[Macro] Diagnose macros declared in nested scopes

Author: DougGregor

include/swift/AST/DiagnosticsSema.def

@@ -6767,6 +6767,8 @@ WARNING(macro_type_access_warn,none,
         "because its %select{parameter|result type}3 uses "
         "%select{a private|a fileprivate|an internal|%error|%error}2 type",
         (bool, AccessLevel, AccessLevel, bool))
+ERROR(macro_in_nested,none,
+      "macro %0 can only be declared at file scope", (DeclName))
 
 //------------------------------------------------------------------------------
 // MARK: Move Only Errors

lib/Sema/TypeCheckDeclPrimary.cpp

@@ -1985,6 +1985,9 @@ class DeclChecker : public DeclVisitor<DeclChecker> {
   void visitMacroDecl(MacroDecl *MD) {
     TypeChecker::checkDeclAttributes(MD);
     checkAccessControl(MD);
+
+    if (!MD->getDeclContext()->isModuleScopeContext())
+      MD->diagnose(diag::macro_in_nested, MD->getName());
   }
 
   void visitMacroExpansionDecl(MacroExpansionDecl *MED) {

test/Macros/macros_diagnostics.swift

@@ -25,6 +25,11 @@ macro m3(_: Int) -> Float = A.B
 macro m4(_: Int) -> Int = A.B // expected-note{{'m4' previously declared here}}
 macro m4(_: Int) -> Int = A.B // expected-error{{invalid redeclaration of 'm4'}}
 
+struct ZZZ {
+  macro m5: Int = A.B
+  // expected-error@-1{{macro 'm5' can only be declared at file scope}}
+}
+
 func test(a: Int, b: Int) {
   // FIXME: Bad diagnostic.
   let s = #stringify<Int, Int>(a + b) // expected-error{{type of expression is ambiguous without more context}}