Rollup merge of rust-lang#125093 - zachs18:rc-into-raw-with-allocator-only, r=Mark-Simulacrum

Add `fn into_raw_with_allocator` to Rc/Arc/Weak.

Split out from rust-lang#119761

Add `fn into_raw_with_allocator` for `Rc`/`rc::Weak`[^1]/`Arc`/`sync::Weak`.
* Pairs with `from_raw_in` (which already exists on all 4 types).
* Name matches `Box::into_raw_with_allocator`.
* Associated fns on `Rc`/`Arc`, methods on `Weak`s.

<details> <summary>Future PR/ACP</summary>

As a follow-on to this PR, I plan to make a PR/ACP later to move `into_raw(_parts)` from `Container<_, A: Allocator>` to only `Container<_, Global>` (where `Container` = `Vec`/`Box`/`Rc`/`rc::Weak`/`Arc`/`sync::Weak`) so that users of non-`Global` allocators have to explicitly handle the allocator when using `into_raw`-like APIs.

The current behaviors of stdlib containers are inconsistent with respect to what happens to the allocator when `into_raw` is called (which does not return the allocator)

| Type | `into_raw` currently callable with | behavior of `into_raw`|
| --- | --- | --- |
| `Box` | any allocator | allocator is [dropped](https://doc.rust-lang.org/nightly/src/alloc/boxed.rs.html#1060) |
| `Vec` | any allocator | allocator is [forgotten](https://doc.rust-lang.org/nightly/src/alloc/vec/mod.rs.html#884) |
| `Arc`/`Rc`/`Weak` | any allocator | allocator is [forgotten](https://doc.rust-lang.org/src/alloc/sync.rs.html#1487)(Arc) [(sync::Weak)](https://doc.rust-lang.org/src/alloc/sync.rs.html#2726) [(Rc)](https://doc.rust-lang.org/src/alloc/rc.rs.html#1352) [(rc::Weak)](https://doc.rust-lang.org/src/alloc/rc.rs.html#2993) |

In my opinion, neither implicitly dropping nor implicitly forgetting the allocator is ideal; dropping it could immediately invalidate the returned pointer, and forgetting it could unintentionally leak memory. My (to-be) proposed solution is to just forbid calling `into_raw(_parts)` on containers with non-`Global` allocators, and require calling `into_raw_with_allocator`(/`Vec::into_raw_parts_with_alloc`)

</details>

[^1]:  Technically, `rc::Weak::into_raw_with_allocator` is not newly added, as it was modified and renamed from `rc::Weak::into_raw_and_alloc`.

Author: matthiaskrgr

library/alloc/src/rc.rs

@@ -1356,6 +1356,33 @@ impl<T: ?Sized, A: Allocator> Rc<T, A> {
         ptr
     }
 
+    /// Consumes the `Rc`, returning the wrapped pointer and allocator.
+    ///
+    /// To avoid a memory leak the pointer must be converted back to an `Rc` using
+    /// [`Rc::from_raw_in`].
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(allocator_api)]
+    /// use std::rc::Rc;
+    /// use std::alloc::System;
+    ///
+    /// let x = Rc::new_in("hello".to_owned(), System);
+    /// let (ptr, alloc) = Rc::into_raw_with_allocator(x);
+    /// assert_eq!(unsafe { &*ptr }, "hello");
+    /// let x = unsafe { Rc::from_raw_in(ptr, alloc) };
+    /// assert_eq!(&*x, "hello");
+    /// ```
+    #[unstable(feature = "allocator_api", issue = "32838")]
+    pub fn into_raw_with_allocator(this: Self) -> (*const T, A) {
+        let this = mem::ManuallyDrop::new(this);
+        let ptr = Self::as_ptr(&this);
+        // Safety: `this` is ManuallyDrop so the allocator will not be double-dropped
+        let alloc = unsafe { ptr::read(&this.alloc) };
+        (ptr, alloc)
+    }
+
     /// Provides a raw pointer to the data.
     ///
     /// The counts are not affected in any way and the `Rc` is not consumed. The pointer is valid
@@ -3024,39 +3051,42 @@ impl<T: ?Sized, A: Allocator> Weak<T, A> {
         result
     }
 
