Diagnose @safe @unsafe when used together

Fixes rdar://147943857.

Author: DougGregor

include/swift/AST/DiagnosticsSema.def

@@ -8233,7 +8233,7 @@ NOTE(sending_function_result_with_sending_param_note, none,
      ())
 
 //------------------------------------------------------------------------------
-// MARK: Strict Safety Diagnostics
+// MARK: Strict Memory Safety Diagnostics
 //------------------------------------------------------------------------------
 NOTE(note_reference_to_unsafe_decl,none,
       "%select{reference|call}0 to unsafe %kind1",
@@ -8272,6 +8272,9 @@ NOTE(decl_storage_mark_safe,none,
       "add '@safe' if this type encapsulates the unsafe storage in "
       "a safe interface", ())
 
+ERROR(safe_and_unsafe_attr,none,
+      "%kindbase0 cannot be both @safe and @unsafe", (const Decl *))
+
 GROUPED_WARNING(unsafe_superclass,StrictMemorySafety,none,
      "%kindbase0 has superclass involving unsafe type %1", 
      (const ValueDecl *, Type))

lib/Sema/TypeCheckAttr.cpp

@@ -192,7 +192,6 @@ class AttributeChecker : public AttributeVisitor<AttributeChecker> {
   IGNORED_ATTR(AllowFeatureSuppression)
   IGNORED_ATTR(PreInverseGenerics)
   IGNORED_ATTR(Safe)
-  IGNORED_ATTR(Unsafe)
 #undef IGNORED_ATTR
 
   void visitABIAttr(ABIAttr *attr) {
@@ -445,6 +444,7 @@ class AttributeChecker : public AttributeVisitor<AttributeChecker> {
   void visitLifetimeAttr(LifetimeAttr *attr);
   void visitAddressableSelfAttr(AddressableSelfAttr *attr);
   void visitAddressableForDependenciesAttr(AddressableForDependenciesAttr *attr);
+  void visitUnsafeAttr(UnsafeAttr *attr);
 };
 
 } // end anonymous namespace
@@ -8140,6 +8140,14 @@ AttributeChecker::visitAddressableForDependenciesAttr(
   }
 }
 
+void AttributeChecker::visitUnsafeAttr(UnsafeAttr *attr) {
+  if (auto safeAttr = D->getAttrs().getAttribute<SafeAttr>()) {
+    D->diagnose(diag::safe_and_unsafe_attr, D)
+      .highlight(attr->getRange())
+      .highlight(safeAttr->getRange());
+  }
+}
+
 namespace {
 
 class ClosureAttributeChecker

test/Unsafe/safe.swift

@@ -268,3 +268,6 @@ struct UnsafeWrapTest {
     }
   }
 }
+
+@safe @unsafe
+struct ConfusedStruct { } // expected-error{{struct 'ConfusedStruct' cannot be both @safe and @unsafe}}