[Sema] @_spiOnly needs to be enabled with a flag

Author: xymus

include/swift/AST/DiagnosticsSema.def

@@ -1902,6 +1902,9 @@ WARNING(spi_attribute_on_import_of_public_module,none,
         "'@_spi' import of %0 will not include any SPI symbols; "
         "%0 was built from the public interface at %1",
         (DeclName, StringRef))
+ERROR(spi_only_imports_not_enabled, none,
+      "'@_spiOnly' requires setting the frontend flag '-experimental-spi-only-imports'",
+      ())
 
 // Opaque return types
 ERROR(opaque_type_invalid_constraint,none,

include/swift/Basic/LangOptions.h

@@ -456,6 +456,9 @@ namespace swift {
     // FrontendOptions.
     bool AllowModuleWithCompilerErrors = false;
 
+    /// Enable using @_spiOnly on import decls.
+    bool EnableSPIOnlyImports = false;
+
     /// A helper enum to represent whether or not we customized the default
     /// ASTVerifier behavior via a frontend flag. By default, we do not
     /// customize.

include/swift/Option/FrontendOptions.td

@@ -317,6 +317,10 @@ def enable_resilience : Flag<["-"], "enable-resilience">,
 def enable_experimental_async_top_level :
   Flag<["-"], "enable-experimental-async-top-level">,
   HelpText<"Enable experimental concurrency in top-level code">;
+
+def experimental_spi_only_imports :
+  Flag<["-"], "experimental-spi-only-imports">,
+  HelpText<"Enable use of @_spiOnly imports">;
 }
 
 // HIDDEN FLAGS

lib/Frontend/CompilerInvocation.cpp

@@ -721,6 +721,8 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
     }
   }
 
+  Opts.EnableSPIOnlyImports = Args.hasArg(OPT_experimental_spi_only_imports);
+
   if (Opts.EnableSwift3ObjCInference) {
     if (const Arg *A = Args.getLastArg(
                                    OPT_warn_swift3_objc_inference_minimal,

lib/Sema/TypeCheckAttr.cpp

@@ -3911,6 +3911,9 @@ AttributeChecker::visitImplementationOnlyAttr(ImplementationOnlyAttr *attr) {
 
 void
 AttributeChecker::visitSPIOnlyAttr(SPIOnlyAttr *attr) {
+  if (!Ctx.LangOpts.EnableSPIOnlyImports) {
+    diagnoseAndRemoveAttr(attr, diag::spi_only_imports_not_enabled);
+  }
 }
 
 void AttributeChecker::visitTypeSequenceAttr(TypeSequenceAttr *attr) {

test/SPI/spi_only_import_flag_check.swift

@@ -0,0 +1,22 @@
+/// Test requiring the flag -experimental-spi-only-imports to use @_spiOnly.
+
+// RUN: %empty-directory(%t)
+// RUN: split-file %s %t
+
+/// Build dummy dependency.
+// RUN: %target-swift-frontend -emit-module %t/Empty.swift \
+// RUN:   -module-name Empty -emit-module-path %t/Empty.swiftmodule
+
+/// Attribute is rejected without the flag.
+// RUN: %target-swift-frontend -emit-module %t/Client.swift -I %t \
+// RUN:   -module-name Client -emit-module-path %t/Client.swiftmodule -verify
+
+/// Attribute is accepted with the flag.
+// RUN: %target-swift-frontend -emit-module %t/Client.swift -I %t \
+// RUN:   -module-name Client -emit-module-path %t/Client.swiftmodule \
+// RUN:   -experimental-spi-only-imports
+
+//--- Empty.swift
+//--- Client.swift
+
+@_spiOnly import Empty // expected-error {{'@_spiOnly' requires setting the frontend flag '-experimental-spi-only-imports'}}