From 3a129b2f55ac9e53ca780974603cfdbf5340dcff Mon Sep 17 00:00:00 2001
From: Gymore <gymore.contact@gmail.com>
Date: Sun, 24 Jan 2021 06:31:26 +0100
Subject: [PATCH 1/3] Implementation of `Result::swap` and tests

---
 library/core/src/result.rs   | 22 ++++++++++++++++++++++
 library/core/tests/result.rs |  7 +++++++
 2 files changed, 29 insertions(+)

diff --git a/library/core/src/result.rs b/library/core/src/result.rs
index d6d1762572928..33ec0c8f33f73 100644
--- a/library/core/src/result.rs
+++ b/library/core/src/result.rs
@@ -592,6 +592,28 @@ impl<T, E> Result<T, E> {
         }
     }
 
+    /// Maps a `Result<T, E>` to a `Result<E, T>` by swapping the success/failure
+    /// semantics of the stored variant.
+    ///
+    /// # Examples
+    ///
+    /// Basic usage:
+    ///
+    /// ```
+    /// #![feature(result_swap)]
+    ///
+    /// assert_eq!(Ok("Error :(").swap(), Err("Error :("));
+    /// assert_eq!(Err("Success!").swap(), Ok("Success!"));
+    /// ```
+    #[inline]
+    #[unstable(feature = "result_swap", issue = "none")]
+    pub fn swap(self) -> Result<E, T> {
+        match self {
+            Ok(t) => Err(t),
+            Err(e) => Ok(e),
+        }
+    }
+
     /////////////////////////////////////////////////////////////////////////
     // Iterator constructors
     /////////////////////////////////////////////////////////////////////////
diff --git a/library/core/tests/result.rs b/library/core/tests/result.rs
index 81660870e95e4..c257368574504 100644
--- a/library/core/tests/result.rs
+++ b/library/core/tests/result.rs
@@ -358,3 +358,10 @@ fn result_opt_conversions() {
 
     assert_eq!(res, Err(BadNumErr))
 }
+
+#[test]
+fn test_result_swap() {
+    assert_eq!(Ok("err").swap(), Err("err"));
+    assert_eq!(Err("ok").swap(), Ok("ok"));
+    assert_eq!(Ok("ok").swap().swap(), Ok("ok"));
+}

From 9c800abed78d95e75d4549a53268e431b9ca5b64 Mon Sep 17 00:00:00 2001
From: Gymore <gymore.contact@gmail.com>
Date: Sun, 24 Jan 2021 07:13:14 +0100
Subject: [PATCH 2/3] Add feature attribute `result_swap` for tests

---
 library/core/tests/lib.rs | 1 +
 1 file changed, 1 insertion(+)

diff --git a/library/core/tests/lib.rs b/library/core/tests/lib.rs
index 98f5982fbb2c1..572db1c74a19b 100644
--- a/library/core/tests/lib.rs
+++ b/library/core/tests/lib.rs
@@ -76,6 +76,7 @@
 #![feature(integer_atomics)]
 #![feature(slice_group_by)]
 #![feature(trusted_random_access)]
+#![feature(result_swap)]
 #![deny(unsafe_op_in_unsafe_fn)]
 
 extern crate test;

From 5381c26bb2e62c0a07c632e055a2b0617b7966ee Mon Sep 17 00:00:00 2001
From: Gymore <gymore.contact@gmail.com>
Date: Sun, 24 Jan 2021 07:52:32 +0100
Subject: [PATCH 3/3] Specify type parameters in the `test_result_swap` test
 function

---
 library/core/tests/result.rs | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/library/core/tests/result.rs b/library/core/tests/result.rs
index c257368574504..4dc3a0c7f0fcb 100644
--- a/library/core/tests/result.rs
+++ b/library/core/tests/result.rs
@@ -361,7 +361,7 @@ fn result_opt_conversions() {
 
 #[test]
 fn test_result_swap() {
-    assert_eq!(Ok("err").swap(), Err("err"));
-    assert_eq!(Err("ok").swap(), Ok("ok"));
-    assert_eq!(Ok("ok").swap().swap(), Ok("ok"));
+    assert_eq!(Ok::<_, u8>("err").swap(), Err("err"));
+    assert_eq!(Err::<u8, _>("ok").swap(), Ok("ok"));
+    assert_eq!(Ok::<_, u8>("ok").swap().swap(), Ok("ok"));
 }