Split StableCompare trait out of StableOrd trait.

StableCompare is a companion trait to `StableOrd`. Some types like `Symbol` can be compared in a cross-session stable way, but their `Ord` implementation is not stable. In such cases, a `StableOrd` implementation can be provided to offer a lightweight way for stable sorting. (The more heavyweight option is to sort via `ToStableHashKey`, but then sorting needs to have access to a stable hashing context and `ToStableHashKey` can also be expensive as in the case of `Symbol` where it has to allocate a `String`.)

compiler/rustc_data_structures/src/stable_hasher.rs

@@ -245,6 +245,32 @@ unsafe impl<T: StableOrd> StableOrd for &T {
     const CAN_USE_UNSTABLE_SORT: bool = T::CAN_USE_UNSTABLE_SORT;
 }
 
+/// This is a companion trait to `StableOrd`. Some types like `Symbol` can be
+/// compared in a cross-session stable way, but their `Ord` implementation is
+/// not stable. In such cases, a `StableOrd` implementation can be provided
+/// to offer a lightweight way for stable sorting. (The more heavyweight option
+/// is to sort via `ToStableHashKey`, but then sorting needs to have access to
+/// a stable hashing context and `ToStableHashKey` can also be expensive as in
+/// the case of `Symbol` where it has to allocate a `String`.)
+///
+/// See the documentation of [StableOrd] for how stable sort order is defined.
+/// The same definition applies here. Be careful when implementing this trait.
+pub trait StableCompare {
+    const CAN_USE_UNSTABLE_SORT: bool;
+
+    fn stable_cmp(&self, other: &Self) -> std::cmp::Ordering;
+}
+
+/// `StableOrd` denotes that the type's `Ord` implementation is stable, so
+/// we can implement `StableCompare` by just delegating to `Ord`.
+impl<T: StableOrd> StableCompare for T {
+    const CAN_USE_UNSTABLE_SORT: bool = T::CAN_USE_UNSTABLE_SORT;
+
+    fn stable_cmp(&self, other: &Self) -> std::cmp::Ordering {
+        self.cmp(other)
+    }
+}
+
 /// Implement HashStable by just calling `Hash::hash()`. Also implement `StableOrd` for the type since
 /// that has the same requirements.
 ///