Add an option to build the runtime with copy-on-write existentials

Author: aschwaighofer

CMakeLists.txt

@@ -270,6 +270,10 @@ option(SWIFT_STDLIB_ENABLE_RESILIENCE
     "Build the standard libraries and overlays with resilience enabled; see docs/LibraryEvolution.rst"
     FALSE)
 
+option(SWIFT_RUNTIME_ENABLE_COW_EXISTENTIALS
+    "Build the runtime with a copy-on-write implementation for opaque existentials"
+    FALSE)
+
 option(SWIFT_STDLIB_USE_NONATOMIC_RC
     "Build the standard libraries and overlays with nonatomic reference count operations enabled"
     FALSE)

cmake/modules/SwiftSource.cmake

@@ -243,6 +243,10 @@ function(_compile_swift_files
     list(APPEND swift_flags "-Xfrontend" "-assume-single-threaded")
   endif()
 
+  if(SWIFT_RUNTIME_ENABLE_COW_EXISTENTIALS)
+    list(APPEND swift_flags "-Xfrontend" "-enable-cow-existentials")
+  endif()
+
   if(SWIFT_STDLIB_ENABLE_SIL_OWNERSHIP AND SWIFTFILE_IS_STDLIB)
     list(APPEND swift_flags "-Xfrontend" "-enable-sil-ownership")
   endif()