diff --git a/include/swift/AST/Decl.h b/include/swift/AST/Decl.h
index a04531ea24313..72c33ec384335 100644
--- a/include/swift/AST/Decl.h
+++ b/include/swift/AST/Decl.h
@@ -1068,6 +1068,14 @@ class alignas(1 << DeclAlignInBits) Decl : public ASTAllocated<Decl>, public Swi
   /// behaviors for it and, if it's an extension, its members.
   bool isObjCImplementation() const;
 
+  /// True if this declaration should never have its implementation made
+  /// available to any client. This overrides cross-module optimization and
+  /// optimizations that might use the implementation, such that the only
+  /// implementation of this function is the one compiled into its owning
+  /// module. Practically speaking, this prohibits serialization of the SIL
+  /// for this definition.
+  bool isNeverEmittedIntoClient() const;
+
   using AuxiliaryDeclCallback = llvm::function_ref<void(Decl *)>;
 
   /// Iterate over the auxiliary declarations for this declaration,
diff --git a/include/swift/AST/DeclAttr.def b/include/swift/AST/DeclAttr.def
index 51ed6e7808f39..8da6f1541ac01 100644
--- a/include/swift/AST/DeclAttr.def
+++ b/include/swift/AST/DeclAttr.def
@@ -201,7 +201,11 @@ SIMPLE_DECL_ATTR(requires_stored_property_inits, RequiresStoredPropertyInits,
   ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove | UnreachableInABIAttr,
   27)
 
-// Unused '28'
+SIMPLE_DECL_ATTR(_neverEmitIntoClient, NeverEmitIntoClient,
+  OnVar | OnSubscript | OnAbstractFunction,
+  UserInaccessible | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove | ForbiddenInABIAttr,
+  28)
+
 // Unused '29'
 
 SIMPLE_DECL_ATTR(nonobjc, NonObjC,
diff --git a/lib/AST/ASTDumper.cpp b/lib/AST/ASTDumper.cpp
index 872793e4c1701..1f4dd877bb151 100644
--- a/lib/AST/ASTDumper.cpp
+++ b/lib/AST/ASTDumper.cpp
@@ -5011,6 +5011,7 @@ class PrintAttribute : public AttributeVisitor<PrintAttribute, void, Label>,
   TRIVIAL_ATTR_PRINTER(NSApplicationMain, ns_application_main)
   TRIVIAL_ATTR_PRINTER(NSCopying, ns_copying)
   TRIVIAL_ATTR_PRINTER(NSManaged, ns_managed)
+  TRIVIAL_ATTR_PRINTER(NeverEmitIntoClient, never_emit_into_client)
   TRIVIAL_ATTR_PRINTER(NoAllocation, no_allocation)
   TRIVIAL_ATTR_PRINTER(NoDerivative, no_derivative)
   TRIVIAL_ATTR_PRINTER(NoEagerMove, no_eager_move)
diff --git a/lib/AST/Decl.cpp b/lib/AST/Decl.cpp
index eaa4855c63abf..2443ff45ae05e 100644
--- a/lib/AST/Decl.cpp
+++ b/lib/AST/Decl.cpp
@@ -2281,6 +2281,10 @@ bool Decl::isObjCImplementation() const {
   return getAttrs().hasAttribute<ObjCImplementationAttr>(/*AllowInvalid=*/true);
 }
 
+bool Decl::isNeverEmittedIntoClient() const {
+  return getAttrs().hasAttribute<NeverEmitIntoClientAttr>();
+}
+
 PatternBindingDecl::PatternBindingDecl(SourceLoc StaticLoc,
                                        StaticSpellingKind StaticSpelling,
                                        SourceLoc VarLoc,
diff --git a/lib/ASTGen/Sources/ASTGen/DeclAttrs.swift b/lib/ASTGen/Sources/ASTGen/DeclAttrs.swift
index 0d7f49b4d03ef..f6a5a96aed226 100644
--- a/lib/ASTGen/Sources/ASTGen/DeclAttrs.swift
+++ b/lib/ASTGen/Sources/ASTGen/DeclAttrs.swift
@@ -267,6 +267,7 @@ extension ASTGenVisitor {
         .MainType,
         .Marker,
         .MoveOnly,
+        .NeverEmitIntoClient,
         .NoAllocation,
         .NoDerivative,
         .NoEagerMove,
diff --git a/lib/SIL/IR/SILDeclRef.cpp b/lib/SIL/IR/SILDeclRef.cpp
index 921f95ae726cf..5a4b131627d2e 100644
--- a/lib/SIL/IR/SILDeclRef.cpp
+++ b/lib/SIL/IR/SILDeclRef.cpp
@@ -887,6 +887,11 @@ SerializedKind_t SILDeclRef::getSerializedKind() const {
   ASSERT(ABIRoleInfo(d).providesAPI()
             && "should not get serialization info from ABI-only decl");
 
