Rollup merge of #90851 - ibraheemdev:downcast-unchecked, r=scottmcm

Add unchecked downcast methods

```rust
impl dyn Any (+ Send + Sync) {
    pub unsafe fn downcast_ref_unchecked<T: Any>(&self) -> &T;
    pub unsafe fn downcast_mut_unchecked<T: Any>(&mut self) -> &mut T;
}

impl<A: Allocator> Box<dyn Any (+ Send + Sync), A> {
    pub unsafe fn downcast_unchecked<T: Any>(&self) -> Box<T, A>;
}
```

2 files changed, 275 insertions, 36 deletions

diff --git a/library/alloc/src/boxed.rs b/library/alloc/src/boxed.rs
index 0b72b3f0ee7..ab41f5646e5 100644
--- a/library/alloc/src/boxed.rs
+++ b/library/alloc/src/boxed.rs
@@ -1518,8 +1518,6 @@ impl<T, const N: usize> TryFrom<Box<[T]>> for Box<[T; N]> {
 }
 
 impl<A: Allocator> Box<dyn Any, A> {
-    #[inline]
-    #[stable(feature = "rust1", since = "1.0.0")]
     /// Attempt to downcast the box to a concrete type.
     ///
     /// # Examples
@@ -1537,21 +1535,48 @@ impl<A: Allocator> Box<dyn Any, A> {
     /// print_if_string(Box::new(my_string));
     /// print_if_string(Box::new(0i8));
     /// ```
+    #[inline]
+    #[stable(feature = "rust1", since = "1.0.0")]
     pub fn downcast<T: Any>(self) -> Result<Box<T, A>, Self> {
-        if self.is::<T>() {
-            unsafe {
-                let (raw, alloc): (*mut dyn Any, _) = Box::into_raw_with_allocator(self);
-                Ok(Box::from_raw_in(raw as *mut T, alloc))
-            }
-        } else {
-            Err(self)
+        if self.is::<T>() { unsafe { Ok(self.downcast_unchecked::<T>()) } } else { Err(self) }
+    }
+
+    /// Downcasts the box to a concrete type.
+    ///
+    /// For a safe alternative see [`downcast`].
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(downcast_unchecked)]
+    ///
+    /// use std::any::Any;
+    ///
+    /// let x: Box<dyn Any> = Box::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) -> Box<T, A> {
+        debug_assert!(self.is::<T>());
+        unsafe {
+            let (raw, alloc): (*mut dyn Any, _) = Box::into_raw_with_allocator(self);
+            Box::from_raw_in(raw as *mut T, alloc)
         }
     }
 }
 
 impl<A: Allocator> Box<dyn Any + Send, A> {
-    #[inline]
-    #[stable(feature = "rust1", since = "1.0.0")]
     /// Attempt to downcast the box to a concrete type.
     ///
     /// # Examples
@@ -1569,21 +1594,48 @@ impl<A: Allocator> Box<dyn Any + Send, A> {
     /// print_if_string(Box::new(my_string));
     /// print_if_string(Box::new(0i8));
     /// ```
+    #[inline]
+    #[stable(feature = "rust1", since = "1.0.0")]
     pub fn downcast<T: Any>(self) -> Result<Box<T, A>, Self> {
-        if self.is::<T>() {
-            unsafe {
-                let (raw, alloc): (*mut (dyn Any + Send), _) = Box::into_raw_with_allocator(self);
-                Ok(Box::from_raw_in(raw as *mut T, alloc))
-            }
-        } else {
-            Err(self)
+        if self.is::<T>() { unsafe { Ok(self.downcast_unchecked::<T>()) } } else { Err(self) }
+    }
+
+    /// Downcasts the box to a concrete type.
+    ///
+    /// For a safe alternative see [`downcast`].
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(downcast_unchecked)]
+    ///
+    /// use std::any::Any;
+    ///
+    /// let x: Box<dyn Any + Send> = Box::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) -> Box<T, A> {
+        debug_assert!(self.is::<T>());
+        unsafe {
+            let (raw, alloc): (*mut (dyn Any + Send), _) = Box::into_raw_with_allocator(self);
+            Box::from_raw_in(raw as *mut T, alloc)
         }
     }
 }
 
