Fix false positives in unused_declaration for protocol extensions (re…

…alm#3000)

CHANGELOG.md

@@ -23,6 +23,10 @@
   [JP Simard](https://github.com/jpsim)
   [#2877](https://github.com/realm/SwiftLint/issues/2877)
 
+* Fix false positives from the `unused_declaration` rule involving
+  functions in protocol extensions.  
+  [JP Simard](https://github.com/jpsim)
+
 ## 0.38.0: Toroidal Agitation
 
 #### Breaking

Rules.md

@@ -24610,6 +24610,28 @@ let kConstant = 0
 _ = kConstant
 ```
 
+```swift
+enum Change<T> {
+  case insert(T)
+  case delete(T)
+}
+
+extension Sequence {
+  func deletes<T>() -> [T] where Element == Change<T> {
+    return compactMap { operation in
+      if case .delete(let value) = operation {
+        return value
+      } else {
+        return nil
+      }
+    }
+  }
+}
+
+let changes = [Change.insert(0), .delete(0)]
+changes.deletes()
+```
+
 ```swift
 struct Item {}
 struct ResponseModel: Codable {

Source/SwiftLintFramework/Rules/Lint/UnusedDeclarationRule.swift

@@ -25,6 +25,27 @@ public struct UnusedDeclarationRule: AutomaticTestableRule, ConfigurationProvide
             _ = kConstant
             """,
             """
+            enum Change<T> {
+              case insert(T)
+              case delete(T)
+            }
+
+            extension Sequence {
+              func deletes<T>() -> [T] where Element == Change<T> {
+                return compactMap { operation in
+                  if case .delete(let value) = operation {
+                    return value
+                  } else {
+                    return nil
+                  }
+                }
+              }
+            }
+
+            let changes = [Change.insert(0), .delete(0)]
+            changes.deletes()
+            """,
+            """
             struct Item {}
             struct ResponseModel: Codable {
                 let items: [Item]
@@ -219,7 +240,10 @@ private extension SwiftLintFile {
     private static func referencedUSR(cursorInfo: SourceKittenDictionary) -> String? {
         if let usr = cursorInfo.usr,
             let kind = cursorInfo.kind,
-            kind.contains("source.lang.swift.ref") {
+            kind.starts(with: "source.lang.swift.ref") {
+            if let synthesizedLocation = usr.range(of: "::SYNTHESIZED::")?.lowerBound {
+                return String(usr.prefix(upTo: synthesizedLocation))
+            }
             return usr
         }