+  // A declaration marked as "never emitted into client" will not have its SIL
+  // serialized, ever.
+  if (d->isNeverEmittedIntoClient())
+    return IsNotSerialized;
+
   // Default and property wrapper argument generators are serialized if the
   // containing declaration is public.
   if (isDefaultArgGenerator() || (isPropertyWrapperBackingInitializer() &&
@@ -1102,6 +1107,11 @@ bool SILDeclRef::declHasNonUniqueDefinition(const ValueDecl *decl) {
   if (!decl->getASTContext().LangOpts.hasFeature(Feature::Embedded))
     return false;
 
+  // If the declaration is marked as @_neverEmitIntoClient, it has a unique
+  // definition.
+  if (decl->isNeverEmittedIntoClient())
+    return false;
+
   // If the declaration is not from the main module, treat its definition as
   // non-unique.
   auto module = decl->getModuleContext();
diff --git a/lib/Sema/TypeCheckAttr.cpp b/lib/Sema/TypeCheckAttr.cpp
index 9e9d65bc8a694..749f8d7899901 100644
--- a/lib/Sema/TypeCheckAttr.cpp
+++ b/lib/Sema/TypeCheckAttr.cpp
@@ -153,6 +153,7 @@ class AttributeChecker : public AttributeVisitor<AttributeChecker> {
 
 #define IGNORED_ATTR(X) void visit##X##Attr(X##Attr *) {}
   IGNORED_ATTR(AlwaysEmitIntoClient)
+  IGNORED_ATTR(NeverEmitIntoClient)
   IGNORED_ATTR(HasInitialValue)
   IGNORED_ATTR(ClangImporterSynthesizedType)
   IGNORED_ATTR(Convenience)
diff --git a/lib/Sema/TypeCheckDeclOverride.cpp b/lib/Sema/TypeCheckDeclOverride.cpp
index fa620318394e9..104bd0beeac61 100644
--- a/lib/Sema/TypeCheckDeclOverride.cpp
+++ b/lib/Sema/TypeCheckDeclOverride.cpp
@@ -1589,6 +1589,7 @@ namespace  {
     UNINTERESTING_ATTR(AccessControl)
     UNINTERESTING_ATTR(Alignment)
     UNINTERESTING_ATTR(AlwaysEmitIntoClient)
+    UNINTERESTING_ATTR(NeverEmitIntoClient)
     UNINTERESTING_ATTR(Borrowed)
     UNINTERESTING_ATTR(Borrowing)
     UNINTERESTING_ATTR(CDecl)
diff --git a/lib/Serialization/SerializeSIL.cpp b/lib/Serialization/SerializeSIL.cpp
index 2bc2d2b2f6903..49254e5106554 100644
--- a/lib/Serialization/SerializeSIL.cpp
+++ b/lib/Serialization/SerializeSIL.cpp
@@ -3575,6 +3575,12 @@ bool SILSerializer::shouldEmitFunctionBody(const SILFunction *F,
   if (F->isAvailableExternally())
     return false;
 
