[Build] Fix ABI ODR violation for C++20 consumers linking against C++…

CMake/FollyConfigChecks.cmake

@@ -164,6 +164,22 @@ check_cxx_source_compiles("
   FOLLY_HAVE_EXTRANDOM_SFMT19937
 )
 
+# Check if [[no_unique_address]] attribute is available at library compile time.
+# This is critical for ABI stability: the attribute changes struct layout,
+# and if headers are compiled with a different C++ standard than the library,
+# ODR violations and memory corruption can occur.
+# See: https://github.com/facebook/folly/issues/2477
+check_cxx_source_compiles("
+  struct Empty {};
+  struct Test {
+    [[no_unique_address]] Empty e;
+    int x;
+  };
+  static_assert(sizeof(Test) == sizeof(int), \"\");
+  int main() { return 0; }"
+  FOLLY_LIBRARY_HAS_NO_UNIQUE_ADDRESS
+)
+
 check_cxx_source_runs("
   #include <stdarg.h>
   #include <stdio.h>

CMake/folly-config.h.cmake

@@ -85,3 +85,14 @@
 #cmakedefine01 FOLLY_HAVE_LIBRT
 
 #cmakedefine01 FOLLY_HAVE_VSOCK
+
+// Record the C++ standard version used to compile the Folly library.
+// This enables ABI compatibility checks when headers are included by
+// applications compiled with a different C++ standard version.
+// See: https://github.com/facebook/folly/issues/2477
+#define FOLLY_LIBRARY_CXX_STANDARD @CMAKE_CXX_STANDARD@
+
+// Record whether [[no_unique_address]] was active when building the library.
+// This attribute changes struct layout and can cause ODR violations if
+// headers are compiled with a different C++ standard than the library.
+#cmakedefine01 FOLLY_LIBRARY_HAS_NO_UNIQUE_ADDRESS

folly/CppAttributes.h

@@ -16,11 +16,22 @@
 
 /**
  * GCC compatible wrappers around clang attributes.
+ *
+ * ABI STABILITY NOTE:
+ * Some attributes (notably [[no_unique_address]]) affect struct layout.
+ * When the Folly library is compiled with one C++ standard and headers are
+ * included from an application compiled with a different standard, ODR
+ * violations can occur, leading to memory corruption and crashes.
+ *
+ * This header includes logic to detect such mismatches and force the headers
+ * to use the same layout as the compiled library.
+ * See: https://github.com/facebook/folly/issues/2477
  */
 
 #pragma once
 
 #include <folly/Portability.h>
+#include <folly/portability/Config.h>
 
 #ifndef __has_attribute
 #define FOLLY_HAS_ATTRIBUTE(x) 0
@@ -115,15 +126,74 @@
  *
  *  sizeof(NonEmpty1); // may be == sizeof(int)
  *  sizeof(NonEmpty2); // must be > sizeof(int)
+ *
+ * ABI STABILITY:
+ * This attribute changes struct layout. When linking against a pre-compiled
+ * Folly library, we MUST use the same layout as the library was compiled with.
+ * If the library was compiled without [[no_unique_address]] support (e.g.,
+ * C++17), we must NOT apply the attribute even if the current compilation
+ * would support it (e.g., C++20). Doing otherwise causes ODR violations,
+ * memory corruption, and crashes.
+ * See: https://github.com/facebook/folly/issues/2477
  */
+
+// First, detect what the current compilation unit supports
 #if FOLLY_HAS_CPP_ATTRIBUTE(no_unique_address)
-#define FOLLY_ATTR_NO_UNIQUE_ADDRESS no_unique_address
+#define FOLLY_CURRENT_HAS_NO_UNIQUE_ADDRESS 1
+#define FOLLY_CURRENT_NO_UNIQUE_ADDRESS_ATTR no_unique_address
 #elif FOLLY_HAS_CPP_ATTRIBUTE(msvc::no_unique_address)
-#define FOLLY_ATTR_NO_UNIQUE_ADDRESS msvc::no_unique_address
+#define FOLLY_CURRENT_HAS_NO_UNIQUE_ADDRESS 1
+#define FOLLY_CURRENT_NO_UNIQUE_ADDRESS_ATTR msvc::no_unique_address
+#else
+#define FOLLY_CURRENT_HAS_NO_UNIQUE_ADDRESS 0
+#endif
+
+// Now determine what to actually use, respecting library ABI
+#if defined(FOLLY_LIBRARY_HAS_NO_UNIQUE_ADDRESS)
+// We have library configuration available - use it for ABI compatibility
+
+#if FOLLY_LIBRARY_HAS_NO_UNIQUE_ADDRESS && FOLLY_CURRENT_HAS_NO_UNIQUE_ADDRESS
+// Both library and current compilation support it - use the attribute
+#define FOLLY_ATTR_NO_UNIQUE_ADDRESS FOLLY_CURRENT_NO_UNIQUE_ADDRESS_ATTR
+
+#elif FOLLY_LIBRARY_HAS_NO_UNIQUE_ADDRESS && \
+    !FOLLY_CURRENT_HAS_NO_UNIQUE_ADDRESS
+// Library was compiled with it, but current compilation doesn't support it.
+// This is an error - the application will have wrong struct layouts.
+#error \
+    "Folly library was compiled with [[no_unique_address]] support, but " \
+        "current compilation does not support it. This will cause ABI " \
+        "incompatibility. Please compile with C++20 or later."
+
+#elif !FOLLY_LIBRARY_HAS_NO_UNIQUE_ADDRESS && \
+    FOLLY_CURRENT_HAS_NO_UNIQUE_ADDRESS
+// Library was compiled WITHOUT [[no_unique_address]], but current compilation
+// supports it. We MUST NOT use the attribute to maintain ABI compatibility.
+// Issue a warning to inform users of this situation.
+#if defined(__GNUC__) || defined(__clang__)
+#pragma message(                                                           \
+    "Warning: Folly library was compiled without [[no_unique_address]] "   \
+    "(likely C++17), but current compilation supports it. Disabling the "  \
+    "attribute to maintain ABI compatibility. Consider recompiling Folly " \
+    "with C++20 for optimal performance. See issue #2477.")
+#endif
+#define FOLLY_ATTR_NO_UNIQUE_ADDRESS /* disabled for ABI compatibility */
+
 #else
+// Neither library nor current compilation support it
 #define FOLLY_ATTR_NO_UNIQUE_ADDRESS
 #endif
 
