Diagnose macros that have multiple freestanding macro roles.

Per SE-0397, a macro may only have a single freestanding macro role,
otherwise we would have an ambiguity in how a particular freestanding
macro would be expanded. Produce an error on such macro declarations.

Fixes rdar://110178899.

Author: DougGregor

include/swift/AST/DiagnosticsSema.def

@@ -7109,6 +7109,8 @@ NOTE(macro_remove_result_type,none,
       ())
 NOTE(macro_make_freestanding_expression,none,
       "make this macro a freestanding expression macro", ())
+ERROR(macro_multiple_freestanding_roles,none,
+      "macro can only have a single freestanding role", ())
 ERROR(macro_expansion_missing_pound,none,
       "expansion of macro %0 requires leading '#'", (DeclName))
 ERROR(macro_expansion_missing_arguments,none,

lib/Sema/TypeCheckDeclPrimary.cpp

@@ -2017,6 +2017,17 @@ class DeclChecker : public DeclVisitor<DeclChecker> {
     llvm_unreachable("should always be type-checked already");
   }
 
+  /// Determine the number of bits set.
+  static unsigned numBitsSet(uint64_t value) {
+    unsigned count = 0;
+    for (uint64_t i : range(0, 63)) {
+      if (value & (uint64_t(1) << i))
+        ++count;
+    }
+
+    return count;
+  }
+
   void visitMacroDecl(MacroDecl *MD) {
     TypeChecker::checkDeclAttributes(MD);
     checkAccessControl(MD);
@@ -2077,6 +2088,13 @@ class DeclChecker : public DeclVisitor<DeclChecker> {
           .fixItRemove(SourceRange(MD->arrowLoc, resultTypeRepr->getEndLoc()));
       }
     }
+
+    // A macro can only have a single freestanding macro role.
+    MacroRoles freestandingRolesInhabited =
+        MD->getMacroRoles() & getFreestandingMacroRoles();
+    if (numBitsSet(freestandingRolesInhabited.toRaw()) > 1) {
+      MD->diagnose(diag::macro_multiple_freestanding_roles);
+    }
   }
 
   void visitMacroExpansionDecl(MacroExpansionDecl *MED) {

test/Macros/macros_diagnostics.swift

@@ -211,3 +211,10 @@ struct SomeType {
 
 @freestanding(declaration) macro nonExpressionReturnsVoid<T>(_: T) -> Void = #externalMacro(module: "A", type: "B")
 // expected-warning@-1{{external macro implementation type}}
+
+
+@freestanding(expression)
+@freestanding(declaration)
+macro multipleFreestandingRoles<T>(_: T) -> Void = #externalMacro(module: "A", type: "B")
+// expected-warning@-1{{external macro implementation type}}
+// expected-error@-2{{macro can only have a single freestanding role}}

test/Macros/parsing.swift

@@ -35,6 +35,7 @@ protocol Q { associatedtype Assoc }
 @freestanding(expression) @freestanding(declaration, names: named(Foo)) @attached(accessor)
 macro m10(_: String) = #externalMacro(module: "A", type: "M4")
 // expected-warning@-1{{external macro implementation type 'A.M4' could not be found for macro 'm10'}}
+// expected-error@-2{{macro can only have a single freestanding role}}
 
 @attached(
   accessor,