caution against ptr-to-int transmutes
diff --git a/library/core/src/intrinsics.rs b/library/core/src/intrinsics.rs
index 8ad4317..4962299 100644
--- a/library/core/src/intrinsics.rs
+++ b/library/core/src/intrinsics.rs
@@ -991,6 +991,12 @@ pub unsafe fn drop_in_place<T: ?Sized>(to_drop: *mut T) {
/// 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. To
+ /// make sure your code is well-defined, the conversion of pointers to integers and back should
+ /// always be done explicitly via casts.
+ ///
/// Turning a `*mut T` into an `&mut T`:
///
/// ```