-    /// Consumes the `Weak<T>` and turns it into a raw pointer.
+    /// Consumes the `Weak<T>`, returning the wrapped pointer and allocator.
     ///
     /// This converts the weak pointer into a raw pointer, while still preserving the ownership of
     /// one weak reference (the weak count is not modified by this operation). It can be turned
-    /// back into the `Weak<T>` with [`from_raw`].
+    /// back into the `Weak<T>` with [`from_raw_in`].
     ///
     /// The same restrictions of accessing the target of the pointer as with
     /// [`as_ptr`] apply.
     ///
     /// # Examples
     ///
     /// ```
+    /// #![feature(allocator_api)]
     /// use std::rc::{Rc, Weak};
+    /// use std::alloc::System;
     ///
-    /// let strong = Rc::new("hello".to_owned());
+    /// let strong = Rc::new_in("hello".to_owned(), System);
     /// let weak = Rc::downgrade(&strong);
-    /// let raw = weak.into_raw();
+    /// let (raw, alloc) = weak.into_raw_with_allocator();
     ///
     /// assert_eq!(1, Rc::weak_count(&strong));
     /// assert_eq!("hello", unsafe { &*raw });
     ///
-    /// drop(unsafe { Weak::from_raw(raw) });
+    /// drop(unsafe { Weak::from_raw_in(raw, alloc) });
     /// assert_eq!(0, Rc::weak_count(&strong));
     /// ```
     ///
-    /// [`from_raw`]: Weak::from_raw
+    /// [`from_raw_in`]: Weak::from_raw_in
     /// [`as_ptr`]: Weak::as_ptr
     #[inline]
     #[unstable(feature = "allocator_api", issue = "32838")]
-    pub fn into_raw_and_alloc(self) -> (*const T, A) {
-        let rc = mem::ManuallyDrop::new(self);
-        let result = rc.as_ptr();
-        let alloc = unsafe { ptr::read(&rc.alloc) };
+    pub fn into_raw_with_allocator(self) -> (*const T, A) {
+        let this = mem::ManuallyDrop::new(self);
+        let result = this.as_ptr();
+        // Safety: `this` is ManuallyDrop so the allocator will not be double-dropped
+        let alloc = unsafe { ptr::read(&this.alloc) };
         (result, alloc)
     }
 

library/alloc/src/sync.rs

@@ -1496,6 +1496,34 @@ impl<T: ?Sized, A: Allocator> Arc<T, A> {
         ptr
     }
 
+    /// Consumes the `Arc`, returning the wrapped pointer and allocator.
+    ///
+    /// To avoid a memory leak the pointer must be converted back to an `Arc` using
+    /// [`Arc::from_raw_in`].
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(allocator_api)]
+    /// use std::sync::Arc;
+    /// use std::alloc::System;
+    ///
+    /// let x = Arc::new_in("hello".to_owned(), System);
+    /// let (ptr, alloc) = Arc::into_raw_with_allocator(x);
+    /// assert_eq!(unsafe { &*ptr }, "hello");
+    /// let x = unsafe { Arc::from_raw_in(ptr, alloc) };
+    /// assert_eq!(&*x, "hello");
+    /// ```
+    #[must_use = "losing the pointer will leak memory"]
+    #[unstable(feature = "allocator_api", issue = "32838")]
+    pub fn into_raw_with_allocator(this: Self) -> (*const T, A) {
+        let this = mem::ManuallyDrop::new(this);
+        let ptr = Self::as_ptr(&this);
+        // Safety: `this` is ManuallyDrop so the allocator will not be double-dropped
+        let alloc = unsafe { ptr::read(&this.alloc) };
+        (ptr, alloc)
+    }
+
     /// Provides a raw pointer to the data.
     ///
     /// The counts are not affected in any way and the `Arc` is not consumed. The pointer is valid for
@@ -2740,6 +2768,45 @@ impl<T: ?Sized, A: Allocator> Weak<T, A> {
         result
     }
 
+    /// Consumes the `Weak<T>`, returning the wrapped pointer and allocator.
+    ///
+    /// This converts the weak pointer into a raw pointer, while still preserving the ownership of
+    /// one weak reference (the weak count is not modified by this operation). It can be turned
+    /// back into the `Weak<T>` with [`from_raw_in`].
+    ///
+    /// The same restrictions of accessing the target of the pointer as with
+    /// [`as_ptr`] apply.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(allocator_api)]
+    /// use std::sync::{Arc, Weak};
+    /// use std::alloc::System;
+    ///
+    /// let strong = Arc::new_in("hello".to_owned(), System);
+    /// let weak = Arc::downgrade(&strong);
+    /// let (raw, alloc) = weak.into_raw_with_allocator();
+    ///
+    /// assert_eq!(1, Arc::weak_count(&strong));
+    /// assert_eq!("hello", unsafe { &*raw });
+    ///
+    /// drop(unsafe { Weak::from_raw_in(raw, alloc) });
+    /// assert_eq!(0, Arc::weak_count(&strong));
+    /// ```
+    ///
+    /// [`from_raw_in`]: Weak::from_raw_in
+    /// [`as_ptr`]: Weak::as_ptr
+    #[must_use = "losing the pointer will leak memory"]
+    #[unstable(feature = "allocator_api", issue = "32838")]
+    pub fn into_raw_with_allocator(self) -> (*const T, A) {
+        let this = mem::ManuallyDrop::new(self);
+        let result = this.as_ptr();
+        // Safety: `this` is ManuallyDrop so the allocator will not be double-dropped
+        let alloc = unsafe { ptr::read(&this.alloc) };
+        (result, alloc)
+    }
+
     /// Converts a raw pointer previously created by [`into_raw`] back into `Weak<T>` in the provided
     /// allocator.
     ///