Don’t add any accessors if accessor macro returned an empty array

If an accessor macro returns an empty array, MacroSystem was failing with internal error messages because it tried ot parse an `AccessorBlockSyntax` from an empty string.

To fix this, check if the expanded source is empty before trying to parse the `AccessorBlockSyntax`.

Author: ahoppen

Sources/SwiftSyntaxMacroExpansion/MacroSystem.swift

@@ -259,7 +259,8 @@ private func expandAccessorMacroWithoutExistingAccessors(
       conformanceList: nil,
       in: context,
       indentationWidth: indentationWidth
-    )
+    ),
+    !expanded.isEmpty
   else {
     return nil
   }

Tests/SwiftSyntaxMacroExpansionTest/MacroSystemTests.swift

@@ -2088,4 +2088,230 @@ final class MacroSystemTests: XCTestCase {
       macros: ["Test": DiagnoseFirstArgument.self]
     )
   }
+
+  func testEmptyAccessorMacro() {
+    struct TestMacro: AccessorMacro {
+      static func expansion(
+        of node: AttributeSyntax,
+        providingAccessorsOf declaration: some DeclSyntaxProtocol,
+        in context: some MacroExpansionContext
+      ) throws -> [AccessorDeclSyntax] {
+        return []
+      }
+    }
+
+    // The compiler will reject this with
+    // 'Expansion of macro 'Test()' did not produce a non-observing accessor'
+    // We consider this a semantic error because swift-syntax doesn't have
+    // knowledge about which accessors are observing and which ones aren't.
+    assertMacroExpansion(
+      "@Test var x: Int",
+      expandedSource: "var x: Int",
+      macros: ["Test": TestMacro.self]
+    )
+
+    assertMacroExpansion(
+      "@Test var x: Int { 1 }",
+      expandedSource: "var x: Int { 1 }",
+      macros: ["Test": TestMacro.self]
+    )
+  }
+
+  func testEmitErrorFromAccessorMacro() {
+    struct TestMacro: AccessorMacro {
+      static func expansion(
+        of node: AttributeSyntax,
+        providingAccessorsOf declaration: some DeclSyntaxProtocol,
+        in context: some MacroExpansionContext
+      ) throws -> [AccessorDeclSyntax] {
+        context.diagnose(Diagnostic(node: node, message: MacroExpansionErrorMessage("test")))
+        return []
+      }
+    }
+
+    assertMacroExpansion(
+      "@Test var x: Int",
+      expandedSource: "var x: Int",
+      diagnostics: [
+        DiagnosticSpec(message: "test", line: 1, column: 1)
+      ],
+      macros: ["Test": TestMacro.self]
+    )
+
+    assertMacroExpansion(
+      "@Test var x: Int { 1 }",
+      expandedSource: "var x: Int { 1 }",
+      diagnostics: [DiagnosticSpec(message: "test", line: 1, column: 1)],
+      macros: ["Test": TestMacro.self]
+    )
+  }
+
+  func testEmptyCodeItemMacro() {
+    struct TestMacro: CodeItemMacro {
+      static func expansion(
+        of node: some FreestandingMacroExpansionSyntax,
+        in context: some MacroExpansionContext
+      ) throws -> [CodeBlockItemSyntax] {
+        return []
+      }
+    }
+
+    assertMacroExpansion(
+      "#test",
+      expandedSource: "",
+      macros: [
+        "test": TestMacro.self
+      ]
+    )
+  }
+
+  func testEmptyDeclarationMacro() {
+    struct TestMacro: DeclarationMacro {
+      static func expansion(
+        of node: some FreestandingMacroExpansionSyntax,
+        in context: some MacroExpansionContext
+      ) throws -> [DeclSyntax] {
+        return []
+      }
+    }
+
+    assertMacroExpansion(
+      "#test",
+      expandedSource: "",
+      macros: [
+        "test": TestMacro.self
+      ]
+    )
+  }
+
+  func testEmptyExtensionMacro() {
+    struct TestMacro: ExtensionMacro {
+      static func expansion(
+        of node: AttributeSyntax,
+        attachedTo declaration: some DeclGroupSyntax,
+        providingExtensionsOf type: some TypeSyntaxProtocol,
+        conformingTo protocols: [TypeSyntax],
+        in context: some MacroExpansionContext
+      ) throws -> [ExtensionDeclSyntax] {
+        return []
+      }
+    }
+
+    assertMacroExpansion(
+      "@Test struct Foo {}",
+      expandedSource: "struct Foo {}",
+      macros: [
+        "Test": TestMacro.self
+      ]
+    )
+  }
+
+  func testEmptyMemberAttributeMacro() {
+    struct TestMacro: MemberAttributeMacro {
+      static func expansion(
+        of node: AttributeSyntax,
+        attachedTo declaration: some DeclGroupSyntax,
+        providingAttributesFor member: some DeclSyntaxProtocol,
+        in context: some MacroExpansionContext
+      ) throws -> [AttributeSyntax] {
+        return []
+      }
+    }
+
+    assertMacroExpansion(
+      """
+      @Test
+      struct Foo {
+        var x: Int
+      }
+      """,
+      expandedSource: """
+        struct Foo {
+          var x: Int
+        }
+        """,
+      macros: [
+        "Test": TestMacro.self
+      ]
+    )
+
+    assertMacroExpansion(
+      """
+      @Test
+      struct Foo {
+      }
+      """,
+      expandedSource: """
+        struct Foo {
+        }
+        """,
+      macros: [
+        "Test": TestMacro.self
+      ]
+    )
+  }
+
+  func testEmptyMemberMacro() {
+    struct TestMacro: MemberMacro {
+      static func expansion(
+        of node: AttributeSyntax,
+        providingMembersOf declaration: some DeclGroupSyntax,
+        in context: some MacroExpansionContext
+      ) throws -> [DeclSyntax] {
+        return []
+      }
+    }
+
+    assertMacroExpansion(
+      """
+      @Test
+      struct Foo {
+        var x: Int
+      }
+      """,
+      expandedSource: """
+        struct Foo {
+          var x: Int
+        }
+        """,
+      macros: [
+        "Test": TestMacro.self
+      ]
+    )
+
+    assertMacroExpansion(
+      """
+      @Test
+      struct Foo {
+      }
+      """,
+      expandedSource: """
+        struct Foo {
+        }
+        """,
+      macros: [
+        "Test": TestMacro.self
+      ]
+    )
+  }
+
+  func testEmptyPeerMacro() {
+    struct TestMacro: PeerMacro {
+      static func expansion(
+        of node: AttributeSyntax,
+        providingPeersOf declaration: some DeclSyntaxProtocol,
+        in context: some MacroExpansionContext
+      ) throws -> [DeclSyntax] {
+        return []
+      }
+    }
+
+    assertMacroExpansion(
+      "@Test var x: Int",
+      expandedSource: "var x: Int",
+      macros: [
+        "Test": TestMacro.self
+      ]
+    )
+  }
 }