[cxx-interop] Make sure C++ span is imported as @unsafe

The C++ span should be a non-escapable type but is imported as escapable
for backward compatibility reason. This is inherently unsafe, so make
sure std::span is imported as such. In the future, we plan to generate
safe overloads using Swift's Span and that will be the preferred way of
using the API.

Author: Gabor Horvath

lib/ClangImporter/ClangImporter.cpp

@@ -5096,8 +5096,6 @@ static const llvm::StringMap<std::vector<int>> STLConditionalEscapableParams{
     {"multimap", {0, 1}},
     {"flat_multimap", {0, 1}},
     {"unordered_multimap", {0, 1}},
-    {"span", {0}},   // TODO: remove when span is non-escapable by default
-    {"mdspan", {0}}, // TODO: remove when mdspan is non-escapable by default
 };
 
 static std::set<StringRef>

lib/ClangImporter/ImportDecl.cpp

@@ -1516,6 +1516,9 @@ namespace {
                                       /*genericparams*/nullptr, DC);
 
       Result->setUnderlyingType(SwiftType);
+      if (SwiftType->isUnsafe())
+        Result->getAttrs().add(new (Impl.SwiftContext)
+                                   UnsafeAttr(/*implicit=*/true));
 
       // Make Objective-C's 'id' unavailable.
       if (Impl.SwiftContext.LangOpts.EnableObjCInterop && isObjCId(Decl)) {

lib/Sema/TypeCheckDecl.cpp

@@ -34,6 +34,7 @@
 #include "swift/AST/Attr.h"
 #include "swift/AST/ClangModuleLoader.h"
 #include "swift/AST/ConformanceLookup.h"
+#include "swift/AST/Decl.h"
 #include "swift/AST/DiagnosticsParse.h"
 #include "swift/AST/ExistentialLayout.h"
 #include "swift/AST/Expr.h"
@@ -47,6 +48,7 @@
 #include "swift/AST/PropertyWrappers.h"
 #include "swift/AST/ProtocolConformance.h"
 #include "swift/AST/SourceFile.h"
+#include "swift/AST/Type.h"
 #include "swift/AST/TypeCheckRequests.h"
 #include "swift/AST/TypeWalker.h"
 #include "swift/Basic/Assertions.h"

test/Interop/Cxx/class/safe-interop-mode.swift

@@ -1,12 +1,13 @@
 
 // RUN: rm -rf %t
 // RUN: split-file %s %t
-// RUN: %target-swift-frontend -typecheck -verify -I %swift_src_root/lib/ClangImporter/SwiftBridging  -I %t/Inputs  %t/test.swift -enable-experimental-feature AllowUnsafeAttribute -enable-experimental-feature WarnUnsafe  -enable-experimental-feature SafeInterop -cxx-interoperability-mode=default -diagnostic-style llvm 2>&1
+// RUN: %target-swift-frontend -typecheck -verify -I %swift_src_root/lib/ClangImporter/SwiftBridging -Xcc -std=c++20 -I %t/Inputs  %t/test.swift -enable-experimental-feature AllowUnsafeAttribute -enable-experimental-feature WarnUnsafe  -enable-experimental-feature SafeInterop -enable-experimental-feature LifetimeDependence -cxx-interoperability-mode=default -diagnostic-style llvm 2>&1
 
 // REQUIRES: objc_interop
 // REQUIRES: swift_feature_AllowUnsafeAttribute
 // REQUIRES: swift_feature_SafeInterop
 // REQUIRES: swift_feature_WarnUnsafe
+// REQUIRES: swift_feature_LifetimeDependence
 
 //--- Inputs/module.modulemap
 module Test {
@@ -16,6 +17,7 @@ module Test {
 
 //--- Inputs/nonescapable.h
 #include "swift/bridging"
+#include <span>
 
 struct SWIFT_NONESCAPABLE View {
     __attribute__((swift_attr("@lifetime(immortal)")))
@@ -50,6 +52,9 @@ struct MyContainer {
     int end() const { return -1; }
 };
 
+using SpanOfInt = Unannotated;
+using SpanOfIntAlias = SpanOfInt;
+
 //--- test.swift
 
 import Test
@@ -75,3 +80,11 @@ func useSafeParams(x: Owner, y: View, z: SafeEscapableAggregate, c: MyContainer)
 
 func useCfType(x: CFArray) {
 }
+
+// expected-note@+1{{make global function 'useCppSpan' @unsafe to indicate that its use is not memory-safe}}
+func useCppSpan(x: SpanOfInt) { // expected-warning{{reference to unsafe type alias 'SpanOfInt'}}
+}
+
+// expected-note@+1{{make global function 'useCppSpan2' @unsafe to indicate that its use is not memory-safe}}
+func useCppSpan2(x: SpanOfIntAlias) { // expected-warning{{reference to unsafe type alias 'SpanOfIntAlias'}}
+}