+  if (F->getDeclRef().hasDecl()) {
+    if (auto decl = F->getDeclRef().getDecl())
+      if (decl->isNeverEmittedIntoClient())
+        return false;
+  }
+
   // If we are asked to serialize everything, go ahead and do it.
   if (ShouldSerializeAll)
     return true;
diff --git a/test/embedded/linkage/never_emit_into_client.swift b/test/embedded/linkage/never_emit_into_client.swift
new file mode 100644
index 0000000000000..83bdf8f5ce388
--- /dev/null
+++ b/test/embedded/linkage/never_emit_into_client.swift
@@ -0,0 +1,81 @@
+// RUN: %empty-directory(%t)
+// RUN: split-file %s %t
+
+// Library module
+
+// SIL checking
+// RUN: %target-swift-frontend %t/Library.swift -parse-as-library -entry-point-function-name Library_main -enable-experimental-feature Embedded -emit-sil -emit-module-path %t/Modules/Library.swiftmodule -o - | %FileCheck -check-prefix LIBRARY-SIL %s
+
+// IR checking to ensure we get the right weak symbols.
+// RUN: %target-swift-frontend %t/Library.swift -parse-as-library -entry-point-function-name Library_main -enable-experimental-feature Embedded -emit-ir -o - | %FileCheck -check-prefix LIBRARY-IR --dump-input-filter all %s
+
+// Application module
+
+// RUN: %target-swift-frontend %t/Application.swift -I %t/Modules -parse-as-library -entry-point-function-name Application_main -enable-experimental-feature Embedded -emit-sil -o - | %FileCheck -check-prefix APPLICATION-SIL %s
+
+// RUN: %target-swift-frontend %t/Application.swift -I %t/Modules -parse-as-library -entry-point-function-name Application_main -enable-experimental-feature Embedded -emit-ir -o - | %FileCheck -check-prefix APPLICATION-IR %s
+
+// REQUIRES: swift_in_compiler
+// REQUIRES: swift_feature_Embedded
+
+//--- Library.swift
+
+// LIBRARY-IR: define {{(protected |dllexport )?}}swiftcc ptr @"$e7Library5helloSaySiGyF"()
+@_neverEmitIntoClient
+public func hello() -> [Int] {
+  getArray()
+}
+
+// LIBRARY-IR: define {{(protected |dllexport )?}}swiftcc ptr @"$e7Library8getArraySaySiGyF"()
+public func getArray() -> [Int] {
+  throughInternal()
+}
+
+// LIBRARY-IR: define {{(protected |dllexport )?}}swiftcc ptr @"$e7Library15throughInternalSaySiGyF"()
+func throughInternal() -> [Int] {
+  throughPrivate()
+}
+
+// LIBRARY-IR: define {{(protected |dllexport )?}}swiftcc ptr @"$e7Library14throughPrivate
+private func throughPrivate() -> [Int] {
+  [5, 6, 7]
+}
+
+// LIBRARY-IR-NOT: unnecessary
+public func unnecessary() -> Int { 5 }
+
+// LIBRARY-IR: define linkonce_odr hidden swiftcc { ptr, ptr } @"$es27_allocateUninitializedArrayySayxG_BptBwlFSi_Tg5"
+
+// LIBRARY-SIL: sil @$e7Library5helloSaySiGyF
+// LIBRARY-SIL: sil @$e7Library8getArraySaySiGyF : $@convention(thin) () -> @owned Array<Int> {
+
+//--- Application.swift
+import Library
+
+@_neverEmitIntoClient
+public func testMe() {
+  _ = hello()
+  _ = getArray()
+}
+
+// Note: "hello" is emitted only into the object file, so there is no definition
+// here.
+
+// APPLICATION-SIL: sil @$e7Library5helloSaySiGyF : $@convention(thin) () -> @owned Array<Int>{{$}}
+// APPLICATION-IR: declare swiftcc ptr @"$e7Library5helloSaySiGyF"()
+
+// Note: "getArray" is not @_neverEmitIntoClient, so it's definition is
+// available.
+
+// APPLICATION-SIL: sil public_external @$e7Library8getArraySaySiGyF : $@convention(thin) () -> @owned Array<Int> {
+// APPLICATION-IR: define linkonce_odr hidden swiftcc ptr @"$e7Library8getArraySaySiGyF"()
+
+// APPLICATION-IR: define{{.*}}@Application_main
+@main
+struct Main {
+  static func main() {
+  }
+}
+
+// APPLICATION-IR: define linkonce_odr hidden swiftcc { ptr, ptr } @"$es27_allocateUninitializedArrayySayxG_BptBwlFSi_Tg5"
+