diff --git a/test/embedded/linkage/Inputs/CHeader.h b/test/embedded/linkage/Inputs/CHeader.h
new file mode 100644
index 0000000000000..d7ef0f7c265cc
--- /dev/null
+++ b/test/embedded/linkage/Inputs/CHeader.h
@@ -0,0 +1,5 @@
+typedef struct {
+  double x, y;
+} Point;
+
+Point getPoint(double x, double y);
diff --git a/test/embedded/linkage/Inputs/SwiftDependency.swift b/test/embedded/linkage/Inputs/SwiftDependency.swift
new file mode 100644
index 0000000000000..2fc3b1624c11b
--- /dev/null
+++ b/test/embedded/linkage/Inputs/SwiftDependency.swift
@@ -0,0 +1,7 @@
+public struct A {
+  private var string: String
+
+  public init() { self.string = "Hello" }
+
+  public func doSomething() { }
+}
diff --git a/test/embedded/linkage/Inputs/module.modulemap b/test/embedded/linkage/Inputs/module.modulemap
new file mode 100644
index 0000000000000..5445080fa9516
--- /dev/null
+++ b/test/embedded/linkage/Inputs/module.modulemap
@@ -0,0 +1,3 @@
+module CDependency {
+  header "CHeader.h"
+}
diff --git a/test/embedded/linkage/implementation_only_hiding.swift b/test/embedded/linkage/implementation_only_hiding.swift
new file mode 100644
index 0000000000000..079c6f02cc7ca
--- /dev/null
+++ b/test/embedded/linkage/implementation_only_hiding.swift
@@ -0,0 +1,73 @@
+// This elaborate test ensures that @_implementationOnly with Embedded Swift
+// can hide some dependencies if you are very, very careful.
+
+// RUN: %empty-directory(%t)
+// RUN: mkdir -p %t/Dependencies
+// RUN: mkdir -p %t/Files
+// RUN: mkdir -p %t/Modules
+
+// Copy Swift file + C header + module map into a separate directory we'll use
+// when building the module whose implementation we want to hide.
+// RUN: cp %S/Inputs/SwiftDependency.swift %t/Dependencies/
+// RUN: cp %S/Inputs/CHeader.h %t/Dependencies/
+// RUN: cp %S/Inputs/module.modulemap %t/Dependencies/
+
+// RUN: split-file %s %t/Files
+
+// Compile the Swift dependencies into that same location.
+// RUN: %target-swift-frontend -parse-as-library -emit-module %t/Dependencies/SwiftDependency.swift -enable-experimental-feature Embedded -o %t/Dependencies/SwiftDependency.swiftmodule
+
+// Build the library (which is supposed to encapsulate those dependencies)
+// against the dependencies.
+// RUN: %target-swift-frontend -parse-as-library -emit-module %t/Files/Library.swift -enable-experimental-feature Embedded -I %t/Dependencies/ -o %t/Modules/Library.swiftmodule
+
+// Remove the dependencies so there is no way we can find them later.
+// RUN: rm -rf %t/Dependencies
+
+// Build the application against the library. This is expected to work because
+// @_neverEmitIntoClient hides the body of test().
+// RUN: %target-swift-frontend -emit-ir -parse-as-library %t/Files/Application.swift -enable-experimental-feature Embedded -I %t/Modules -o %t/Application.ir
+
+// Build the application against the library, but intentionally trigger
+// deserialization of some serialized SIL that refers to an implementation-only
+// dependency. Right now, these fail spectacularly. Over time, we want them to
+// become compile-time errors or start working.
+// RUN: not --crash %target-swift-frontend -emit-ir -parse-as-library %t/Files/Application.swift -enable-experimental-feature Embedded -I %t/Modules -o %t/Application.ir -DBAD_C_USAGE
+// RUN: not --crash %target-swift-frontend -emit-ir -parse-as-library %t/Files/Application.swift -enable-experimental-feature Embedded -I %t/Modules -o %t/Application.ir -DBAD_SWIFT_USAGE
+
+// REQUIRES: swift_in_compiler
+// REQUIRES: swift_feature_Embedded
+
+//--- Library.swift
+@_implementationOnly import CDependency
+@_implementationOnly import SwiftDependency
+
+@_neverEmitIntoClient
+public func test() {
+  _ = getPoint(3.14159, 2.71828)
+  A().doSomething()
+}
+
+public func badCLibraryUsage() {
+  _ = getPoint(3.14159, 2.71828)
+}
+
+public func badSwiftLibraryUsage() {
+  A().doSomething()
+}
+
+//--- Application.swift
+
+import Library
+
+public func useTest() {
+  test()
+
+#if BAD_C_USAGE
+  badCLibraryUsage()
+#endif
+
+#if BAD_SWIFT_USAGE
+  badSwiftLibraryUsage()
+#endif
+}