U>> for Box<T> {}
+impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<BoxPtr<U>> for BoxPtr<T> {}
 
 #[stable(feature = "boxed_slice_from_iter", since = "1.32.0")]
 impl<A> FromIterator<A> for Box<[A]> {

src/liballoc/collections/btree/node.rs

@@ -34,7 +34,7 @@
 use core::cmp::Ordering;
 use core::marker::PhantomData;
 use core::mem::{self, MaybeUninit};
-use core::ptr::{self, NonNull, Unique};
+use core::ptr::{self, NonNull};
 use core::slice;
 
 use crate::alloc::{AllocRef, Global, Layout};
@@ -118,20 +118,20 @@ impl<K, V> InternalNode<K, V> {
 /// of nodes it actually contains, and, partially due to this lack of information,
 /// has no destructor.
 struct BoxedNode<K, V> {
-    ptr: Unique<LeafNode<K, V>>,
+    ptr: NonNull<LeafNode<K, V>>,
 }
 
 impl<K, V> BoxedNode<K, V> {
     fn from_leaf(node: Box<LeafNode<K, V>>) -> Self {
-        BoxedNode { ptr: Box::into_unique(node) }
+        BoxedNode { ptr: Box::into_raw_non_null(node) }
     }
 
     fn from_internal(node: Box<InternalNode<K, V>>) -> Self {
-        BoxedNode { ptr: Box::into_unique(node).cast() }
+        BoxedNode { ptr: Box::into_raw_non_null(node).cast() }
     }
 
     unsafe fn from_ptr(ptr: NonNull<LeafNode<K, V>>) -> Self {
-        BoxedNode { ptr: Unique::from(ptr) }
+        BoxedNode { ptr: NonNull::from(ptr) }
     }
 
     fn as_ptr(&self) -> NonNull<LeafNode<K, V>> {

src/liballoc/lib.rs

@@ -105,7 +105,6 @@
 #![feature(optin_builtin_traits)]
 #![feature(or_patterns)]
 #![feature(pattern)]
-#![feature(ptr_internals)]
 #![feature(ptr_offset_from)]
 #![feature(rustc_attrs)]
 #![feature(receiver_trait)]

src/liballoc/raw_vec.rs

@@ -3,9 +3,10 @@
 
 use core::alloc::MemoryBlock;
 use core::cmp;
+use core::marker::PhantomData;
 use core::mem::{self, ManuallyDrop, MaybeUninit};
 use core::ops::Drop;
-use core::ptr::{NonNull, Unique};
+use core::ptr::NonNull;
 use core::slice;
 
 use crate::alloc::{
@@ -25,14 +26,15 @@ mod tests;
 /// involved. This type is excellent for building your own data structures like Vec and VecDeque.
 /// In particular:
 ///
-/// * Produces `Unique::empty()` on zero-sized types.
-/// * Produces `Unique::empty()` on zero-length allocations.
-/// * Avoids freeing `Unique::empty()`.
+/// * Produces `NonNull::dangling()` on zero-sized types.
+/// * Produces `NonNull::dangling()` on zero-length allocations.
+/// * Avoids freeing `NonNull::dangling()`.
 /// * Catches all overflows in capacity computations (promotes them to "capacity overflow" panics).
 /// * Guards against 32-bit systems allocating more than isize::MAX bytes.
 /// * Guards against overflowing your length.
 /// * Calls `handle_alloc_error` for fallible allocations.
-/// * Contains a `ptr::Unique` and thus endows the user with all related benefits.
+/// * Contains a `ptr::NonNull` and thus endows the user with enum layout optimizations.
+/// * Is `Send` and `Sync` when items are.
 /// * Uses the excess returned from the allocator to use the largest available capacity.
 ///
 /// This type does not in anyway inspect the memory that it manages. When dropped it *will*
@@ -44,11 +46,24 @@ mod tests;
 /// `Box<[T]>`, since `capacity()` won't yield the length.
 #[allow(missing_debug_implementations)]
 pub struct RawVec<T, A: AllocRef = Global> {
-    ptr: Unique<T>,
+    ptr: NonNull<T>,
     cap: usize,
     alloc: A,
+
+    // NOTE: this marker has no consequences for variance, but is necessary
+    // for dropck to understand that we logically own a `T`.
+    //
+    // For details, see:
+    // https://github.com/rust-lang/rfcs/blob/master/text/0769-sound-generic-drop.md#phantom-data
+    _phantom: PhantomData<T>,
 }
 
