Rollup merge of #76198 - CDirkx:const-ordering, r=dtolnay

Make some Ordering methods const

Resubmission of [PR#75463](https://github.com/rust-lang/rust/pull/75463) as per [PR#76172](https://github.com/rust-lang/rust/pull/76172).

Constify the following methods of `core::cmp::Ordering`:
 - `reverse`
 - `then`

Insta-stabilizes these methods as const under the `const_ordering` feature, as their implementation is a trivial match and the recent stabilization of #49146 (Allow `if` and `match` in constants).
Note: the `const_ordering` feature has never actually been used as these methods have not been `#[rustc_const_unstable]`.

Tracking issue:  #76113

library/core/src/cmp.rs

@@ -356,8 +356,9 @@ impl Ordering {
     /// ```
     #[inline]
     #[must_use]
+    #[rustc_const_stable(feature = "const_ordering", since = "1.48.0")]
     #[stable(feature = "rust1", since = "1.0.0")]
-    pub fn reverse(self) -> Ordering {
+    pub const fn reverse(self) -> Ordering {
         match self {
             Less => Greater,
             Equal => Equal,
@@ -394,8 +395,9 @@ impl Ordering {
     /// ```
     #[inline]
     #[must_use]
+    #[rustc_const_stable(feature = "const_ordering", since = "1.48.0")]
     #[stable(feature = "ordering_chaining", since = "1.17.0")]
-    pub fn then(self, other: Ordering) -> Ordering {
+    pub const fn then(self, other: Ordering) -> Ordering {
         match self {
             Equal => other,
             _ => self,

library/core/tests/cmp.rs

@@ -1,4 +1,7 @@
-use core::cmp::{self, Ordering::*};
+use core::cmp::{
+    self,
+    Ordering::{self, *},
+};
 
 #[test]
 fn test_int_totalord() {
@@ -116,3 +119,16 @@ fn test_user_defined_eq() {
     assert!(SketchyNum { num: 37 } == SketchyNum { num: 34 });
     assert!(SketchyNum { num: 25 } != SketchyNum { num: 57 });
 }
+
+#[test]
+fn ordering_const() {
+    // test that the methods of `Ordering` are usable in a const context
+
+    const ORDERING: Ordering = Greater;
+
+    const REVERSE: Ordering = ORDERING.reverse();
+    assert_eq!(REVERSE, Less);
+
+    const THEN: Ordering = Equal.then(ORDERING);
+    assert_eq!(THEN, Greater);
+}