stdlib: Symlink libswiftCore.dylib to libswiftCompatibility62.dylib

Author: slavapestov

stdlib/public/core/CMakeLists.txt

@@ -424,6 +424,47 @@ add_swift_target_library(swiftCore
                     zippered
                  )
 
+# Backward deployment of Span on Apple platforms is implemented via the
+# libswiftCompatibility62.dylib shim.
+#
+# When a binary built with an older deployment target is run with a new
+# standard library, the symbols that were previously in the backward
+# compatibility shim are now in the standard library. To ensure the
+# dynamic linker continues to find these symbols, the compatibility
+# shim must be replaced by a symlink to the standard library.
+#
+# Since we're building the "new" standard library here, we create this
+# symlink in the same directory, from libswiftCompatibility62.dylib to
+# libswiftCore.dylib.
+#
+# The sources for the real libswiftCompatibility62.dylib are found in
+# stdlib/toolchain/Compatibility62. When running a binary with an old
+# standard library, this dynamic library must be present at run time.
+#
+if(CMAKE_SYSTEM_NAME STREQUAL "Darwin")
+  foreach(sdk ${SWIFT_SDKS})
+    set(lib_dir "${SWIFT_SDK_${sdk}_LIB_SUBDIR}")
+    set(lib_path "${SWIFTLIB_DIR}/${lib_dir}")
+    set(compat_lib_name "${lib_path}/libswiftCompatibility62.dylib")
+
+    # This doesn't depend on libswiftCore.dylib because we don't actually need
+    # for it to exist to create the symlink, nor is there any need to recreate
+    # the symlink if the dylib changes.
+    add_custom_command_target(unused_var
+                              CUSTOM_TARGET_NAME "swiftCompatibility62-symlink-${lib_dir}"
+                              OUTPUT "${compat_lib_name}"
+                              COMMAND ${CMAKE_COMMAND} "-E" "create_symlink" "${lib_path}/libswiftCore.dylib" "${compat_lib_name}")
+    foreach(ARCH ${SWIFT_SDK_${sdk}_ARCHITECTURES})
+      add_dependencies("swiftCore-${lib_dir}-${ARCH}" "swiftCompatibility62-symlink-${lib_dir}")
+    endforeach()
+
+    swift_install_symlink_component(stdlib
+      LINK_NAME libswiftCompatibility62.dylib
+      TARGET libswiftCore.dylib
+      DESTINATION "lib/swift/${lib_dir}")
+  endforeach()
+endif()
+
 # Embedded standard library - embedded libraries are built as .swiftmodule only,
 # i.e. there is no .o or .a file produced (no binary code is actually produced)
 # and only users of a library are going to actually compile any needed code.