Clarify and restrict when `{Arc,Rc}::get_mut_unchecked` is allowed.

2 files changed, 10 insertions, 8 deletions

diff --git a/library/alloc/src/rc.rs b/library/alloc/src/rc.rs
index f3cbfe27b3e..1e9ed3889e8 100644
--- a/library/alloc/src/rc.rs
+++ b/library/alloc/src/rc.rs
@@ -1091,10 +1091,11 @@ impl<T: ?Sized> Rc<T> {
     ///
     /// # Safety
     ///
-    /// Any other `Rc` or [`Weak`] pointers to the same allocation must not be dereferenced
-    /// for the duration of the returned borrow.
-    /// This is trivially the case if no such pointers exist,
-    /// for example immediately after `Rc::new`.
+    /// If any other `Rc` or [`Weak`] pointers to the same allocation exist, then
+    /// they must be must not be dereferenced or have active borrows for the duration
+    /// of the returned borrow, and their inner type must be exactly the same as the
+    /// inner type of this Rc (including lifetimes). This is trivially the case if no
+    /// such pointers exist, for example immediately after `Rc::new`.
     ///
     /// # Examples
     ///
diff --git a/library/alloc/src/sync.rs b/library/alloc/src/sync.rs
index 37e07eb5998..2bb5030c51e 100644
--- a/library/alloc/src/sync.rs
+++ b/library/alloc/src/sync.rs
@@ -1630,10 +1630,11 @@ impl<T: ?Sized> Arc<T> {
     ///
     /// # Safety
     ///
-    /// Any other `Arc` or [`Weak`] pointers to the same allocation must not be dereferenced
-    /// for the duration of the returned borrow.
-    /// This is trivially the case if no such pointers exist,
-    /// for example immediately after `Arc::new`.
+    /// If any other `Arc` or [`Weak`] pointers to the same allocation exist, then
+    /// they must be must not be dereferenced or have active borrows for the duration
+    /// of the returned borrow, and their inner type must be exactly the same as the
+    /// inner type of this Rc (including lifetimes). This is trivially the case if no
+    /// such pointers exist, for example immediately after `Arc::new`.
     ///
     /// # Examples
     ///