Workaround Swift 6 bug for missing related references to external ove…

…rriden declarations. Closes #820

CHANGELOG.md

@@ -12,6 +12,7 @@
 
 - Enums with the `@main` attribute are now retained.
 - Unused public/exported imports are excluded from the results even if unused in the declaring file as the exported symbols may be referenced in other files, and thus removing the import would result in a build failure.
+- Fixed issue in Swift 6 where declarations that override external type members are incorrectly identified as unused.
 
 ## 2.21.0 (2024-06-15)
 

Sources/SourceGraph/Elements/Declaration.swift

@@ -164,6 +164,10 @@ public final class Declaration {
             functionKinds.union(variableKinds).union(globalKinds)
         }
 
+        static var overrideKinds: Set<Kind> {
+            [.functionMethodInstance, .varInstance]
+        }
+
         public var isAccessorKind: Bool {
             rawValue.hasPrefix("function.accessor")
         }

Sources/SourceGraph/Mutators/ExternalOverrideRetainer.swift

@@ -0,0 +1,46 @@
+import Configuration
+import Foundation
+import Shared
+
+/// Retains instance functions/vars that override external declarations.
+///
+/// It's not possible to determine if a declaration that overrides an external declaration is used, as the
+/// external implementation may call the overridden declaration.
+final class ExternalOverrideRetainer: SourceGraphMutator {
+    private let graph: SourceGraph
+    private let isSwift6FixEnabled: Bool
+
+    required init(graph: SourceGraph, configuration _: Configuration, swiftVersion: SwiftVersion) {
+        self.graph = graph
+        isSwift6FixEnabled = swiftVersion.version.isVersion(greaterThanOrEqualTo: "6.0")
+    }
+
+    func mutate() {
+        for decl in graph.declarations(ofKinds: Declaration.Kind.overrideKinds) {
+            guard decl.isOverride else { continue }
+
+            var didIdentifyRelatedRef = false
+
+            for relatedRef in decl.related {
+                if relatedRef.kind == decl.kind,
+                   relatedRef.name == decl.name,
+                   relatedRef.location == decl.location
+                {
+                    didIdentifyRelatedRef = true
+
+                    if graph.explicitDeclaration(withUsr: relatedRef.usr) == nil {
+                        // The related decl is external.
+                        graph.markRetained(decl)
+                    }
+
+                    break
+                }
+            }
+
+            // https://github.com/swiftlang/swift/issues/76628
+            if !didIdentifyRelatedRef, isSwift6FixEnabled {
+                graph.markRetained(decl)
+            }
+        }
+    }
+}

Sources/SourceGraph/SourceGraphMutatorRunner.swift

@@ -42,6 +42,7 @@ public final class SourceGraphMutatorRunner {
         ResultBuilderRetainer.self,
         CapitalSelfFunctionCallRetainer.self,
         CodablePropertyRetainer.self,
+        ExternalOverrideRetainer.self,
 
         AncestralReferenceEliminator.self,
         AssignOnlyPropertyReferenceEliminator.self,