[Sema] Error on resilient importing publicly a non-resilient module

Importing a non-library-evolution enabled module from a
library-evolution enabled module will lead to generating a
swiftinterface that can't be rebuilt by a different compiler.
Make it a hard error when using access level imports, and keep it as a
warning for @_implementationOnly imports.

Author: xymus

include/swift/AST/DiagnosticsSema.def

@@ -1011,10 +1011,10 @@ ERROR(import_restriction_conflict,none,
       "%select{implementation-only|SPI only|exported}2 ",
       (Identifier, unsigned, unsigned))
 
-WARNING(module_not_compiled_with_library_evolution,none,
-        "module %0 was not compiled with library evolution support; "
-        "using it means binary compatibility for %1 can't be guaranteed",
-        (Identifier, Identifier))
+ERROR(module_not_compiled_with_library_evolution,none,
+      "module %0 was not compiled with library evolution support; "
+      "using it means binary compatibility for %1 can't be guaranteed",
+      (Identifier, Identifier))
 
 ERROR(module_allowable_client_violation,none,
       "module %0 doesn't allow importation from module %1",

lib/Sema/ImportResolution.cpp

@@ -737,7 +737,8 @@ void UnboundImport::validateAllowableClient(ModuleDecl *importee,
 
 void UnboundImport::validateResilience(NullablePtr<ModuleDecl> topLevelModule,
                                        SourceFile &SF) {
-  if (import.options.contains(ImportFlags::ImplementationOnly))
+  if (import.options.contains(ImportFlags::ImplementationOnly) ||
+      import.accessLevel < AccessLevel::Public)
     return;
 
   // Per getTopLevelModule(), we'll only get nullptr here for non-Swift modules,
@@ -760,12 +761,20 @@ void UnboundImport::validateResilience(NullablePtr<ModuleDecl> topLevelModule,
       topLevelModule.get()->isResilient())
     return;
 
-  ctx.Diags.diagnose(import.module.getModulePath().front().Loc,
-                     diag::module_not_compiled_with_library_evolution,
-                     topLevelModule.get()->getName(),
-                     SF.getParentModule()->getName());
-  // FIXME: Once @_implementationOnly is a real feature, we should have a fix-it
-  // to add it.
+  auto inFlight = ctx.Diags.diagnose(import.module.getModulePath().front().Loc,
+                                     diag::module_not_compiled_with_library_evolution,
+                                     topLevelModule.get()->getName(),
+                                     SF.getParentModule()->getName());
+
+  if (ctx.LangOpts.hasFeature(Feature::AccessLevelOnImport)) {
+    SourceLoc attrLoc = import.accessLevelLoc;
+    if (attrLoc.isValid())
+      inFlight.fixItReplace(attrLoc, "internal");
+    else
+      inFlight.fixItInsert(import.importLoc, "internal ");
+  } else {
+    inFlight.limitBehavior(DiagnosticBehavior::Warning);
+  }
 }
 
 void UnboundImport::diagnoseInvalidAttr(DeclAttrKind attrKind,

test/Sema/access-level-and-non-resilient-import.swift

@@ -0,0 +1,51 @@
+// RUN: %empty-directory(%t)
+// RUN: split-file %s %t
+
+/// Build the libraries.
+// RUN: %target-swift-frontend -emit-module %t/DefaultLib.swift -o %t
+// RUN: %target-swift-frontend -emit-module %t/PublicLib.swift -o %t
+// RUN: %target-swift-frontend -emit-module %t/PackageLib.swift -o %t
+// RUN: %target-swift-frontend -emit-module %t/InternalLib.swift -o %t
+// RUN: %target-swift-frontend -emit-module %t/FileprivateLib.swift -o %t
+// RUN: %target-swift-frontend -emit-module %t/PrivateLib.swift -o %t
+
+/// A resilient client will error on public imports.
+// RUN: %target-swift-frontend -typecheck %t/Client_Swift5.swift -I %t \
+// RUN:   -enable-library-evolution -swift-version 5 \
+// RUN:   -enable-experimental-feature AccessLevelOnImport -verify
+// RUN: %target-swift-frontend -typecheck %t/Client_Swift6.swift -I %t \
+// RUN:   -enable-library-evolution -swift-version 6 \
+// RUN:   -enable-experimental-feature AccessLevelOnImport -verify
+
+/// A non-resilient client doesn't complain.
+// RUN: %target-swift-frontend -typecheck %t/Client_Swift5.swift -I %t \
+// RUN:   -swift-version 5 \
+// RUN:   -enable-experimental-feature AccessLevelOnImport
+// RUN: %target-swift-frontend -typecheck %t/Client_Swift6.swift -I %t \
+// RUN:   -swift-version 6 \
+// RUN:   -enable-experimental-feature AccessLevelOnImport
+
+//--- DefaultLib.swift
+//--- PublicLib.swift
+//--- PackageLib.swift
+//--- InternalLib.swift
+//--- FileprivateLib.swift
+//--- PrivateLib.swift
+
+//--- Client_Swift5.swift
+
+import DefaultLib // expected-error {{module 'DefaultLib' was not compiled with library evolution support; using it means binary compatibility for 'Client_Swift5' can't be guaranteed}} {{1-1=internal }}
+public import PublicLib // expected-error {{module 'PublicLib' was not compiled with library evolution support; using it means binary compatibility for 'Client_Swift5' can't be guaranteed}} {{1-7=internal}}
+package import PackageLib
+internal import InternalLib
+fileprivate import FileprivateLib
+private import PrivateLib
+
+//--- Client_Swift6.swift
+
+import DefaultLib
+public import PublicLib // expected-error {{module 'PublicLib' was not compiled with library evolution support; using it means binary compatibility for 'Client_Swift6' can't be guaranteed}} {{1-7=internal}}
+package import PackageLib
+internal import InternalLib
+fileprivate import FileprivateLib
+private import PrivateLib