[embedded] Relax assert in SILLinkerVisitor::maybeAddFunctionToWorklist to allow external forward-declarations even in regular Swift, add tests

include/swift/SIL/SILFunction.h

@@ -1354,6 +1354,20 @@ public:
     WasmImportModuleAndField = std::make_pair(module, field);
   }
 
+  bool isExternForwardDeclaration() const {
+    if (isExternalDeclaration()) {
+      if (auto declContext = getDeclContext()) {
+        if (auto decl = declContext->getAsDecl()) {
+          if (decl->getAttrs().hasAttribute<ExternAttr>())
+            return true;
+          if (decl->getAttrs().hasAttribute<SILGenNameAttr>())
+            return true;
+        }
+      }
+    }
+    return false;
+  }
+
   /// Returns true if this function belongs to a declaration that returns
   /// an opaque result type with one or more availability conditions that are
   /// allowed to produce a different underlying type at runtime.

lib/SIL/IR/Linker.cpp

@@ -102,7 +102,7 @@ void SILLinkerVisitor::maybeAddFunctionToWorklist(SILFunction *F,
                                                   bool setToSerializable) {
   SILLinkage linkage = F->getLinkage();
   assert((!setToSerializable || F->hasValidLinkageForFragileRef() ||
-         hasSharedVisibility(linkage) || Mod.getOptions().EmbeddedSwift) &&
+         hasSharedVisibility(linkage) || F->isExternForwardDeclaration()) &&
          "called function has wrong linkage for serialized function");
                                          
   if (!F->isExternalDeclaration()) {

test/SIL/modules-extern.swift

@@ -0,0 +1,26 @@
+// RUN: %empty-directory(%t)
+// RUN: %{python} %utils/split_file.py -o %t %s
+
+// RUN: %target-swift-frontend -emit-module -o %t/MyModule.swiftmodule %t/MyModule.swift -parse-as-library
+// RUN: %target-swift-frontend -c -I %t %t/Main.swift -o %t/a.o
+
+// BEGIN MyModule.swift
+
+@_extern(c)
+@_alwaysEmitIntoClient
+func some_c_api()
+
+@_transparent
+public func publicFuncInAModule() {
+  some_c_api()
+}
+
+// BEGIN Main.swift
+
+import MyModule
+
+@_extern(c)
+func some_c_api()
+
+some_c_api()
+publicFuncInAModule()

test/SIL/modules-extern2.swift

@@ -0,0 +1,26 @@
+// RUN: %empty-directory(%t)
+// RUN: %{python} %utils/split_file.py -o %t %s
+
+// RUN: %target-swift-frontend -emit-module -o %t/MyModule.swiftmodule %t/MyModule.swift -parse-as-library
+// RUN: %target-swift-frontend -c -I %t %t/Main.swift -o %t/a.o
+
+// BEGIN MyModule.swift
+
+@_silgen_name("some_forward_declared_api")
+@_alwaysEmitIntoClient
+func some_forward_declared_api()
+
+@_transparent
+public func publicFuncInAModule() {
+  some_forward_declared_api()
+}
+
+// BEGIN Main.swift
+
+import MyModule
+
+@_silgen_name("some_forward_declared_api")
+func some_forward_declared_api()
+
+some_forward_declared_api()
+publicFuncInAModule()

test/embedded/modules-extern.swift

@@ -0,0 +1,33 @@
+// RUN: %empty-directory(%t)
+// RUN: %{python} %utils/split_file.py -o %t %s
+
+// RUN: %target-swift-frontend -emit-module -o %t/MyModule.swiftmodule %t/MyModule.swift -enable-experimental-feature Embedded -parse-as-library
+// RUN: %target-swift-frontend -c -I %t %t/Main.swift -enable-experimental-feature Embedded -o %t/a.o
+
+// REQUIRES: swift_in_compiler
+// REQUIRES: OS=macosx || OS=linux-gnu
+
+// BEGIN MyModule.swift
+
+@_extern(c)
+func some_c_api()
+
+@_transparent
+public func publicFuncInAModule() {
+  internalFuncInAModule()
+}
+
+@usableFromInline
+internal func internalFuncInAModule() {
+  some_c_api()
+}
+
+// BEGIN Main.swift
+
+import MyModule
+
+@_extern(c)
+func some_c_api()
+
+some_c_api()
+publicFuncInAModule()