+#else
+// No library configuration available (header-only use or building Folly itself)
+// Use whatever the current compilation supports
+#if FOLLY_CURRENT_HAS_NO_UNIQUE_ADDRESS
+#define FOLLY_ATTR_NO_UNIQUE_ADDRESS FOLLY_CURRENT_NO_UNIQUE_ADDRESS_ATTR
+#else
+#define FOLLY_ATTR_NO_UNIQUE_ADDRESS
+#endif
+#endif
+
 #if FOLLY_HAS_CPP_ATTRIBUTE(clang::no_destroy)
 #define FOLLY_ATTR_CLANG_NO_DESTROY clang::no_destroy
 #else

folly/Portability.h

@@ -34,6 +34,40 @@
 
 static_assert(FOLLY_CPLUSPLUS >= 201703L, "__cplusplus >= 201703L");
 
+/**
+ * ABI Compatibility Check
+ *
+ * When linking against a pre-compiled Folly library, the C++ standard version
+ * used by the application should match the library to avoid ODR violations.
+ * Key layout-affecting features differ between C++17 and C++20:
+ *   - [[no_unique_address]] attribute availability and behavior
+ *   - Coroutine support structures
+ *   - std::span vs folly::span selection
+ *
+ * See: https://github.com/facebook/folly/issues/2477
+ */
+#if defined(FOLLY_LIBRARY_CXX_STANDARD) && !defined(FOLLY_SKIP_ABI_CHECK)
+// Determine if there's a C++ standard version mismatch that could affect ABI
+#if (FOLLY_LIBRARY_CXX_STANDARD < 20 && FOLLY_CPLUSPLUS >= 202002L) || \
+    (FOLLY_LIBRARY_CXX_STANDARD >= 20 && FOLLY_CPLUSPLUS < 202002L)
+// There's a C++17/C++20 boundary crossing, which affects [[no_unique_address]]
+#if defined(__GNUC__) || defined(__clang__)
+#pragma message( \
+     "Warning: Folly library was compiled with C++" \
+     FOLLY_PP_STRINGIZE(FOLLY_LIBRARY_CXX_STANDARD) \
+     ", but application is using a different C++ standard. " \
+     "This may cause ABI issues. See issue #2477. " \
+     "Define FOLLY_SKIP_ABI_CHECK to suppress this warning.")
+#endif
+#endif
+#endif
+
+// Helper macro for stringizing (used in the warning above)
+#ifndef FOLLY_PP_STRINGIZE
+#define FOLLY_PP_STRINGIZE_IMPL(x) #x
+#define FOLLY_PP_STRINGIZE(x) FOLLY_PP_STRINGIZE_IMPL(x)
+#endif
+
 #if defined(__GNUC__) && !defined(__clang__)
 #if defined(FOLLY_CONFIG_TEMPORARY_DOWNGRADE_GCC)
 static_assert(__GNUC__ >= 9, "__GNUC__ >= 9");