 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
@@ -1601,15 +1653,44 @@ impl<A: Allocator> Box<dyn Any + Send + Sync, A> {
     /// print_if_string(Box::new(my_string));
     /// print_if_string(Box::new(0i8));
     /// ```
+    #[inline]
+    #[stable(feature = "box_send_sync_any_downcast", since = "1.51.0")]
     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)
+        if self.is::<T>() { unsafe { Ok(self.downcast_unchecked::<T>()) } } else { Err(self) }
+    }
+
+    /// Downcasts the box to a concrete type.
+    ///
+    /// For a safe alternative see [`downcast`].
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(downcast_unchecked)]
+    ///
+    /// use std::any::Any;
+    ///
+    /// let x: Box<dyn Any + Send + Sync> = Box::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) -> Box<T, A> {
+        debug_assert!(self.is::<T>());
+        unsafe {
+            let (raw, alloc): (*mut (dyn Any + Send + Sync), _) =
+                Box::into_raw_with_allocator(self);
+            Box::from_raw_in(raw as *mut T, alloc)
         }
     }
 }
diff --git a/library/core/src/any.rs b/library/core/src/any.rs
index 1fd5aa27fce..72528185707 100644
--- a/library/core/src/any.rs
+++ b/library/core/src/any.rs
@@ -164,7 +164,7 @@ impl fmt::Debug for dyn Any + Send + Sync {
 }
 
 impl dyn Any {
-    /// Returns `true` if the boxed type is the same as `T`.
+    /// Returns `true` if the inner type is the same as `T`.
     ///
     /// # Examples
     ///
@@ -195,7 +195,7 @@ impl dyn Any {
         t == concrete
     }
 
-    /// Returns some reference to the boxed value if it is of type `T`, or
+    /// Returns some reference to the inner value if it is of type `T`, or
     /// `None` if it isn't.
     ///
     /// # Examples
@@ -221,13 +221,13 @@ impl dyn Any {
             // SAFETY: just checked whether we are pointing to the correct type, and we can rely on
             // that check for memory safety because we have implemented Any for all types; no other
             // impls can exist as they would conflict with our impl.
-            unsafe { Some(&*(self as *const dyn Any as *const T)) }
+            unsafe { Some(self.downcast_ref_unchecked()) }
         } else {
             None
         }
     }
 
-    /// Returns some mutable reference to the boxed value if it is of type `T`, or
+    /// Returns some mutable reference to the inner value if it is of type `T`, or
     /// `None` if it isn't.
     ///
     /// # Examples
@@ -257,15 +257,73 @@ impl dyn Any {
             // SAFETY: just checked whether we are pointing to the correct type, and we can rely on
             // that check for memory safety because we have implemented Any for all types; no other
             // impls can exist as they would conflict with our impl.
-            unsafe { Some(&mut *(self as *mut dyn Any as *mut T)) }
+            unsafe { Some(self.downcast_mut_unchecked()) }
         } else {
             None
         }
     }
+
+    /// Returns a reference to the inner value as type `dyn T`.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(downcast_unchecked)]
+    ///
+    /// use std::any::Any;
+    ///
+    /// let x: Box<dyn Any> = Box::new(1_usize);
+    ///
+    /// unsafe {
+    ///     assert_eq!(*x.downcast_ref_unchecked::<usize>(), 1);
+    /// }
+    /// ```
+    ///
+    /// # Safety
+    ///
+    /// The contained value must be of type `T`. Calling this method
+    /// with the incorrect type is *undefined behavior*.
+    #[unstable(feature = "downcast_unchecked", issue = "90850")]
+    #[inline]
+    pub unsafe fn downcast_ref_unchecked<T: Any>(&self) -> &T {
+        debug_assert!(self.is::<T>());
+        // SAFETY: caller guarantees that T is the correct type
+        unsafe { &*(self as *const dyn Any as *const T) }
+    }
+
+    /// Returns a mutable reference to the inner value as type `dyn T`.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(downcast_unchecked)]
+    ///
+    /// use std::any::Any;
+    ///
+    /// let mut x: Box<dyn Any> = Box::new(1_usize);
+    ///
+    /// unsafe {
+    ///     *x.downcast_mut_unchecked::<usize>() += 1;
+    /// }
+    ///
+    /// assert_eq!(*x.downcast_ref::<usize>().unwrap(), 2);
+    /// ```
+    ///
+    /// # Safety
+    ///
+    /// The contained value must be of type `T`. Calling this method
+    /// with the incorrect type is *undefined behavior*.
+    #[unstable(feature = "downcast_unchecked", issue = "90850")]
+    #[inline]
+    pub unsafe fn downcast_mut_unchecked<T: Any>(&mut self) -> &mut T {
+        debug_assert!(self.is::<T>());
+        // SAFETY: caller guarantees that T is the correct type
+        unsafe { &mut *(self as *mut dyn Any as *mut T) }
+    }
 }
 
 impl dyn Any + Send {
-    /// Forwards to the method defined on the type `Any`.
+    /// Forwards to the method defined on the type `dyn Any`.
     ///
     /// # Examples
     ///
@@ -289,7 +347,7 @@ impl dyn Any + Send {
         <dyn Any>::is::<T>(self)
     }
 
