[Sema] Error on implementation-only imported types in SPI with @_spiOnly

This used to be just a warning because it happened to work when clients
used only swiftmodules. Some projects have been using it as an
alternative to @_spiOnly imports. Now that we have an official feature
we can complete the diagnositc and error on implementation-only imports
types used in SPI.

Author: xymus

lib/Sema/TypeCheckAccess.cpp

@@ -1548,7 +1548,9 @@ swift::getDisallowedOriginKind(const Decl *decl,
   if (howImported != RestrictedImportKind::None) {
     // Temporarily downgrade implementation-only exportability in SPI to
     // a warning.
-    if (where.isSPI())
+    if (where.isSPI() &&
+        where.getFragileFunctionKind().kind == FragileFunctionKind::None &&
+        !SF->getASTContext().LangOpts.EnableSPIOnlyImports)
       downgradeToWarning = DowngradeToWarning::Yes;
 
     if (where.isSPI() && howImported == RestrictedImportKind::SPIOnly)

test/SPI/report-ioi-in-spi.swift

@@ -0,0 +1,60 @@
+// RUN: %empty-directory(%t)
+// RUN: split-file %s %t
+
+// RUN: %target-swift-frontend -emit-module %t/Lib.swift -I %t \
+// RUN:   -module-name Lib -emit-module-path %t/Lib.swiftmodule \
+// RUN:   -swift-version 5
+
+/// Use of IOI types in SPI signatures is an error with -experimental-spi-only-imports
+// RUN: %target-swift-frontend -emit-module %t/ClientSPIOnlyMode.swift -I %t \
+// RUN:   -swift-version 5 -verify \
+// RUN:   -experimental-spi-only-imports
+
+/// Use of IOI types in SPI signatures is a warning without -experimental-spi-only-imports
+// RUN: %target-swift-frontend -emit-module %t/ClientDefaultMode.swift -I %t \
+// RUN:   -swift-version 5 -verify
+
+/// This is a warning in swiftinterfaces
+// R UN: %target-swift-typecheck-module-from-interface(%t/Client.private.swiftinterface) \
+// R UN:   -I %t -module-name Client
+
+//--- Lib.swift
+
+public struct IOIStruct {
+    public init() {}
+}
+
+//--- ClientSPIOnlyMode.swift
+
+@_implementationOnly import Lib
+
+@_spi(X) public func spiClient(s: IOIStruct) -> IOIStruct { // expected-error 2 {{cannot use struct 'IOIStruct' here; 'Lib' has been imported as implementation-only}}
+    return IOIStruct()
+}
+
+@_spi(X) @inlinable public func inlinableClient(s: IOIStruct) -> IOIStruct { // expected-error 2 {{struct 'IOIStruct' cannot be used in an '@inlinable' function because 'Lib' was imported implementation-only}}
+    return IOIStruct() // expected-error {{struct 'IOIStruct' cannot be used in an '@inlinable' function because 'Lib' was imported implementation-only}}
+    // expected-error @-1 {{initializer 'init()' cannot be used in an '@inlinable' function because 'Lib' was imported implementation-only}}
+}
+
+//--- ClientDefaultMode.swift
+
+@_implementationOnly import Lib
+
+@_spi(X) public func spiClient(s: IOIStruct) -> IOIStruct { // expected-warning 2 {{cannot use struct 'IOIStruct' here; 'Lib' has been imported as implementation-only}}
+    return IOIStruct()
+}
+
+@_spi(X) @inlinable public func inlinableClient(s: IOIStruct) -> IOIStruct { // expected-error 2 {{struct 'IOIStruct' cannot be used in an '@inlinable' function because 'Lib' was imported implementation-only}}
+    return IOIStruct() // expected-error {{struct 'IOIStruct' cannot be used in an '@inlinable' function because 'Lib' was imported implementation-only}}
+    // expected-error @-1 {{initializer 'init()' cannot be used in an '@inlinable' function because 'Lib' was imported implementation-only}}
+}
+
+//--- Client.private.swiftinterface
+
+// swift-interface-format-version: 1.0
+// swift-compiler-version: Swift version 5.8-dev effective-4.1.50
+// swift-module-flags: -swift-version 4 -module-name Client
+@_implementationOnly import Lib
+
+@_spi(X) public func spiClient() -> IOIStruct { fatalError() }