Merging upstream version 1.72.1+dfsg1.

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 151 insertions, 17 deletions

diff --git a/library/alloc/src/rc.rs b/library/alloc/src/rc.rs
index 38a711ac7..b3305b8ca 100644
--- a/library/alloc/src/rc.rs
+++ b/library/alloc/src/rc.rs
@@ -258,19 +258,19 @@ use core::iter;
 use core::marker::{PhantomData, Unsize};
 #[cfg(not(no_global_oom_handling))]
 use core::mem::size_of_val;
-use core::mem::{self, align_of_val_raw, forget};
-use core::ops::{CoerceUnsized, Deref, DispatchFromDyn, Receiver};
+use core::mem::{self, align_of_val_raw, forget, ManuallyDrop};
+use core::ops::{CoerceUnsized, Deref, DerefMut, DispatchFromDyn, Receiver};
 use core::panic::{RefUnwindSafe, UnwindSafe};
 #[cfg(not(no_global_oom_handling))]
 use core::pin::Pin;
-use core::ptr::{self, NonNull};
+use core::ptr::{self, drop_in_place, NonNull};
 #[cfg(not(no_global_oom_handling))]
 use core::slice::from_raw_parts_mut;
 
 #[cfg(not(no_global_oom_handling))]
 use crate::alloc::handle_alloc_error;
 #[cfg(not(no_global_oom_handling))]
-use crate::alloc::{box_free, WriteCloneIntoRaw};
+use crate::alloc::WriteCloneIntoRaw;
 use crate::alloc::{AllocError, Allocator, Global, Layout};
 use crate::borrow::{Cow, ToOwned};
 #[cfg(not(no_global_oom_handling))]
@@ -1169,7 +1169,7 @@ impl<T: ?Sized> Rc<T> {
     #[inline]
     #[stable(feature = "ptr_eq", since = "1.17.0")]
     /// Returns `true` if the two `Rc`s point to the same allocation in a vein similar to
-    /// [`ptr::eq`]. See [that function][`ptr::eq`] for caveats when comparing `dyn Trait` pointers.
+    /// [`ptr::eq`]. This function ignores the metadata of  `dyn Trait` pointers.
     ///
     /// # Examples
     ///
@@ -1184,7 +1184,7 @@ impl<T: ?Sized> Rc<T> {
     /// assert!(!Rc::ptr_eq(&five, &other_five));
     /// ```
     pub fn ptr_eq(this: &Self, other: &Self) -> bool {
-        this.ptr.as_ptr() == other.ptr.as_ptr()
+        this.ptr.as_ptr() as *const () == other.ptr.as_ptr() as *const ()
     }
 }
 
@@ -1442,23 +1442,21 @@ impl<T: ?Sized> Rc<T> {
     }
 
     #[cfg(not(no_global_oom_handling))]
-    fn from_box(v: Box<T>) -> Rc<T> {
+    fn from_box(src: Box<T>) -> Rc<T> {
         unsafe {
-            let (box_unique, alloc) = Box::into_unique(v);
-            let bptr = box_unique.as_ptr();
-
-            let value_size = size_of_val(&*bptr);
-            let ptr = Self::allocate_for_ptr(bptr);
+            let value_size = size_of_val(&*src);
+            let ptr = Self::allocate_for_ptr(&*src);
 
             // Copy value as bytes
             ptr::copy_nonoverlapping(
-                bptr as *const T as *const u8,
+                &*src as *const T as *const u8,
                 &mut (*ptr).value as *mut _ as *mut u8,
                 value_size,
             );
 
             // Free the allocation without dropping its contents
-            box_free(box_unique, alloc);
+            let src = Box::from_raw(Box::into_raw(src) as *mut mem::ManuallyDrop<T>);
+            drop(src);
 
             Self::from_ptr(ptr)
         }
@@ -2468,8 +2466,8 @@ impl<T: ?Sized> Weak<T> {
     }
 
     /// Returns `true` if the two `Weak`s point to the same allocation similar to [`ptr::eq`], or if
-    /// both don't point to any allocation (because they were created with `Weak::new()`). See [that
-    /// function][`ptr::eq`] for caveats when comparing `dyn Trait` pointers.
+    /// both don't point to any allocation (because they were created with `Weak::new()`). However,
+    /// this function ignores the metadata of  `dyn Trait` pointers.
     ///
     /// # Notes
     ///
@@ -2510,7 +2508,7 @@ impl<T: ?Sized> Weak<T> {
     #[must_use]
     #[stable(feature = "weak_ptr_eq", since = "1.39.0")]
     pub fn ptr_eq(&self, other: &Self) -> bool {
-        self.ptr.as_ptr() == other.ptr.as_ptr()
+        ptr::eq(self.ptr.as_ptr() as *const (), other.ptr.as_ptr() as *const ())
     }
 }
 
@@ -2746,3 +2744,139 @@ fn data_offset_align(align: usize) -> usize {
     let layout = Layout::new::<RcBox<()>>();
     layout.size() + layout.padding_needed_for(align)
 }
+
+/// A uniquely owned `Rc`
+///
+/// This represents an `Rc` that is known to be uniquely owned -- that is, have exactly one strong
+/// reference. Multiple weak pointers can be created, but attempts to upgrade those to strong
+/// references will fail unless the `UniqueRc` they point to has been converted into a regular `Rc`.
+///
+/// Because they are uniquely owned, the contents of a `UniqueRc` can be freely mutated. A common
+/// use case is to have an object be mutable during its initialization phase but then have it become
+/// immutable and converted to a normal `Rc`.
+///
+/// This can be used as a flexible way to create cyclic data structures, as in the example below.
+///
+/// ```
+/// #![feature(unique_rc_arc)]
+/// use std::rc::{Rc, Weak, UniqueRc};
+///
+/// struct Gadget {
+///     #[allow(dead_code)]
+///     me: Weak<Gadget>,
+/// }
+///
+/// fn create_gadget() -> Option<Rc<Gadget>> {
+///     let mut rc = UniqueRc::new(Gadget {
+///         me: Weak::new(),
+///     });
+///     rc.me = UniqueRc::downgrade(&rc);
+///     Some(UniqueRc::into_rc(rc))
+/// }
+///
+/// create_gadget().unwrap();
+/// ```
+///
+/// An advantage of using `UniqueRc` over [`Rc::new_cyclic`] to build cyclic data structures is that
+/// [`Rc::new_cyclic`]'s `data_fn` parameter cannot be async or return a [`Result`]. As shown in the
+/// previous example, `UniqueRc` allows for more flexibility in the construction of cyclic data,
+/// including fallible or async constructors.
+#[unstable(feature = "unique_rc_arc", issue = "112566")]
+#[derive(Debug)]
+pub struct UniqueRc<T> {
+    ptr: NonNull<RcBox<T>>,
+    phantom: PhantomData<RcBox<T>>,
+}
+
+impl<T> UniqueRc<T> {
+    /// Creates a new `UniqueRc`
+    ///
+    /// Weak references to this `UniqueRc` can be created with [`UniqueRc::downgrade`]. Upgrading
+    /// these weak references will fail before the `UniqueRc` has been converted into an [`Rc`].
+    /// After converting the `UniqueRc` into an [`Rc`], any weak references created beforehand will
+    /// point to the new [`Rc`].
+    #[cfg(not(no_global_oom_handling))]
+    #[unstable(feature = "unique_rc_arc", issue = "112566")]
+    pub fn new(value: T) -> Self {
+        Self {
+            ptr: Box::leak(Box::new(RcBox {
+                strong: Cell::new(0),
+                // keep one weak reference so if all the weak pointers that are created are dropped
+                // the UniqueRc still stays valid.
+                weak: Cell::new(1),
+                value,
+            }))
+            .into(),
+            phantom: PhantomData,
+        }
+    }
+
+    /// Creates a new weak reference to the `UniqueRc`
+    ///
+    /// Attempting to upgrade this weak reference will fail before the `UniqueRc` has been converted
+    /// to a [`Rc`] using [`UniqueRc::into_rc`].
+    #[unstable(feature = "unique_rc_arc", issue = "112566")]
+    pub fn downgrade(this: &Self) -> Weak<T> {
+        // SAFETY: This pointer was allocated at creation time and we guarantee that we only have
+        // one strong reference before converting to a regular Rc.
+        unsafe {
+            this.ptr.as_ref().inc_weak();
+        }
+        Weak { ptr: this.ptr }
+    }
+
+    /// Converts the `UniqueRc` into a regular [`Rc`]
+    ///
+    /// This consumes the `UniqueRc` and returns a regular [`Rc`] that contains the `value` that
+    /// is passed to `into_rc`.
+    ///
+    /// Any weak references created before this method is called can now be upgraded to strong
+    /// references.
+    #[unstable(feature = "unique_rc_arc", issue = "112566")]
+    pub fn into_rc(this: Self) -> Rc<T> {
+        let mut this = ManuallyDrop::new(this);
+        // SAFETY: This pointer was allocated at creation time so we know it is valid.
+        unsafe {
+            // Convert our weak reference into a strong reference
+            this.ptr.as_mut().strong.set(1);
+            Rc::from_inner(this.ptr)
+        }
+    }
+}
+
+#[unstable(feature = "unique_rc_arc", issue = "112566")]
+impl<T> Deref for UniqueRc<T> {
+    type Target = T;
+
+    fn deref(&self) -> &T {
+        // SAFETY: This pointer was allocated at creation time so we know it is valid.
+        unsafe { &self.ptr.as_ref().value }
+    }
+}
+
+#[unstable(feature = "unique_rc_arc", issue = "112566")]
+impl<T> DerefMut for UniqueRc<T> {
+    fn deref_mut(&mut self) -> &mut T {
+        // SAFETY: This pointer was allocated at creation time so we know it is valid. We know we
+        // have unique ownership and therefore it's safe to make a mutable reference because
+        // `UniqueRc` owns the only strong reference to itself.
+        unsafe { &mut (*self.ptr.as_ptr()).value }
+    }
+}
+
+#[unstable(feature = "unique_rc_arc", issue = "112566")]
+unsafe impl<#[may_dangle] T> Drop for UniqueRc<T> {
+    fn drop(&mut self) {
+        unsafe {
+            // destroy the contained object
+            drop_in_place(DerefMut::deref_mut(self));
+
+            // remove the implicit "strong weak" pointer now that we've destroyed the contents.
+            self.ptr.as_ref().dec_weak();
+
+            if self.ptr.as_ref().weak() == 0 {
+                Global.deallocate(self.ptr.cast(), Layout::for_value(self.ptr.as_ref()));
+            }
+        }
+    }
+}