Add test for Result::into_err

library/core/tests/result.rs

@@ -225,6 +225,28 @@ fn infallible_op2() -> Result<isize, MyNeverToken> {
     assert_eq!(infallible_op2().into_ok(), 667);
 }
 
+#[test]
+pub fn test_into_err() {
+    fn until_error_op() -> Result<!, isize> {
+        Err(666)
+    }
+
+    assert_eq!(until_error_op().into_err(), 666);
+
+    enum MyNeverToken {}
+    impl From<MyNeverToken> for ! {
+        fn from(never: MyNeverToken) -> ! {
+            match never {}
+        }
+    }
+
+    fn until_error_op2() -> Result<MyNeverToken, isize> {
+        Err(667)
+    }
+
+    assert_eq!(until_error_op2().into_err(), 667);
+}
+
 #[test]
 fn test_try() {
     fn try_result_some() -> Option<u8> {