propose assert_ne

text/0000-assert_ne.md

@@ -0,0 +1,66 @@
+- Feature Name: Assert Not Equals Macro (`assert_ne`)
+- Start Date: (2016-06-17)
+- RFC PR: (leave this empty)
+- Rust Issue: (leave this empty)
+
+# Summary
+[summary]: #summary
+
+`assert_ne` is a macro that takes 2 arguments and panics if they are equal. It
+works and is implemented identically to `assert_eq` and serves as its complement.
+This proposal also includes a `debug_asset_ne`, matching `debug_assert_eq`.
+
+# Motivation
+[motivation]: #motivation
+
+This feature, among other reasons, makes testing more readable and consistent as
+it complements `asset_eq`. It gives the same style panic message as `assert_eq`,
+which eliminates the need to write it yourself.
+
+# Detailed design
+[design]: #detailed-design
+
+This feature has exactly the same design and implementation as `assert_eq`.
+
+Here is the definition:
+
+```rust
+macro_rules! assert_ne {
+    ($left:expr , $right:expr) => ({
+        match (&$left, &$right) {
+            (left_val, right_val) => {
+                if *left_val == *right_val {
+                    panic!("assertion failed: `(left != right)` \
+                           (left: `{:?}`, right: `{:?}`)", left_val, right_val)
+                }
+            }
+        }
+    })
+}
+```
+
+This is complemented by a `debug_assert_ne` (similar to `debug_assert_eq`):
+
+```rust
+macro_rules! debug_assert_ne {
+    ($($arg:tt)*) => (if cfg!(debug_assertions) { assert_ne!($($arg)*); })
+}
+```
+
+# Drawbacks
+[drawbacks]: #drawbacks
+
+Any addition to the standard library will need to be maintained forever, so it is
+worth weighing the maintenance cost of this over the value add. Given that it is so
+similar to `assert_eq`, I believe the weight of this drawback is low.
+
+# Alternatives
+[alternatives]: #alternatives
+
+Alternatively, users implement this feature themselves, or use the crate `assert_ne`
+that I published.
+
+# Unresolved questions
+[unresolved]: #unresolved-questions
+
+None at this moment.