Add a cast method to raw pointers.

This is similar to `NonNull::cast`.

Compared to the `as` operator (which has a wide range of meanings
depending on the input and output types), a call to this method:

* Can only go from a raw pointer to a raw pointer
* Cannot change the pointer’s `const`ness

… even when the pointed types are inferred based on context.

src/libcore/ptr.rs

@@ -974,6 +974,13 @@ impl<T: ?Sized> *const T {
         (self as *const u8) == null()
     }
 
+    /// Cast to a pointer to a different type
+    #[unstable(feature = "ptr_cast", issue = "60602")]
+    #[inline]
+    pub const fn cast<U>(self) -> *const U {
+        self as _
+    }
+
     /// Returns `None` if the pointer is null, or else returns a reference to
     /// the value wrapped in `Some`.
     ///
@@ -1593,6 +1600,13 @@ impl<T: ?Sized> *mut T {
         (self as *mut u8) == null_mut()
     }
 
+    /// Cast to a pointer to a different type
+    #[unstable(feature = "ptr_cast", issue = "60602")]
+    #[inline]
+    pub const fn cast<U>(self) -> *mut U {
+        self as _
+    }
+
     /// Returns `None` if the pointer is null, or else returns a reference to
     /// the value wrapped in `Some`.
     ///