Merge pull request #73878 from hborla/invalid-accessor-macro

[Macros] Don't allow macros to add accessors to `let` variables.

Author: hborla

include/swift/AST/DiagnosticsSema.def

@@ -7635,6 +7635,9 @@ ERROR(external_macro_arg_not_type_name,none,
 ERROR(invalid_decl_in_macro_expansion,none,
       "macro expansion cannot introduce %0",
       (DescriptiveDeclKind))
+ERROR(let_accessor_expansion,none,
+      "cannot expand accessors on variable declared with 'let'",
+      ())
 ERROR(invalid_main_type_in_macro_expansion,none,
       "macro expansion cannot introduce '@main' type",
       ())

lib/Sema/TypeCheckMacros.cpp

@@ -739,12 +739,12 @@ static void validateMacroExpansion(SourceFile *expansionBuffer,
   };
 
   for (auto item : expansionBuffer->getTopLevelItems()) {
+    auto &ctx = expansionBuffer->getASTContext();
     auto *decl = item.dyn_cast<Decl *>();
     if (!decl) {
       if (role != MacroRole::CodeItem &&
           role != MacroRole::Preamble &&
           role != MacroRole::Body) {
-        auto &ctx = expansionBuffer->getASTContext();
         ctx.Diags.diagnose(item.getStartLoc(),
                            diag::expected_macro_expansion_decls);
       }
@@ -753,8 +753,18 @@ static void validateMacroExpansion(SourceFile *expansionBuffer,
     }
 
     // Certain macro roles can generate special declarations.
-    if ((isa<AccessorDecl>(decl) && role == MacroRole::Accessor) ||
-        (isa<ExtensionDecl>(decl) && role == MacroRole::Conformance)) {
+    if (isa<AccessorDecl>(decl) && role == MacroRole::Accessor) {
+      auto *var = dyn_cast<VarDecl>(attachedTo);
+      if (var && var->isLet()) {
+        ctx.Diags.diagnose(var->getLoc(),
+                           diag::let_accessor_expansion)
+          .warnUntilSwiftVersion(6);
+      }
+
+      continue;
+    }
+
+    if (isa<ExtensionDecl>(decl) && role == MacroRole::Conformance) {
       continue;
     }
 

test/Macros/Inputs/syntax_macro_definitions.swift

@@ -2482,3 +2482,12 @@ public struct AllLexicalContextsMacro: DeclarationMacro {
   }
 }
 
+public struct AddGetterMacro: AccessorMacro {
+  public static func expansion(
+    of node: AttributeSyntax,
+    providingAccessorsOf declaration: some DeclSyntaxProtocol,
+    in context: some MacroExpansionContext
+  ) throws -> [AccessorDeclSyntax] {
+    return ["get { 0 }"]
+  }
+}

test/Macros/accessor_macros.swift

@@ -163,3 +163,14 @@ struct MultipleVars {
   // expected-error@-1 2{{accessor macro 'AddWillSet()' can only apply to a single variable}}
 }
 #endif
+
+@attached(accessor)
+macro addGetterMacro() =
+    #externalMacro(module: "MacroDefinition", type: "AddGetterMacro")
+
+#if TEST_DIAGNOSTICS
+struct S {
+  @addGetterMacro let x: Int
+  // expected-warning@-1 {{cannot expand accessors on variable declared with 'let'; this is an error in the Swift 6 language mode}}
+}
+#endif