Create rule S6524: Collection should be immutable if contents is not …

…changed (#1637)

rules/S6524/kotlin/metadata.json

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+{
+  "title": "Collection should be immutable if contents is not changed",
+  "type": "CODE_SMELL",
+  "code": {
+    "impacts": {
+      "MAINTAINABILITY": "MEDIUM"
+    },
+    "attribute": "CLEAR"
+  },
+  "status": "ready",
+  "remediation": {
+    "func": "Constant\/Issue",
+    "constantCost": "5min"
+  },
+  "tags": [
+  ],
+  "defaultSeverity": "Minor",
+  "ruleSpecification": "RSPEC-6524",
+  "sqKey": "S6524",
+  "scope": "All",
+  "defaultQualityProfiles": ["Sonar way"],
+  "quickfix": "unknown"
+}

rules/S6524/kotlin/rule.adoc

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+== Why is this an issue?
+
+If a mutable collection type is used but no mutating functions such as `add` or `remove` are ever called,
+and the collection instance does not leave the scope of the function,
+it can be replaced with the corresponding immutable collection type.
+
+This is similar to why `val` should be used instead of `var` for local variables that are never re-assigned.
+
+=== What is the potential impact?
+
+==== Readability and Understanding
+
+If an immutable collection type is used, it is evident to the readers that its content is never changed.
+This makes it easier to understand the code because readers do not need to keep track of possible state changes of the collection.
+
+==== Performance
+
+In some cases, optimized implementation variants of collection classes can be used when the collection is immutable.
+
+==== Wrong code
+
+Developers might intend for a collection to remain unchanged and have their code relying on that constraint.
+For example, a map could be expected to contain specific elements.
+Changing the contents of a collection breaks that constraint.
+Also, users of an API might otherwise downcast an immutable collection they got from a library
+into a mutable collection, and so cause unforeseen side effects.
+
+Declare collections that remain unchanged as immutable to avoid these mistakes.
+
+== How to fix it
+
+Replace mutable collection type names such as `MutableList` or `MutableMap`
+with their immutable equivalents, such as `List` or `map`.
+
+Replace builder functions that return mutable collection instances,
+such as `mutableListOf` with their immutable counterparts, such as `listOf`.
+
+=== Code examples
+
+==== Noncompliant code example
+
+[source,kotlin,diff-id=1,diff-type=noncompliant]
+----
+fun sum123(): Int {
+    val list = mutableListOf(1,2,3) // Noncompliant, can be immutable
+    return list.reduce { acc, it -> acc + it}
+}
+----
+
+==== Compliant solution
+
+[source,kotlin,diff-id=1,diff-type=compliant]
+----
+fun sum123(): Int {
+    val list = listOf(1,2,3) // Compliant
+    return list.reduce { acc, it -> acc + it}
+}
+----
+
+==== Noncompliant code example
+
+[source,kotlin,diff-id=2,diff-type=noncompliant]
+----
+fun sumList(list: MutableList<Int>): Int { // Noncompliant, can be immutable
+    return list.reduce { acc, it -> acc + it}
+}
+----
+
+==== Compliant solution
+
+[source,kotlin,diff-id=2,diff-type=compliant]
+----
+fun sumList(list: List<Int>): Int { // Compliant
+    return list.reduce { acc, it -> acc + it}
+}
+----
+
+==== Noncompliant code example
+
+[source,kotlin,diff-id=3,diff-type=noncompliant]
+----
+fun MutableList<Int>.sum(): Int { // Noncompliant, can be immutable
+    return reduce { acc, it -> acc + it}
+}
+----
+
+==== Compliant solution
+
+[source,kotlin,diff-id=3,diff-type=compliant]
+----
+fun List<Int>.sum(): Int { // Compliant
+    return reduce { acc, it -> acc + it}
+}
+----
+
+== Resources
+
+=== Articles & blog posts
+
+* https://www.baeldung.com/kotlin/immutable-collections[Baeldung, Kotlin Immutable Collections]

rules/S6524/metadata.json

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+{
+}