Auto merge of #126326 - eggyal:ununsafe-StableOrd, r=michaelwoerister

Un-unsafe the `StableOrd` trait

Whilst incorrect implementations of this trait can cause miscompilation, they cannot cause memory unsafety in rustc.

[Discussed on Zulip](https://rust-lang.zulipchat.com/#narrow/stream/182449-t-compiler.2Fhelp/topic/Policy.20of.20.60unsafe.60.20within.20the.20compiler).

cc [MCP 533](https://github.com/rust-lang/compiler-team/issues/533), #105175, `@michaelwoerister`

r? `@Nilstrieb`

compiler/rustc_abi/src/lib.rs

@@ -427,11 +427,13 @@ pub struct Size {
     raw: u64,
 }
 
-// Safety: Ord is implement as just comparing numerical values and numerical values
-// are not changed by (de-)serialization.
 #[cfg(feature = "nightly")]
-unsafe impl StableOrd for Size {
+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