warn against using intrinsics that leave the scope of our memory model

Author: RalfJung

library/core/src/intrinsics.rs

@@ -341,6 +341,7 @@ extern "rust-intrinsic" {
     /// [`Ordering::Relaxed`] as the `order`. For example, [`AtomicBool::load`].
     #[rustc_nounwind]
     pub fn atomic_load_relaxed<T: Copy>(src: *const T) -> T;
+    /// Do NOT use this intrinsic; "unordered" operations do not exist in our memory model!
     #[rustc_nounwind]
     pub fn atomic_load_unordered<T: Copy>(src: *const T) -> T;
 
@@ -365,6 +366,7 @@ extern "rust-intrinsic" {
     /// [`Ordering::Relaxed`] as the `order`. For example, [`AtomicBool::store`].
     #[rustc_nounwind]
     pub fn atomic_store_relaxed<T: Copy>(dst: *mut T, val: T);
+    /// Do NOT use this intrinsic; "unordered" operations do not exist in our memory model!
     #[rustc_nounwind]
     pub fn atomic_store_unordered<T: Copy>(dst: *mut T, val: T);
 
@@ -2312,6 +2314,8 @@ extern "rust-intrinsic" {
 
     /// Emits a `!nontemporal` store according to LLVM (see their docs).
     /// Probably will never become stable.
+    ///
+    /// Do NOT use this intrinsic; "nontemporal" operations do not exist in our memory model!
     #[rustc_nounwind]
     pub fn nontemporal_store<T>(ptr: *mut T, val: T);