Merge pull request #1581 from marcelofabri/fix-1525

Don't trigger `discarded_notification_center_observer` when the observer is returned

CHANGELOG.md

@@ -27,6 +27,12 @@
   [Marcelo Fabri](https://github.com/marcelofabri)
   [#1532](https://github.com/realm/SwiftLint/issues/1532)
 
+* Don't trigger violations from `discarded_notification_center_observer` rule 
+  when the observer is being returned from a function that is not marked 
+  as `@discardableResult`.  
+  [Marcelo Fabri](https://github.com/marcelofabri)
+  [#1525](https://github.com/realm/SwiftLint/issues/1525)
+
 ##### Bug Fixes
 
 * None.

Source/SwiftLintFramework/Rules/DiscardedNotificationCenterObserverRule.swift

@@ -21,11 +21,17 @@ public struct DiscardedNotificationCenterObserverRule: ASTRule, ConfigurationPro
                      "returned should be stored so it can be removed later.",
         nonTriggeringExamples: [
             "let foo = nc.addObserver(forName: .NSSystemTimeZoneDidChange, object: nil, queue: nil) { }\n",
-            "let foo = nc.addObserver(forName: .NSSystemTimeZoneDidChange, object: nil, queue: nil, using: { })\n"
+            "let foo = nc.addObserver(forName: .NSSystemTimeZoneDidChange, object: nil, queue: nil, using: { })\n",
+            "func foo() -> Any {\n" +
+            "   return nc.addObserver(forName: .NSSystemTimeZoneDidChange, object: nil, queue: nil, using: { })\n" +
+            "}\n"
         ],
         triggeringExamples: [
             "↓nc.addObserver(forName: .NSSystemTimeZoneDidChange, object: nil, queue: nil) { }\n",
-            "↓nc.addObserver(forName: .NSSystemTimeZoneDidChange, object: nil, queue: nil, using: { })\n"
+            "↓nc.addObserver(forName: .NSSystemTimeZoneDidChange, object: nil, queue: nil, using: { })\n",
+            "@discardableResult func foo() -> Any {\n" +
+            "   return ↓nc.addObserver(forName: .NSSystemTimeZoneDidChange, object: nil, queue: nil, using: { })\n" +
+            "}\n"
         ]
     )
 
@@ -57,6 +63,35 @@ public struct DiscardedNotificationCenterObserverRule: ASTRule, ConfigurationPro
             return []
         }
 
+        if let lastMatch = file.match(pattern: "\\breturn\\s+", with: [.keyword], range: range).last,
+            lastMatch.location == range.length - lastMatch.length,
+            let lastFunction = file.structure.functions(forByteOffset: offset).last,
+            !lastFunction.enclosedSwiftAttributes.contains("source.decl.attribute.discardableResult") {
+            return []
+        }
+
         return [offset]
     }
 }
+
+private extension Structure {
+    func functions(forByteOffset byteOffset: Int) -> [[String: SourceKitRepresentable]] {
+        var results = [[String: SourceKitRepresentable]]()
+
+        func parse(_ dictionary: [String: SourceKitRepresentable]) {
+            guard let offset = dictionary.offset,
+                let byteRange = dictionary.length.map({ NSRange(location: offset, length: $0) }),
+                NSLocationInRange(byteOffset, byteRange) else {
+                    return
+            }
+
+            if let kind = dictionary.kind.flatMap(SwiftDeclarationKind.init),
+                SwiftDeclarationKind.functionKinds().contains(kind) {
+                results.append(dictionary)
+            }
+            dictionary.substructure.forEach(parse)
+        }
+        parse(dictionary)
+        return results
+    }
+}