Rollup merge of #138303 - DiuDiu777:rc-fix, r=Mark-Simulacrum

Fix Ptr inconsistency in {Rc,Arc}

### PR Description
This pr aims to address the problem discussed on [zulip](https://rust-lang.zulipchat.com/#narrow/channel/219381-t-libs/topic/Inconsistency.20in.20.7BRc.2CArc.7D's.20ptr.20requirements/with/504259637).

### Problem Clarification
As this post presents, the `{Rc, Arc}::{in/de-crement_strong_count_/in}` do not limit the layout of the memory that `ptr` points to, while internally `Rc::from_raw_in` is called directly.

UB doesn't just appear when the strong count is decremented to zero. Miri also detects the UB of `out-of-bounds pointer use` when increment strong count is called on a pointer with an incorrect layout(shown as below).

```rust
use std::rc::Rc;
#[repr(align(8))]
struct Aligned8(u64);

#[repr(align(16))]
struct Aligned16(u64);

fn main() {
    let rc: Rc<Aligned8> = Rc::new(Aligned8(42));
    let raw_ptr = Rc::into_raw(rc);

    unsafe {
        Rc::increment_strong_count(raw_ptr as *const Aligned16);
    }
}
```

Miri output:
```
error: Undefined Behavior: out-of-bounds pointer use: expected a pointer to 32 bytes of memory, but got alloc954 which is only 24 bytes from the end of the allocation
```

