Create rule S7421: Closures of type Fn(...) -> Ord should not return the unit type (#4762)

* Create rule S7421

* Update RSPEC

* Change severity to Critical

---------

Co-authored-by: yassin-kammoun-sonarsource <yassin-kammoun-sonarsource@users.noreply.github.com>
Co-authored-by: yassin-kammoun-sonarsource <yassin.kammoun@sonarsource.com>

rules/S7421/metadata.json

@@ -0,0 +1,2 @@
+{
+}

rules/S7421/rust/metadata.json

@@ -0,0 +1,24 @@
+{
+  "title": "Closures of `type Fn(...) -> Ord` should not return the unit type",
+  "type": "BUG",
+  "status": "ready",
+  "remediation": {
+    "func": "Constant\/Issue",
+    "constantCost": "5min"
+  },
+  "tags": [
+    "clippy"
+  ],
+  "defaultSeverity": "Critical",
+  "ruleSpecification": "RSPEC-7421",
+  "sqKey": "S7421",
+  "scope": "All",
+  "defaultQualityProfiles": ["Sonar way"],
+  "quickfix": "unknown",
+  "code": {
+    "impacts": {
+      "RELIABILITY": "HIGH"
+    },
+    "attribute": "LOGICAL"
+  }
+}

rules/S7421/rust/rule.adoc

@@ -0,0 +1,26 @@
+== Why is this an issue?
+
+Returning the unit type from a closure expecting an `Ord` type is likely a mistake caused by an extra semi-colon. This is because the unit type implements `Ord` but doesn't serve the intended sorting purpose, making the code misleading and potentially incorrect.
+
+=== Code examples
+
+==== Noncompliant code example
+
+[source,rust,diff-id=1,diff-type=noncompliant]
+----
+let mut twins = vec![(1, 1), (2, 2)];
+twins.sort_by_key(|x| { x.1; }); // Noncompliant: Closure returns unit type due to unnecessary semi-colon.
+----
+
+==== Compliant solution
+
+[source,rust,diff-id=1,diff-type=compliant]
+----
+let mut twins = vec![(1, 1), (2, 2)];
+twins.sort_by_key(|x| x.1); // Compliant: Closure correctly returns an `Ord` type.
+----
+
+== Resources
+=== Documentation
+
+* Clippy Lints - https://rust-lang.github.io/rust-clippy/master/index.html#unit_return_expecting_ord