Implemented const casts of raw pointers

This adds `as_mut()` method for `*const T` and `as_const()` for `*mut T`
which are intended to make casting of consts safer. This was discussed
in the [internals discussion][discussion].

[discussion]: https://internals.rust-lang.org/t/casting-constness-can-be-risky-heres-a-simple-fix/15933

library/core/src/ptr/const_ptr.rs

@@ -48,6 +48,16 @@ impl<T: ?Sized> *const T {
         self as _
     }
 
+    /// Changes constness without changing the type.
+    ///
+    /// This is a bit safer than `as` because it wouldn't silently change the type if the code is
+    /// refactored.
+    #[unstable(feature = "ptr_const_cast", issue = "92675")]
+    #[rustc_const_unstable(feature = "ptr_const_cast", issue = "92675")]
+    pub const fn as_mut(self) -> *mut T {
+        self as _
+    }
+
     /// Casts a pointer to its raw bits.
     ///
     /// This is equivalent to `as usize`, but is more specific to enhance readability.

library/core/src/ptr/mut_ptr.rs

@@ -47,6 +47,20 @@ impl<T: ?Sized> *mut T {
         self as _
     }
 
+    /// Changes constness without changing the type.
+    ///
+    /// This is a bit safer than `as` because it wouldn't silently change the type if the code is
+    /// refactored.
+    ///
+    /// While not strictly required (`*mut T` coerces to `*const T`), this is provided for symmetry
+    /// with `as_mut()` on `*const T` and may have documentation value if used instead of implicit
+    /// coercion.
+    #[unstable(feature = "ptr_const_cast", issue = "92675")]
+    #[rustc_const_unstable(feature = "ptr_const_cast", issue = "92675")]
+    pub const fn as_const(self) -> *const T {
+        self as _
+    }
+
     /// Casts a pointer to its raw bits.
     ///
     /// This is equivalent to `as usize`, but is more specific to enhance readability.