2 files changed, 44 insertions, 20 deletions

diff --git a/library/alloc/src/rc.rs b/library/alloc/src/rc.rs
index fc1cee28d03..619d9f258e3 100644
--- a/library/alloc/src/rc.rs
+++ b/library/alloc/src/rc.rs
@@ -1327,11 +1327,14 @@ impl<T: ?Sized> Rc<T> {
     ///
     /// # Safety
     ///
-    /// The pointer must have been obtained through `Rc::into_raw`, the
-    /// associated `Rc` instance must be valid (i.e. the strong count must be at
+    /// The pointer must have been obtained through `Rc::into_raw` and must satisfy the
+    /// same layout requirements specified in [`Rc::from_raw_in`][from_raw_in].
+    /// The associated `Rc` instance must be valid (i.e. the strong count must be at
     /// least 1) for the duration of this method, and `ptr` must point to a block of memory
     /// allocated by the global allocator.
     ///
+    /// [from_raw_in]: Rc::from_raw_in
+    ///
     /// # Examples
     ///
     /// ```
@@ -1360,12 +1363,15 @@ impl<T: ?Sized> Rc<T> {
     ///
     /// # Safety
     ///
-    /// The pointer must have been obtained through `Rc::into_raw`, the
-    /// associated `Rc` instance must be valid (i.e. the strong count must be at
+    /// The pointer must have been obtained through `Rc::into_raw`and must satisfy the
+    /// same layout requirements specified in [`Rc::from_raw_in`][from_raw_in].
+    /// The associated `Rc` instance must be valid (i.e. the strong count must be at
     /// least 1) when invoking this method, and `ptr` must point to a block of memory
     /// allocated by the global allocator. This method can be used to release the final `Rc` and
     /// backing storage, but **should not** be called after the final `Rc` has been released.
     ///
+    /// [from_raw_in]: Rc::from_raw_in
+    ///
     /// # Examples
     ///
     /// ```
@@ -1623,10 +1629,13 @@ impl<T: ?Sized, A: Allocator> Rc<T, A> {
     ///
     /// # Safety
     ///
-    /// The pointer must have been obtained through `Rc::into_raw`, the
-    /// associated `Rc` instance must be valid (i.e. the strong count must be at
+    /// The pointer must have been obtained through `Rc::into_raw` and must satisfy the
+    /// same layout requirements specified in [`Rc::from_raw_in`][from_raw_in].
+    /// The associated `Rc` instance must be valid (i.e. the strong count must be at
     /// least 1) for the duration of this method, and `ptr` must point to a block of memory
-    /// allocated by `alloc`
+    /// allocated by `alloc`.
+    ///
+    /// [from_raw_in]: Rc::from_raw_in
     ///
     /// # Examples
     ///
@@ -1665,11 +1674,14 @@ impl<T: ?Sized, A: Allocator> Rc<T, A> {
     ///
     /// # Safety
     ///
-    /// The pointer must have been obtained through `Rc::into_raw`, the
-    /// associated `Rc` instance must be valid (i.e. the strong count must be at
+    /// The pointer must have been obtained through `Rc::into_raw`and must satisfy the
+    /// same layout requirements specified in [`Rc::from_raw_in`][from_raw_in].
+    /// The associated `Rc` instance must be valid (i.e. the strong count must be at
     /// least 1) when invoking this method, and `ptr` must point to a block of memory
-    /// allocated by `alloc`. This method can be used to release the final `Rc` and backing storage,
-    /// but **should not** be called after the final `Rc` has been released.
+    /// allocated by `alloc`. This method can be used to release the final `Rc` and
+    /// backing storage, but **should not** be called after the final `Rc` has been released.
+    ///
+    /// [from_raw_in]: Rc::from_raw_in
     ///
     /// # Examples
     ///
diff --git a/library/alloc/src/sync.rs b/library/alloc/src/sync.rs
index 4999319f618..104cb35c23b 100644
--- a/library/alloc/src/sync.rs
+++ b/library/alloc/src/sync.rs
@@ -1453,11 +1453,14 @@ impl<T: ?Sized> Arc<T> {
     ///
     /// # Safety
     ///
-    /// The pointer must have been obtained through `Arc::into_raw`, and the
-    /// associated `Arc` instance must be valid (i.e. the strong count must be at
+    /// The pointer must have been obtained through `Arc::into_raw` and must satisfy the
+    /// same layout requirements specified in [`Arc::from_raw_in`][from_raw_in].
+    /// The associated `Arc` instance must be valid (i.e. the strong count must be at
     /// least 1) for the duration of this method, and `ptr` must point to a block of memory
     /// allocated by the global allocator.
     ///
+    /// [from_raw_in]: Arc::from_raw_in
+    ///
     /// # Examples
     ///
     /// ```
@@ -1488,13 +1491,16 @@ impl<T: ?Sized> Arc<T> {
     ///
     /// # Safety
     ///
-    /// The pointer must have been obtained through `Arc::into_raw`, and the
-    /// associated `Arc` instance must be valid (i.e. the strong count must be at
+    /// The pointer must have been obtained through `Arc::into_raw` and must satisfy the
+    /// same layout requirements specified in [`Arc::from_raw_in`][from_raw_in].
+    /// The associated `Arc` instance must be valid (i.e. the strong count must be at
     /// least 1) when invoking this method, and `ptr` must point to a block of memory
     /// allocated by the global allocator. This method can be used to release the final
     /// `Arc` and backing storage, but **should not** be called after the final `Arc` has been
     /// released.
     ///
+    /// [from_raw_in]: Arc::from_raw_in
+    ///
     /// # Examples
     ///
     /// ```
@@ -1806,11 +1812,14 @@ impl<T: ?Sized, A: Allocator> Arc<T, A> {
     ///
     /// # Safety
     ///
-    /// The pointer must have been obtained through `Arc::into_raw`, and the
-    /// associated `Arc` instance must be valid (i.e. the strong count must be at
-    /// least 1) for the duration of this method,, and `ptr` must point to a block of memory
+    /// The pointer must have been obtained through `Arc::into_raw` and must satisfy the
+    /// same layout requirements specified in [`Arc::from_raw_in`][from_raw_in].
+    /// The associated `Arc` instance must be valid (i.e. the strong count must be at
+    /// least 1) for the duration of this method, and `ptr` must point to a block of memory
     /// allocated by `alloc`.
     ///
+    /// [from_raw_in]: Arc::from_raw_in
+    ///
     /// # Examples
     ///
     /// ```
@@ -1850,13 +1859,16 @@ impl<T: ?Sized, A: Allocator> Arc<T, A> {
     ///
     /// # Safety
     ///
-    /// The pointer must have been obtained through `Arc::into_raw`,  the
-    /// associated `Arc` instance must be valid (i.e. the strong count must be at
+    /// The pointer must have been obtained through `Arc::into_raw` and must satisfy the
+    /// same layout requirements specified in [`Arc::from_raw_in`][from_raw_in].
+    /// The associated `Arc` instance must be valid (i.e. the strong count must be at
     /// least 1) when invoking this method, and `ptr` must point to a block of memory
     /// allocated by `alloc`. This method can be used to release the final
     /// `Arc` and backing storage, but **should not** be called after the final `Arc` has been
     /// released.
     ///
+    /// [from_raw_in]: Arc::from_raw_in
+    ///
     /// # Examples
     ///
     /// ```