From 12014d29b89557836a22fa46d74101371d81daf1 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Sebastian=20Dr=C3=B6ge?= <sebastian@centricular.com>
Date: Tue, 12 Jan 2021 16:03:29 +0200
Subject: [PATCH] Add Box::downcast() for dyn Any + Send + Sync

---
 library/alloc/src/boxed.rs | 33 +++++++++++++++++++++++++++++++++
 1 file changed, 33 insertions(+)

diff --git a/library/alloc/src/boxed.rs b/library/alloc/src/boxed.rs
index 33b812ec59ff9..b4dd5d92c6a13 100644
--- a/library/alloc/src/boxed.rs
+++ b/library/alloc/src/boxed.rs
@@ -1364,6 +1364,39 @@ impl<A: Allocator> Box<dyn Any + Send, A> {
     }
 }
 
+impl<A: Allocator> Box<dyn Any + Send + Sync, A> {
+    #[inline]
+    #[stable(feature = "box_send_sync_any_downcast", since = "1.51.0")]
+    /// Attempt to downcast the box to a concrete type.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::any::Any;
+    ///
+    /// fn print_if_string(value: Box<dyn Any + Send + Sync>) {
+    ///     if let Ok(string) = value.downcast::<String>() {
+    ///         println!("String ({}): {}", string.len(), string);
+    ///     }
+    /// }
+    ///
+    /// let my_string = "Hello World".to_string();
+    /// print_if_string(Box::new(my_string));
+    /// print_if_string(Box::new(0i8));
+    /// ```
+    pub fn downcast<T: Any>(self) -> Result<Box<T, A>, Self> {
+        if self.is::<T>() {
+            unsafe {
+                let (raw, alloc): (*mut (dyn Any + Send + Sync), _) =
+                    Box::into_raw_with_allocator(self);
+                Ok(Box::from_raw_in(raw as *mut T, alloc))
+            }
+        } else {
+            Err(self)
+        }
+    }
+}
+
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T: fmt::Display + ?Sized, A: Allocator> fmt::Display for Box<T, A> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {