Add ignore_coding_keys parameter to nesting

This prevents nested CodingKeys (which may be necessary
for functioning Codable conformance) from triggering
nesting violations

CHANGELOG.md

@@ -12,8 +12,10 @@
 
 ### Enhancements
 
-* None.
-
+* Add `ignore_coding_keys` parameter to `nesting` rule.  
+  [braker1nine](https://github.com/braker1nine)
+  [#5641](https://github.com/realm/SwiftLint/issues/5641)
+
 ### Bug Fixes
 
 * None.

Source/SwiftLintBuiltInRules/Rules/Metrics/NestingRule.swift

@@ -64,7 +64,9 @@ private extension NestingRule {
         }
 
         override func visit(_ node: EnumDeclSyntax) -> SyntaxVisitorContinueKind {
-            validate(forFunction: false, triggeringToken: node.enumKeyword)
+            if !configuration.ignoreCodingKeys || !node.definesCodingKeys {
+                validate(forFunction: false, triggeringToken: node.enumKeyword)
+            }
             return .visitChildren
         }
 

Source/SwiftLintBuiltInRules/Rules/RuleConfigurations/NestingConfiguration.swift

@@ -15,6 +15,8 @@ struct NestingConfiguration: RuleConfiguration {
     private(set) var alwaysAllowOneTypeInFunctions = false
     @ConfigurationElement(key: "ignore_typealiases_and_associatedtypes")
     private(set) var ignoreTypealiasesAndAssociatedtypes = false
+    @ConfigurationElement(key: "ignore_coding_keys")
+    private(set) var ignoreCodingKeys = false
 
     func severity(with config: Severity, for level: Int) -> ViolationSeverity? {
         if let error = config.error, level > error {

Tests/BuiltInRulesTests/NestingRuleTests.swift

@@ -555,4 +555,46 @@ final class NestingRuleTests: SwiftLintTestCase {
 
         verifyRule(description, ruleConfiguration: ["ignore_typealiases_and_associatedtypes": true])
     }
+
+    func testNestingWithoutCodingKeys() {
+        var nonTriggeringExamples = NestingRule.description.nonTriggeringExamples
+        nonTriggeringExamples.append(contentsOf: [
+            .init("""
+                struct Outer {
+                    struct Inner {
+                        enum CodingKeys: String, CodingKey {
+                            case id
+                        }
+                    }
+                }
+                """
+             ),
+        ])
+
+        var triggeringExamples = NestingRule.description.triggeringExamples
+        triggeringExamples.append(contentsOf: [
+            .init("""
+                struct Outer {
+                    struct Inner {
+                        ↓enum Example: String, CodingKey {
+                            case id
+                        }
+                    }
+                }
+            """),
+            .init("""
+                struct Outer {
+                  enum CodingKeys: String, CodingKey {
+                    case id
+                    
+                    ↓struct S {}
+                  }
+                }
+            """)
+        ])
+
+        let description = NestingRule.description.with(nonTriggeringExamples: nonTriggeringExamples, triggeringExamples: triggeringExamples)
+
+        verifyRule(description, ruleConfiguration: ["ignore_coding_keys": true ])
+    }
 }