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`.

(cherry picked from commit 46c02e9)

Author: ahoppen

Sources/SwiftSyntaxMacroExpansion/MacroSystem.swift

@@ -267,7 +267,8 @@ private func expandAccessorMacroWithoutExistingAccessors(
       conformanceList: nil,
       in: context,
       indentationWidth: indentationWidth
-    )
+    ),
+    !expanded.isEmpty
   else {
     return nil
   }

Tests/SwiftSyntaxMacroExpansionTest/AccessorMacroTests.swift

@@ -18,7 +18,9 @@
 // macros are invoked.                                                      //
 //==========================================================================//
 
+import SwiftDiagnostics
 import SwiftSyntax
+import SwiftSyntaxMacroExpansion
 import SwiftSyntaxMacros
 import SwiftSyntaxMacrosTestSupport
 import XCTest
@@ -261,4 +263,61 @@ final class AccessorMacroTests: XCTestCase {
       indentationWidth: indentationWidth
     )
   }
+
+  func testEmpty() {
+    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 testEmitErrorFromMacro() {
+    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]
+    )
+  }
 }

Tests/SwiftSyntaxMacroExpansionTest/CodeItemMacroTests.swift

@@ -133,4 +133,23 @@ final class CodeItemMacroTests: XCTestCase {
       indentationWidth: indentationWidth
     )
   }
+
+  func testEmpty() {
+    struct TestMacro: CodeItemMacro {
+      static func expansion(
+        of node: some FreestandingMacroExpansionSyntax,
+        in context: some MacroExpansionContext
+      ) throws -> [CodeBlockItemSyntax] {
+        return []
+      }
+    }
+
+    assertMacroExpansion(
+      "#test",
+      expandedSource: "",
+      macros: [
+        "test": TestMacro.self
+      ]
+    )
+  }
 }

Tests/SwiftSyntaxMacroExpansionTest/DeclarationMacroTests.swift

@@ -389,4 +389,22 @@ final class DeclarationMacroTests: XCTestCase {
     )
   }
 
+  func testEmpty() {
+    struct TestMacro: DeclarationMacro {
+      static func expansion(
+        of node: some FreestandingMacroExpansionSyntax,
+        in context: some MacroExpansionContext
+      ) throws -> [DeclSyntax] {
+        return []
+      }
+    }
+
+    assertMacroExpansion(
+      "#test",
+      expandedSource: "",
+      macros: [
+        "test": TestMacro.self
+      ]
+    )
+  }
 }

Tests/SwiftSyntaxMacroExpansionTest/ExtensionMacroTests.swift

@@ -106,4 +106,26 @@ final class ExtensionMacroTests: XCTestCase {
       indentationWidth: indentationWidth
     )
   }
+
+  func testEmpty() {
+    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
+      ]
+    )
+  }
 }

Tests/SwiftSyntaxMacroExpansionTest/MemberAttributeMacroTests.swift

@@ -354,4 +354,48 @@ final class MemberAttributeMacroTests: XCTestCase {
     )
   }
 
+  func testEmpty() {
+    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
+      ]
+    )
+  }
 }

Tests/SwiftSyntaxMacroExpansionTest/MemberMacroTests.swift

@@ -255,4 +255,48 @@ final class MemberMacroTests: XCTestCase {
     )
   }
 
+  func testEmpty() {
+    struct TestMacro: MemberMacro {
+      static func expansion(
+        of node: AttributeSyntax,
+        providingMembersOf declaration: some DeclGroupSyntax,
+        conformingTo protocols: [TypeSyntax],
+        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
+      ]
+    )
+  }
 }

Tests/SwiftSyntaxMacroExpansionTest/PeerMacroTests.swift

@@ -173,4 +173,24 @@ final class PeerMacroTests: XCTestCase {
       macros: ["Test": TestMacro.self]
     )
   }
+
+  func testEmpty() {
+    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
+      ]
+    )
+  }
 }