Retain @dynamicMemberLookup subscript functions. Closes #684

CHANGELOG.md

@@ -10,7 +10,7 @@
 
 ##### Bug Fixes
 
-- None.
+- Subscript functions required by `@dynamicMemberLookup` are now retained.
 
 ## 2.17.1 (2023-12-04)
 

Sources/PeripheryKit/SourceGraph/Mutators/DynamicMemberLookupReferenceBuilder.swift

@@ -0,0 +1,18 @@
+import Foundation
+import Shared
+
+final class DynamicMemberLookupReferenceBuilder: SourceGraphMutator {
+    private let graph: SourceGraph
+
+    required init(graph: SourceGraph, configuration: Configuration) {
+        self.graph = graph
+    }
+
+    func mutate() throws {
+        for decl in graph.declarations(ofKind: .functionSubscript) {
+            if decl.name == "subscript(dynamicMember:)", decl.parent?.attributes.contains("dynamicMemberLookup") ?? false {
+                graph.markRetained(decl)
+            }
+        }
+    }
+}

Sources/PeripheryKit/SourceGraph/SourceGraphMutatorRunner.swift

@@ -30,6 +30,7 @@ public final class SourceGraphMutatorRunner {
         DefaultConstructorReferenceBuilder.self,
         ComplexPropertyAccessorReferenceBuilder.self,
         EnumCaseReferenceBuilder.self,
+        DynamicMemberLookupReferenceBuilder.self,
 
         UnusedParameterRetainer.self,
         AssetReferenceRetainer.self,

Tests/Fixtures/RetentionFixtures/testRetainsDynamicMemberLookupSubscript.swift

@@ -0,0 +1,10 @@
+@dynamicMemberLookup
+public struct FixtureStruct7 {
+    subscript(dynamicMember member: String) -> String {
+        ""
+    }
+
+    subscript(other: String) -> String {
+        ""
+    }
+}

Tests/PeripheryTests/RetentionTest.swift

@@ -1032,6 +1032,15 @@ final class RetentionTest: FixtureSourceGraphTestCase {
         }
     }
 
+    func testRetainsDynamicMemberLookupSubscript() {
+        analyze(retainPublic: true) {
+            assertReferenced(.struct("FixtureStruct7")) {
+                self.assertReferenced(.functionSubscript("subscript(dynamicMember:)"))
+                self.assertNotReferenced(.functionSubscript("subscript(_:)"))
+            }
+        }
+    }
+
     // MARK: - Assign-only properties
 
     func testSimplePropertyAssignedButNeverRead() {

Tests/Shared/DeclarationDescription.swift

@@ -70,6 +70,10 @@ struct DeclarationDescription: CustomStringConvertible {
         self.init(kind: .functionDestructor, name: name, line: line)
     }
 
+    static func functionSubscript(_ name: String, line: Int? = nil) -> Self {
+        self.init(kind: .functionSubscript, name: name, line: line)
+    }
+
     static func varStatic(_ name: String, line: Int? = nil) -> Self {
         self.init(kind: .varStatic, name: name, line: line)
     }