Fix the declaration of shim visibility on ELF platforms.

Declarations with protected visibility are assumed to be defined
within the current linkage unit, so we have to use default visibility
if we don't know that we're building that.  Teach the shim
visibility header to only use protected visibility when the
__SWIFT_CURRENT_DYLIB macro is defined, and define it when building
the standard library.  Eventually we should change
SWIFT_RUNTIME_STDLIB_INTERFACE and SWIFT_RUNTIME_EXPORT to be
parameterized by the defining dylib so that this works for all the
overlay stubs, too; for now, special-casing swiftCore is necessary
to fix the LInux build.

Author: rjmccall

cmake/modules/SwiftSource.cmake

@@ -79,6 +79,10 @@ function(handle_swift_sources
     # <rdar://problem/15972329>
     list(APPEND swift_compile_flags "-force-single-frontend-invocation")
 
+    if(SWIFTSOURCES_IS_STDLIB_CORE)
+      list(APPEND swift_compile_flags "-Xcc" "-D__SWIFT_CURRENT_DYLIB=swiftCore")
+    endif()
+
     _compile_swift_files(
         dependency_target
         OUTPUT ${swift_obj}

stdlib/public/SwiftShims/Visibility.h

@@ -36,15 +36,29 @@
 # define SWIFT_END_NULLABILITY_ANNOTATIONS
 #endif
 
+// TODO: support using shims headers in overlays by parameterizing
+// SWIFT_RUNTIME_EXPORT on the library it's exported from, then setting
+// protected vs. default based on the current value of __SWIFT_CURRENT_DYLIB.
+
 /// Attribute used to export symbols from the runtime.
 #if __MACH__
 # define SWIFT_RUNTIME_EXPORT __attribute__((__visibility__("default")))
 #elif __ELF__
+
 // Use protected visibility for ELF, since we don't want Swift symbols to be
 // interposable. The relative relocations we form to metadata aren't
 // valid in ELF shared objects, and leaving them relocatable at load time
 // defeats the purpose of the relative references.
+//
+// Protected visibility on a declaration is interpreted to mean that the
+// symbol is defined in the current dynamic library, so if we're building
+// something else, we need to fall back on using default visibility.
+#ifdef __SWIFT_CURRENT_DYLIB
 # define SWIFT_RUNTIME_EXPORT __attribute__((__visibility__("protected")))
+#else
+# define SWIFT_RUNTIME_EXPORT __attribute__((__visibility__("default")))
+#endif
+
 #elif __CYGWIN__
 # define SWIFT_RUNTIME_EXPORT 
 #else

stdlib/public/runtime/CMakeLists.txt

@@ -22,6 +22,9 @@ if("${CMAKE_SYSTEM_NAME}" STREQUAL "CYGWIN")
   set(swift_runtime_port_sources
       CygwinPort.cpp)
 endif()
+
+list(APPEND swift_runtime_compile_flags
+     "-D__SWIFT_CURRENT_DYLIB=swiftCore")
 
 set(swift_runtime_objc_sources)
 set(swift_runtime_unicode_normalization_sources)