-    /// Forwards to the method defined on the type `Any`.
+    /// Forwards to the method defined on the type `dyn Any`.
     ///
     /// # Examples
     ///
@@ -313,7 +371,7 @@ impl dyn Any + Send {
         <dyn Any>::downcast_ref::<T>(self)
     }
 
-    /// Forwards to the method defined on the type `Any`.
+    /// Forwards to the method defined on the type `dyn Any`.
     ///
     /// # Examples
     ///
@@ -340,6 +398,60 @@ impl dyn Any + Send {
     pub fn downcast_mut<T: Any>(&mut self) -> Option<&mut T> {
         <dyn Any>::downcast_mut::<T>(self)
     }
+
+    /// Forwards to the method defined on the type `dyn Any`.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(downcast_unchecked)]
+    ///
+    /// use std::any::Any;
+    ///
+    /// let x: Box<dyn Any> = Box::new(1_usize);
+    ///
+    /// unsafe {
+    ///     assert_eq!(*x.downcast_ref_unchecked::<usize>(), 1);
+    /// }
+    /// ```
+    ///
+    /// # Safety
+    ///
+    /// Same as the method on the type `dyn Any`.
+    #[unstable(feature = "downcast_unchecked", issue = "90850")]
+    #[inline]
+    pub unsafe fn downcast_ref_unchecked<T: Any>(&self) -> &T {
+        // SAFETY: guaranteed by caller
+        unsafe { <dyn Any>::downcast_ref_unchecked::<T>(self) }
+    }
+
+    /// Forwards to the method defined on the type `dyn Any`.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(downcast_unchecked)]
+    ///
+    /// use std::any::Any;
+    ///
+    /// let mut x: Box<dyn Any> = Box::new(1_usize);
+    ///
+    /// unsafe {
+    ///     *x.downcast_mut_unchecked::<usize>() += 1;
+    /// }
+    ///
+    /// assert_eq!(*x.downcast_ref::<usize>().unwrap(), 2);
+    /// ```
+    ///
+    /// # Safety
+    ///
+    /// Same as the method on the type `dyn Any`.
+    #[unstable(feature = "downcast_unchecked", issue = "90850")]
+    #[inline]
+    pub unsafe fn downcast_mut_unchecked<T: Any>(&mut self) -> &mut T {
+        // SAFETY: guaranteed by caller
+        unsafe { <dyn Any>::downcast_mut_unchecked::<T>(self) }
+    }
 }
 
 impl dyn Any + Send + Sync {
@@ -418,6 +530,52 @@ impl dyn Any + Send + Sync {
     pub fn downcast_mut<T: Any>(&mut self) -> Option<&mut T> {
         <dyn Any>::downcast_mut::<T>(self)
     }
+
+    /// Forwards to the method defined on the type `Any`.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(downcast_unchecked)]
+    ///
+    /// use std::any::Any;
+    ///
+    /// let x: Box<dyn Any> = Box::new(1_usize);
+    ///
+    /// unsafe {
+    ///     assert_eq!(*x.downcast_ref_unchecked::<usize>(), 1);
+    /// }
+    /// ```
+    #[unstable(feature = "downcast_unchecked", issue = "90850")]
+    #[inline]
+    pub unsafe fn downcast_ref_unchecked<T: Any>(&self) -> &T {
+        // SAFETY: guaranteed by caller
+        unsafe { <dyn Any>::downcast_ref_unchecked::<T>(self) }
+    }
+
+    /// Forwards to the method defined on the type `Any`.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(downcast_unchecked)]
+    ///
+    /// use std::any::Any;
+    ///
+    /// let mut x: Box<dyn Any> = Box::new(1_usize);
+    ///
+    /// unsafe {
+    ///     *x.downcast_mut_unchecked::<usize>() += 1;
+    /// }
+    ///
+    /// assert_eq!(*x.downcast_ref::<usize>().unwrap(), 2);
+    /// ```
+    #[unstable(feature = "downcast_unchecked", issue = "90850")]
+    #[inline]
+    pub unsafe fn downcast_mut_unchecked<T: Any>(&mut self) -> &mut T {
+        // SAFETY: guaranteed by caller
+        unsafe { <dyn Any>::downcast_mut_unchecked::<T>(self) }
+    }
 }
 
 ///////////////////////////////////////////////////////////////////////////////