Rollup merge of rust-lang#124980 - zachs18:rc-allocator, r=Amanieu

Generalize `fn allocator` for Rc/Arc.

Split out from rust-lang#119761

- For `Rc`/`Arc`, the existing associated `fn`s are changed to allow unsized pointees.
 - For `Weak`s, new methods are added.

````@rustbot```` label +A-allocators

library/alloc/src/rc.rs

@@ -665,16 +665,6 @@ impl<T> Rc<T> {
 }
 
 impl<T, A: Allocator> Rc<T, A> {
-    /// Returns a reference to the underlying allocator.
-    ///
-    /// Note: this is an associated function, which means that you have
-    /// to call it as `Rc::allocator(&r)` instead of `r.allocator()`. This
-    /// is so that there is no conflict with a method on the inner type.
-    #[inline]
-    #[unstable(feature = "allocator_api", issue = "32838")]
-    pub fn allocator(this: &Self) -> &A {
-        &this.alloc
-    }
     /// Constructs a new `Rc` in the provided allocator.
     ///
     /// # Examples
@@ -1331,6 +1321,17 @@ impl<T: ?Sized> Rc<T> {
 }
 
 impl<T: ?Sized, A: Allocator> Rc<T, A> {
+    /// Returns a reference to the underlying allocator.
+    ///
+    /// Note: this is an associated function, which means that you have
+    /// to call it as `Rc::allocator(&r)` instead of `r.allocator()`. This
+    /// is so that there is no conflict with a method on the inner type.
+    #[inline]
+    #[unstable(feature = "allocator_api", issue = "32838")]
+    pub fn allocator(this: &Self) -> &A {
+        &this.alloc
+    }
+
     /// Consumes the `Rc`, returning the wrapped pointer.
     ///
     /// To avoid a memory leak the pointer must be converted back to an `Rc` using
@@ -2994,6 +2995,13 @@ impl<T: ?Sized> Weak<T> {
 }
 
 impl<T: ?Sized, A: Allocator> Weak<T, A> {
+    /// Returns a reference to the underlying allocator.
+    #[inline]
+    #[unstable(feature = "allocator_api", issue = "32838")]
+    pub fn allocator(&self) -> &A {
+        &self.alloc
+    }
+
     /// Returns a raw pointer to the object `T` pointed to by this `Weak<T>`.
     ///
     /// The pointer is valid only if there are some strong references. The pointer may be dangling,

library/alloc/src/sync.rs

@@ -677,16 +677,6 @@ impl<T> Arc<T> {
 }
 
 impl<T, A: Allocator> Arc<T, A> {
-    /// Returns a reference to the underlying allocator.
-    ///
-    /// Note: this is an associated function, which means that you have
-    /// to call it as `Arc::allocator(&a)` instead of `a.allocator()`. This
-    /// is so that there is no conflict with a method on the inner type.
-    #[inline]
-    #[unstable(feature = "allocator_api", issue = "32838")]
-    pub fn allocator(this: &Self) -> &A {
-        &this.alloc
-    }
     /// Constructs a new `Arc<T>` in the provided allocator.
     ///
     /// # Examples
@@ -1470,6 +1460,17 @@ impl<T: ?Sized> Arc<T> {
 }
 
 impl<T: ?Sized, A: Allocator> Arc<T, A> {
+    /// Returns a reference to the underlying allocator.
+    ///
+    /// Note: this is an associated function, which means that you have
+    /// to call it as `Arc::allocator(&a)` instead of `a.allocator()`. This
+    /// is so that there is no conflict with a method on the inner type.
+    #[inline]
+    #[unstable(feature = "allocator_api", issue = "32838")]
+    pub fn allocator(this: &Self) -> &A {
+        &this.alloc
+    }
+
     /// Consumes the `Arc`, returning the wrapped pointer.
     ///
     /// To avoid a memory leak the pointer must be converted back to an `Arc` using
@@ -2715,6 +2716,13 @@ impl<T: ?Sized> Weak<T> {
 }
 
 impl<T: ?Sized, A: Allocator> Weak<T, A> {
+    /// Returns a reference to the underlying allocator.
+    #[inline]
+    #[unstable(feature = "allocator_api", issue = "32838")]
+    pub fn allocator(&self) -> &A {
+        &self.alloc
+    }
+
     /// Returns a raw pointer to the object `T` pointed to by this `Weak<T>`.
     ///
     /// The pointer is valid only if there are some strong references. The pointer may be dangling,