Disallow attaching peer macro to variable with multiple bindings in assertMacroExpansion

The compiler currently invokes `expansion` twice with the exact same syntax node (rdar://114996981), which causes the same member to be generated twice, effectively disallowing the application of peer macro on variables with multiple bindings. Disallow them in `assertMacroExpansion` with a nice error message.

Author: ahoppen

Sources/SwiftSyntaxMacroExpansion/MacroSystem.swift

@@ -187,6 +187,10 @@ private func expandPeerMacroMember(
   in context: some MacroExpansionContext,
   indentationWidth: Trivia
 ) throws -> MemberBlockItemListSyntax? {
+  if let variable = attachedTo.as(VariableDeclSyntax.self), variable.bindings.count > 1 {
+    throw MacroApplicationError.peerMacroOnVariableWithMultipleBindings
+  }
+
   guard
     let expanded = expandAttachedMacro(
       definition: definition,
@@ -215,6 +219,10 @@ private func expandPeerMacroCodeItem(
   in context: some MacroExpansionContext,
   indentationWidth: Trivia
 ) throws -> CodeBlockItemListSyntax? {
+  if let variable = attachedTo.as(VariableDeclSyntax.self), variable.bindings.count > 1 {
+    throw MacroApplicationError.peerMacroOnVariableWithMultipleBindings
+  }
+
   guard
     let expanded = expandAttachedMacro(
       definition: definition,
@@ -424,6 +432,7 @@ let diagnosticDomain: String = "SwiftSyntaxMacroExpansion"
 
 private enum MacroApplicationError: DiagnosticMessage, Error {
   case accessorMacroOnVariableWithMultipleBindings
+  case peerMacroOnVariableWithMultipleBindings
   case malformedAccessor
 
   var diagnosticID: MessageID {
@@ -435,15 +444,12 @@ private enum MacroApplicationError: DiagnosticMessage, Error {
   var message: String {
     switch self {
     case .accessorMacroOnVariableWithMultipleBindings:
-      return """
-        swift-syntax applies macros syntactically and there is no way to represent a variable \
-        declaration with multiple bindings that have accessors syntactically. \
-        While the compiler allows this expansion, swift-syntax cannot represent it and thus \
-        disallows it.
-        """
+      return "accessor macro can only be applied to a single variable"
+    case .peerMacroOnVariableWithMultipleBindings:
+      return "peer macro can only be applied to a single variable"
     case .malformedAccessor:
       return """
-        Macro returned a malformed accessor. Accessors should start with an introducer like 'get' or 'set'.
+        macro returned a malformed accessor. Accessors should start with an introducer like 'get' or 'set'.
         """
     }
   }
@@ -606,11 +612,12 @@ private class MacroApplication<Context: MacroExpansionContext>: SyntaxRewriter {
   }
 
   override func visit(_ node: VariableDeclSyntax) -> DeclSyntax {
+    var node = super.visit(node).cast(VariableDeclSyntax.self)
+
     guard !macroAttributes(attachedTo: DeclSyntax(node), ofType: AccessorMacro.Type.self).isEmpty else {
-      return super.visit(node).cast(DeclSyntax.self)
+      return DeclSyntax(node)
     }
 
-    var node = super.visit(node).cast(VariableDeclSyntax.self)
     guard node.bindings.count == 1, let binding = node.bindings.first else {
       context.addDiagnostics(from: MacroApplicationError.accessorMacroOnVariableWithMultipleBindings, node: node)
       return DeclSyntax(node)

Tests/SwiftSyntaxMacroExpansionTest/AccessorMacroTests.swift

@@ -229,7 +229,7 @@ final class AccessorMacroTests: XCTestCase {
       diagnostics: [
         DiagnosticSpec(
           message:
-            "swift-syntax applies macros syntactically and there is no way to represent a variable declaration with multiple bindings that have accessors syntactically. While the compiler allows this expansion, swift-syntax cannot represent it and thus disallows it.",
+            "accessor macro can only be applied to a single variable",
           line: 1,
           column: 1,
           severity: .error

Tests/SwiftSyntaxMacroExpansionTest/PeerMacroTests.swift

@@ -130,4 +130,47 @@ final class PeerMacroTests: XCTestCase {
     )
   }
 
+  func testPeerMacroOnVariableWithMultipleBindings() {
+    struct TestMacro: PeerMacro {
+      static func expansion(
+        of node: AttributeSyntax,
+        providingPeersOf declaration: some DeclSyntaxProtocol,
+        in context: some MacroExpansionContext
+      ) throws -> [DeclSyntax] {
+        return ["var baz: Int = 0"]
+      }
+    }
+
+    assertMacroExpansion(
+      """
+      @Test
+      let a = 17, b = 12
+      """,
+      expandedSource: """
+        let a = 17, b = 12
+        """,
+      diagnostics: [
+        DiagnosticSpec(message: "peer macro can only be applied to a single variable", line: 1, column: 1)
+      ],
+      macros: ["Test": TestMacro.self]
+    )
+
+    assertMacroExpansion(
+      """
+      struct Foo {
+        @Test
+        let a = 17, b = 12
+      }
+      """,
+      expandedSource: """
+        struct Foo {
+          let a = 17, b = 12
+        }
+        """,
+      diagnostics: [
+        DiagnosticSpec(message: "peer macro can only be applied to a single variable", line: 2, column: 3)
+      ],
+      macros: ["Test": TestMacro.self]
+    )
+  }
 }