Generate Equatable conformance for empty FlagContainers

Sources/VexilMacros/FlagContainerMacro.swift

@@ -18,7 +18,7 @@
 
 public enum FlagContainerMacro {}
 
 extension FlagContainerMacro: MemberMacro {
 
     public static func expansion(
         of node: AttributeSyntax,
@@ -78,11 +78,6 @@
             shouldGenerateConformance.equatable = false
         }
 
-        // We also can't generate Equatable conformance if there is no variables to generate them
-        if shouldGenerateConformance.equatable, declaration.memberBlock.variables.isEmpty {
-            shouldGenerateConformance.equatable = false
-        }
-
         // Check that conformance doesn't already exist, or that we are inside a unit test.
         // The latter is a workaround for https://github.com/apple/swift-syntax/issues/2031
         guard shouldGenerateConformance.flagContainer else {
@@ -159,8 +154,8 @@
                     inheritanceClause: .init(inheritedTypes: [ .init(type: TypeSyntax(stringLiteral: "Equatable")) ])
                 ) {
                     var variables = declaration.memberBlock.storedVariables
-                    if variables.isEmpty == false {
-                        try FunctionDeclSyntax("func ==(lhs: \(type), rhs: \(type)) -> Bool") {
+                    try FunctionDeclSyntax("func ==(lhs: \(type), rhs: \(type)) -> Bool") {
+                        if variables.isEmpty == false {
                             if let lastBinding = variables.removeLast().bindings.first?.pattern {
                                 for variable in variables {
                                     if let binding = variable.bindings.first?.pattern {
@@ -172,9 +167,14 @@
                                 }
                                 ExprSyntax("lhs.\(lastBinding.trimmed) == rhs.\(lastBinding.trimmed)")
                             }
+
+                            // If there are no stored properties just return `true`. This matches the synthesised Equatable
+                            // behaviour described in https://github.com/swiftlang/swift-evolution/blob/main/proposals/0185-synthesize-equatable-hashable.md#implementation-details
+                        } else {
+                            "true"
                         }
-                        .with(\.modifiers, Array(scopes) + [ DeclModifierSyntax(name: .keyword(.static)) ])
                     }
+                    .with(\.modifiers, Array(scopes) + [ DeclModifierSyntax(name: .keyword(.static)) ])
                 },
             ]
         }

Tests/VexilMacroTests/EquatableFlagContainerMacroTests.swift

@@ -20,7 +20,7 @@ import XCTest
 
 final class EquatableFlagContainerMacroTests: XCTestCase {
 
-    func testDoesntGenerateWhenEmpty() throws {
+    func testGeneratesWhenEmpty() throws {
         assertMacroExpansion(
             """
             @FlagContainer
@@ -50,6 +50,12 @@ final class EquatableFlagContainerMacroTests: XCTestCase {
                     [:]
                 }
             }
+
+            extension TestFlags: Equatable {
+                static func ==(lhs: TestFlags, rhs: TestFlags) -> Bool {
+                    true
+                }
+            }
             """,
             macros: [
                 "FlagContainer": FlagContainerMacro.self,

Tests/VexilMacroTests/FlagContainerMacroTests.swift

@@ -50,6 +50,12 @@ final class FlagContainerMacroTests: XCTestCase {
                     [:]
                 }
             }
+
+            extension TestFlags: Equatable {
+                static func ==(lhs: TestFlags, rhs: TestFlags) -> Bool {
+                    true
+                }
+            }
             """,
             macros: [
                 "FlagContainer": FlagContainerMacro.self,
@@ -87,6 +93,12 @@ final class FlagContainerMacroTests: XCTestCase {
                     [:]
                 }
             }
+
+            extension TestFlags: Equatable {
+                public static func ==(lhs: TestFlags, rhs: TestFlags) -> Bool {
+                    true
+                }
+            }
             """,
             macros: [
                 "FlagContainer": FlagContainerMacro.self,
@@ -124,6 +136,12 @@ final class FlagContainerMacroTests: XCTestCase {
                     [:]
                 }
             }
+
+            extension TestFlags: Equatable {
+                static func ==(lhs: TestFlags, rhs: TestFlags) -> Bool {
+                    true
+                }
+            }
             """,
             macros: [
                 "FlagContainer": FlagContainerMacro.self,

Tests/VexilTests/EquatableTests.swift

@@ -176,4 +176,13 @@ private struct DoubleSubgroupFlags {
     @Flag("Third level test flag")
     var thirdLevelFlag = false
 
+    @FlagGroup("Empty flags")
+    var empty: EmptyFlags
+
+}
+
+// Support for empty Flag Containers
+@FlagContainer
+private struct EmptyFlags {
+    // Intentionally left blank
 }