caution against ptr-to-int transmutes

library/core/src/intrinsics.rs

@@ -991,6 +991,16 @@ extern "rust-intrinsic" {
     /// let ptr_num_cast = ptr as *const i32 as usize;
     /// ```
     ///
+    /// Note that using `transmute` to turn a pointer to a `usize` is (as noted above) [undefined
+    /// behavior][ub] in `const` contexts. Also outside of consts, this operation might not behave
+    /// as expected -- this is touching on many unspecified aspects of the Rust memory model.
+    /// Depending on what the code is doing, the following alternatives are preferrable to
+    /// pointer-to-integer transmutation:
+    /// - If the code just wants to store data of arbitrary type in some buffer and needs to pick a
+    ///   type for that buffer, it can use [`MaybeUninit`][mem::MaybeUninit].
+    /// - If the code actually wants to work on the address the pointer points to, it can use `as`
+    ///   casts or [`ptr.addr()`][pointer::addr].
+    ///
     /// Turning a `*mut T` into an `&mut T`:
     ///
     /// ```