+#[stable(feature = "rust1", since = "1.0.0")]
+unsafe impl<T: Send, A: AllocRef + Send> Send for RawVec<T, A> {}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+unsafe impl<T: Sync, A: AllocRef + Sync> Sync for RawVec<T, A> {}
+
 impl<T> RawVec<T, Global> {
     /// HACK(Centril): This exists because `#[unstable]` `const fn`s needn't conform
     /// to `min_const_fn` and so they cannot be called in `min_const_fn`s either.
@@ -125,7 +140,7 @@ impl<T, A: AllocRef> RawVec<T, A> {
     /// the returned `RawVec`.
     pub const fn new_in(alloc: A) -> Self {
         // `cap: 0` means "unallocated". zero-sized types are ignored.
-        Self { ptr: Unique::empty(), cap: 0, alloc }
+        Self { ptr: NonNull::dangling(), cap: 0, alloc, _phantom: PhantomData }
     }
 
     /// Like `with_capacity`, but parameterized over the choice of
@@ -154,6 +169,7 @@ impl<T, A: AllocRef> RawVec<T, A> {
                 ptr: memory.ptr.cast().into(),
                 cap: Self::capacity_from_bytes(memory.size),
                 alloc,
+                _phantom: PhantomData,
             }
         }
     }
@@ -168,11 +184,11 @@ impl<T, A: AllocRef> RawVec<T, A> {
     /// If the `ptr` and `capacity` come from a `RawVec` created via `a`, then this is guaranteed.
     #[inline]
     pub unsafe fn from_raw_parts_in(ptr: *mut T, capacity: usize, a: A) -> Self {
-        Self { ptr: Unique::new_unchecked(ptr), cap: capacity, alloc: a }
+        Self { ptr: NonNull::new_unchecked(ptr), cap: capacity, alloc: a, _phantom: PhantomData }
     }
 
     /// Gets a raw pointer to the start of the allocation. Note that this is
-    /// `Unique::empty()` if `capacity == 0` or `T` is zero-sized. In the former case, you must
+    /// `NonNull::dangling()` if `capacity == 0` or `T` is zero-sized. In the former case, you must
     /// be careful.
     pub fn ptr(&self) -> *mut T {
         self.ptr.as_ptr()

src/liballoc/rc.rs

@@ -984,8 +984,8 @@ impl<T: ?Sized> Rc<T> {
 
     fn from_box(v: Box<T>) -> Rc<T> {
         unsafe {
-            let box_unique = Box::into_unique(v);
-            let bptr = box_unique.as_ptr();
+            let box_non_null = Box::into_raw_non_null(v);
+            let bptr = box_non_null.as_ptr();
 
             let value_size = size_of_val(&*bptr);
             let ptr = Self::allocate_for_ptr(bptr);
@@ -998,7 +998,7 @@ impl<T: ?Sized> Rc<T> {
             );
 
             // Free the allocation without dropping its contents
-            box_free(box_unique);
+            box_free(box_non_null);
 
             Self::from_ptr(ptr)
         }

src/liballoc/sync.rs

@@ -862,8 +862,8 @@ impl<T: ?Sized> Arc<T> {
 
     fn from_box(v: Box<T>) -> Arc<T> {
         unsafe {
-            let box_unique = Box::into_unique(v);
-            let bptr = box_unique.as_ptr();
+            let box_non_null = Box::into_raw_non_null(v);
+            let bptr = box_non_null.as_ptr();
 
             let value_size = size_of_val(&*bptr);
             let ptr = Self::allocate_for_ptr(bptr);
@@ -876,7 +876,7 @@ impl<T: ?Sized> Arc<T> {
             );
 
             // Free the allocation without dropping its contents
-            box_free(box_unique);
+            box_free(box_non_null);
 
             Self::from_ptr(ptr)
         }

src/libcore/ptr/mod.rs

@@ -91,10 +91,6 @@ mod non_null;
 #[stable(feature = "nonnull", since = "1.25.0")]
 pub use non_null::NonNull;
 
-mod unique;
-#[unstable(feature = "ptr_internals", issue = "none")]
-pub use unique::Unique;
-
 mod const_ptr;
 mod mut_ptr;
 

src/libcore/ptr/non_null.rs

@@ -5,7 +5,6 @@ use crate::hash;
 use crate::marker::Unsize;
 use crate::mem;
 use crate::ops::{CoerceUnsized, DispatchFromDyn};
-use crate::ptr::Unique;
 
 // ignore-tidy-undocumented-unsafe
 
@@ -201,14 +200,6 @@ impl<T: ?Sized> hash::Hash for NonNull<T> {
     }
 }
 
-#[unstable(feature = "ptr_internals", issue = "none")]
-impl<T: ?Sized> From<Unique<T>> for NonNull<T> {
-    #[inline]
-    fn from(unique: Unique<T>) -> Self {
-        unsafe { NonNull::new_unchecked(unique.as_ptr()) }
-    }
-}
-
 #[stable(feature = "nonnull", since = "1.25.0")]
 impl<T: ?Sized> From<&mut T> for NonNull<T> {
     #[inline]