From 1ac54401d1b7224f1b8744b2b1f130d4d3cdfe34 Mon Sep 17 00:00:00 2001
From: Ibraheem Ahmed <ibraheem@ibraheem.ca>
Date: Sun, 1 May 2022 15:12:20 -0400
Subject: [PATCH] add `{Arc, Rc}::downcast_unchecked`

---
 library/alloc/src/rc.rs   | 40 ++++++++++++++++++++++++++++++++--
 library/alloc/src/sync.rs | 45 ++++++++++++++++++++++++++++++++++++---
 2 files changed, 80 insertions(+), 5 deletions(-)

diff --git a/library/alloc/src/rc.rs b/library/alloc/src/rc.rs
index 3065169e5e2..47808d5f86a 100644
--- a/library/alloc/src/rc.rs
+++ b/library/alloc/src/rc.rs
@@ -1241,8 +1241,6 @@ impl<T: Clone> Rc<T> {
 }
 
 impl Rc<dyn Any> {
-    #[inline]
-    #[stable(feature = "rc_downcast", since = "1.29.0")]
     /// Attempt to downcast the `Rc<dyn Any>` to a concrete type.
     ///
     /// # Examples
@@ -1261,6 +1259,8 @@ impl Rc<dyn Any> {
     /// print_if_string(Rc::new(my_string));
     /// print_if_string(Rc::new(0i8));
     /// ```
+    #[inline]
+    #[stable(feature = "rc_downcast", since = "1.29.0")]
     pub fn downcast<T: Any>(self) -> Result<Rc<T>, Rc<dyn Any>> {
         if (*self).is::<T>() {
             unsafe {
@@ -1272,6 +1272,42 @@ impl Rc<dyn Any> {
             Err(self)
         }
     }
+
+    /// Downcasts the `Rc<dyn Any>` to a concrete type.
+    ///
+    /// For a safe alternative see [`downcast`].
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(downcast_unchecked)]
+    ///
+    /// use std::any::Any;
+    /// use std::rc::Rc;
+    ///
+    /// let x: Rc<dyn Any> = Rc::new(1_usize);
+    ///
+    /// unsafe {
+    ///     assert_eq!(*x.downcast_unchecked::<usize>(), 1);
+    /// }
+    /// ```
+    ///
+    /// # Safety
+    ///
+    /// The contained value must be of type `T`. Calling this method
+    /// with the incorrect type is *undefined behavior*.
+    ///
+    ///
+    /// [`downcast`]: Self::downcast
+    #[inline]
+    #[unstable(feature = "downcast_unchecked", issue = "90850")]
+    pub unsafe fn downcast_unchecked<T: Any>(self) -> Rc<T> {
+        unsafe {
+            let ptr = self.ptr.cast::<RcBox<T>>();
+            mem::forget(self);
+            Rc::from_inner(ptr)
+        }
+    }
 }
 
 impl<T: ?Sized> Rc<T> {
diff --git a/library/alloc/src/sync.rs b/library/alloc/src/sync.rs
index 7e7670aad64..2d9c7ff4f6b 100644
--- a/library/alloc/src/sync.rs
+++ b/library/alloc/src/sync.rs
@@ -1691,8 +1691,6 @@ unsafe impl<#[may_dangle] T: ?Sized> Drop for Arc<T> {
 }
 
 impl Arc<dyn Any + Send + Sync> {
-    #[inline]
-    #[stable(feature = "rc_downcast", since = "1.29.0")]
     /// Attempt to downcast the `Arc<dyn Any + Send + Sync>` to a concrete type.
     ///
     /// # Examples
@@ -1711,9 +1709,11 @@ impl Arc<dyn Any + Send + Sync> {
     /// print_if_string(Arc::new(my_string));
     /// print_if_string(Arc::new(0i8));
     /// ```
+    #[inline]
+    #[stable(feature = "rc_downcast", since = "1.29.0")]
     pub fn downcast<T>(self) -> Result<Arc<T>, Self>
     where
-        T: Any + Send + Sync + 'static,
+        T: Any + Send + Sync,
     {
         if (*self).is::<T>() {
             unsafe {
@@ -1725,6 +1725,45 @@ impl Arc<dyn Any + Send + Sync> {
             Err(self)
         }
     }
+
+    /// Downcasts the `Arc<dyn Any + Send + Sync>` to a concrete type.
+    ///
+    /// For a safe alternative see [`downcast`].
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(downcast_unchecked)]
+    ///
+    /// use std::any::Any;
+    /// use std::sync::Arc;
+    ///
+    /// let x: Arc<dyn Any + Send + Sync> = Arc::new(1_usize);
+    ///
+    /// unsafe {
+    ///     assert_eq!(*x.downcast_unchecked::<usize>(), 1);
+    /// }
+    /// ```
+    ///
+    /// # Safety
+    ///
+    /// The contained value must be of type `T`. Calling this method
+    /// with the incorrect type is *undefined behavior*.
+    ///
+    ///
+    /// [`downcast`]: Self::downcast
+    #[inline]
+    #[unstable(feature = "downcast_unchecked", issue = "90850")]
+    pub unsafe fn downcast_unchecked<T>(self) -> Arc<T>
+    where
+        T: Any + Send + Sync,
+    {
+        unsafe {
+            let ptr = self.ptr.cast::<ArcInner<T>>();
+            mem::forget(self);
+            Arc::from_inner(ptr)
+        }
+    }
 }
 
 impl<T> Weak<T> {