Ensure careful consideration is given by impls

Added an associated `const THIS_IMPLEMENTATION_HAS_BEEN_TRIPLE_CHECKED`
to the `StableOrd` trait to ensure that implementors carefully consider
whether the trait's contract is upheld, as incorrect implementations can
cause miscompilations.

compiler/rustc_abi/src/lib.rs

@@ -425,11 +425,13 @@ pub struct Size {
     raw: u64,
 }
 
-// Ord is implement as just comparing numerical values and numerical values
-// are not changed by (de-)serialization.
 #[cfg(feature = "nightly")]
 impl StableOrd for Size {
     const CAN_USE_UNSTABLE_SORT: bool = true;
+
+    // `Ord` is implemented as just comparing numerical values and numerical values
+    // are not changed by (de-)serialization.
+    const THIS_IMPLEMENTATION_HAS_BEEN_TRIPLE_CHECKED: () = ();
 }
 
 // This is debug-printed a lot in larger structs, don't waste too much space there