[WIP] Warn if a configured rule is not enabled

CHANGELOG.md

@@ -47,6 +47,10 @@
   [Cody Winton](https://github.com/codytwinton)
   [#1327](https://github.com/realm/SwiftLint/issues/1327)
 
+* Warn if a configured rule is not enabled.  
+  [Marcelo Fabri](https://github.com/marcelofabri)
+  [#1350](https://github.com/realm/SwiftLint/issues/1350)
+
 ##### Bug Fixes
 
 * Fix false positive on `redundant_discardable_let` rule when using

Source/SwiftLintFramework/Models/Configuration.swift

@@ -103,6 +103,12 @@ public struct Configuration: Equatable {
                 return optInRules.contains(id) || !(rule is OptInRule)
             }
         }
+
+        if !enableAllRules {
+            validateConfiguredRulesAreEnabled(configuredRules: configuredRules, rules: rules,
+                                              whitelistRules: whitelistRules, optInRules: optInRules,
+                                              disabledRules: disabledRules)
+        }
     }
 
     public init?(dict: [String: Any], ruleList: RuleList = masterRuleList, enableAllRules: Bool = false,
@@ -217,6 +223,32 @@ public struct Configuration: Equatable {
         }
         return self
     }
+
+    private func validateConfiguredRulesAreEnabled(configuredRules: [Rule], rules: [Rule],
+                                                   whitelistRules: [String], optInRules: [String],
+                                                   disabledRules: [String]) {
+        for rule in configuredRules {
+            guard !rules.contains(where: { type(of: $0) == type(of: rule) }) else {
+                continue
+            }
+
+            let identifier = type(of: rule).description.identifier
+            let message = "Found a configuration for \(identifier)"
+            guard whitelistRules.isEmpty else {
+                queuedPrintError("\(message), but it is not present on " +
+                                 "\(ConfigurationKey.whitelistRules.rawValue)")
+                continue
+            }
+
+            if rule is OptInRule, !optInRules.contains(identifier) {
+                queuedPrintError("\(message), but it is not enabled on " +
+                                 "\(ConfigurationKey.optInRules.rawValue)")
+            } else {
+                queuedPrintError("\(message), but it is disabled on " +
+                                 "\(ConfigurationKey.disabledRules.rawValue)")
+            }
+        }
+    }
 }
 
 private func validateRuleIdentifiers(configuredRules: [Rule], disabledRules: [String]) -> [String] {

Source/SwiftLintFramework/Models/RuleList.swift

@@ -41,6 +41,7 @@ public struct RuleList {
 
         for (key, configuration) in dictionary {
             guard let identifier = identifier(for: key), let ruleType = list[identifier] else {
+                queuedPrintError("Unknown rule for identifier: '\(key)'. Maybe there's a typo?")
                 continue
             }
             guard rules[identifier] == nil else {