Create rule S7423: Unit values should not be compared (#4764)

* Create rule S7423

* Update RSPEC

---------

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

rules/S7423/metadata.json

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+{
+}

rules/S7423/rust/metadata.json

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+{
+  "title": "Unit values should not be compared",
+  "type": "BUG",
+  "status": "ready",
+  "remediation": {
+    "func": "Constant\/Issue",
+    "constantCost": "5min"
+  },
+  "tags": [
+    "clippy"
+  ],
+  "defaultSeverity": "Major",
+  "ruleSpecification": "RSPEC-7423",
+  "sqKey": "S7423",
+  "scope": "All",
+  "defaultQualityProfiles": ["Sonar way"],
+  "quickfix": "unknown",
+  "code": {
+    "impacts": {
+      "RELIABILITY": "MEDIUM"
+    },
+    "attribute": "LOGICAL"
+  }
+}

rules/S7423/rust/rule.adoc

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+== Why is this an issue?
+
+The unit type `()` is always equal to itself and using it in comparisons is either redundant or a mistake, often caused by accidental semicolon placement.
+
+=== Code examples
+
+==== Noncompliant code example
+
+[source,rust,diff-id=1,diff-type=noncompliant]
+----
+fn foo() {}
+fn bar() {}
+fn baz() {}
+
+if {
+    foo();
+} == {
+    bar();
+} {
+    baz();
+} // Noncompliant: Comparing unit values.
+----
+
+==== Compliant solution
+
+[source,rust,diff-id=1,diff-type=compliant]
+----
+fn foo() {}
+fn bar() {}
+fn baz() {}
+
+foo();
+bar();
+baz(); // Compliant: No unit comparison.
+----
+
+== Resources
+=== Documentation
+
+* Clippy Lints - https://rust-lang.github.io/rust-clippy/master/index.html#unit_cmp