From ea5dc0909ea1ed4135f1bdecbaa3423051be578d Mon Sep 17 00:00:00 2001
From: CDirkx <christiaan@dirkx.com>
Date: Tue, 1 Sep 2020 15:44:00 +0200
Subject: [PATCH 1/2] Make some Ordering methods const

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

Stabilizes these methods as const under the `const_ordering` feature.
Also adds a test for these methods in a const context.

Possible because of #49146 (Allow `if` and `match` in constants).
---
 library/core/src/cmp.rs              |  6 ++++--
 src/test/ui/consts/const-ordering.rs | 15 +++++++++++++++
 2 files changed, 19 insertions(+), 2 deletions(-)
 create mode 100644 src/test/ui/consts/const-ordering.rs

diff --git a/library/core/src/cmp.rs b/library/core/src/cmp.rs
index 3953c73319fe4..dde442aa7b52d 100644
--- a/library/core/src/cmp.rs
+++ b/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,
diff --git a/src/test/ui/consts/const-ordering.rs b/src/test/ui/consts/const-ordering.rs
new file mode 100644
index 0000000000000..454f2da00df9e
--- /dev/null
+++ b/src/test/ui/consts/const-ordering.rs
@@ -0,0 +1,15 @@
+// run-pass
+
+use std::cmp::Ordering;
+
+// the following methods of core::cmp::Ordering are const:
+//  - reverse
+//  - then
+
+fn main() {
+    const REVERSE : Ordering = Ordering::Greater.reverse();
+    assert_eq!(REVERSE, Ordering::Less);
+
+    const THEN : Ordering = Ordering::Equal.then(REVERSE);
+    assert_eq!(THEN, Ordering::Less);
+}

From 79d563c819483eaf6e67b6aaaef9d0ca6030337d Mon Sep 17 00:00:00 2001
From: Christiaan Dirkx <christiaan@dirkx.email>
Date: Fri, 4 Sep 2020 00:40:11 +0200
Subject: [PATCH 2/2] Move const tests for `Ordering` to `library\core`

Part of #76268
---
 library/core/tests/cmp.rs            | 18 +++++++++++++++++-
 src/test/ui/consts/const-ordering.rs | 15 ---------------
 2 files changed, 17 insertions(+), 16 deletions(-)
 delete mode 100644 src/test/ui/consts/const-ordering.rs

diff --git a/library/core/tests/cmp.rs b/library/core/tests/cmp.rs
index 4086917780fa4..835289daf715a 100644
--- a/library/core/tests/cmp.rs
+++ b/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);
+}
diff --git a/src/test/ui/consts/const-ordering.rs b/src/test/ui/consts/const-ordering.rs
deleted file mode 100644
index 454f2da00df9e..0000000000000
--- a/src/test/ui/consts/const-ordering.rs
+++ /dev/null
@@ -1,15 +0,0 @@
-// run-pass
-
-use std::cmp::Ordering;
-
-// the following methods of core::cmp::Ordering are const:
-//  - reverse
-//  - then
-
-fn main() {
-    const REVERSE : Ordering = Ordering::Greater.reverse();
-    assert_eq!(REVERSE, Ordering::Less);
-
-    const THEN : Ordering = Ordering::Equal.then(REVERSE);
-    assert_eq!(THEN, Ordering::Less);
-}