Create rule S7422: Unit values should not be hashed (#4763)

* Create rule S7422

* Update RSPEC

* Update snippets

---------

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

rules/S7422/metadata.json

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+{
+}

rules/S7422/rust/metadata.json

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+{
+  "title": "Unit values should not be hashed",
+  "type": "BUG",
+  "status": "ready",
+  "remediation": {
+    "func": "Constant\/Issue",
+    "constantCost": "5min"
+  },
+  "tags": [
+    "clippy"
+  ],
+  "defaultSeverity": "Major",
+  "ruleSpecification": "RSPEC-7422",
+  "sqKey": "S7422",
+  "scope": "All",
+  "defaultQualityProfiles": ["Sonar way"],
+  "quickfix": "unknown",
+  "code": {
+    "impacts": {
+      "RELIABILITY": "HIGH"
+    },
+    "attribute": "LOGICAL"
+  }
+}

rules/S7422/rust/rule.adoc

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+== Why is this an issue?
+
+Hashing a unit value doesn't accomplish anything because the implementation of `Hash` for `()` is a no-op. This can lead to confusion or misleading code, as it implies some hashing operation is actually taking place, when in reality it does nothing.
+
+=== Code examples
+
+==== Noncompliant code example
+
+[source,rust,diff-id=1,diff-type=noncompliant]
+----
+enum Foo { Empty, WithValue(u8) }
+use Foo::*;
+
+let mut state = DefaultHasher::new();
+let my_enum = Foo::Empty;
+
+match my_enum {
+    Empty => ().hash(&mut state), // Noncompliant
+    WithValue(x) => x.hash(&mut state),
+}
+----
+
+==== Compliant solution
+
+[source,rust,diff-id=1,diff-type=compliant]
+----
+enum Foo { Empty, WithValue(u8) }
+use Foo::*;
+
+let mut state = DefaultHasher::new();
+let my_enum = Foo::Empty;
+
+match my_enum {
+    Empty => 0_u8.hash(&mut state), // Compliant
+    WithValue(x) => x.hash(&mut state),
+}
+----
+
+== Resources
+=== Documentation
+
+* Clippy Lints - https://rust-lang.github.io/rust-clippy/master/index.html#unit_hash