[libc++] Add _LIBCPP_ENABLE_NODISCARD and _LIBCPP_NODISCARD_EXT to allow pre-C++2a [[nodiscard]]

Summary:
The `[[nodiscard]]` attribute is intended to help users find bugs where
function return values are ignored when they shouldn't be. After C++17 the
C++ standard has started to declared such library functions as `[[nodiscard]]`.
However, this application is limited and applies only to dialects after C++17.
Users who want help diagnosing misuses of STL functions may desire a more
liberal application of `[[nodiscard]]`.

For this reason libc++ provides an extension that does just that! The
extension must be enabled by defining `_LIBCPP_ENABLE_NODISCARD`. The extended
applications of `[[nodiscard]]` takes two forms:

1. Backporting `[[nodiscard]]` to entities declared as such by the
   standard in newer dialects, but not in the present one.

2. Extended applications of `[[nodiscard]]`, at the libraries discretion,
   applied to entities never declared as such by the standard.

Users may also opt-out of additional applications `[[nodiscard]]` using
additional macros.

Applications of the first form, which backport `[[nodiscard]]` from a newer
dialect may be disabled using macros specific to the dialect it was added. For
example `_LIBCPP_DISABLE_NODISCARD_AFTER_CXX17`.

Applications of the second form, which are pure extensions, may be disabled
by defining `_LIBCPP_DISABLE_NODISCARD_EXT`.

This patch was originally written by me (Roman Lebedev),
then but then reworked by Eric Fiselier.

Reviewers: mclow.lists, thakis, EricWF

Reviewed By: thakis, EricWF

Subscribers: llvm-commits, mclow.lists, lebedev.ri, EricWF, rjmccall, Quuxplusone, cfe-commits, christof

Differential Revision: https://reviews.llvm.org/D45179

llvm-svn: 342808

libcxx/docs/UsingLibcxx.rst

@@ -226,6 +226,21 @@ thread safety annotations.
   replacement scenarios from working, e.g. replacing `operator new` and
   expecting a non-replaced `operator new[]` to call the replaced `operator new`.
 
+**_LIBCPP_ENABLE_NODISCARD**:
+  Allow the library to add ``[[nodiscard]]`` attributes to entities not specified
+  as ``[[nodiscard]]`` by the current language dialect. This includes
+  backporting applications of ``[[nodiscard]]`` from newer dialects and
+  additional extended applications at the discretion of the library. All
+  additional applications of ``[[nodiscard]]`` are disabled by default.
+  See :ref:`Extended Applications of [[nodiscard]] <nodiscard extension>` for
+  more information.
+
+**_LIBCPP_DISABLE_NODISCARD_EXT**:
+  This macro prevents the library from applying ``[[nodiscard]]`` to entities
+  purely as an extension. See :ref:`Extended Applications of [[nodiscard]] <nodiscard extension>`
+  for more information.
+
+
 C++17 Specific Configuration Macros
 -----------------------------------
 **_LIBCPP_ENABLE_CXX17_REMOVED_FEATURES**:
@@ -238,3 +253,58 @@ C++17 Specific Configuration Macros
 
 **_LIBCPP_ENABLE_CXX17_REMOVED_AUTO_PTR**:
   This macro is used to re-enable `std::auto_ptr` in C++17.
+
+C++2a Specific Configuration Macros:
+------------------------------------
+**_LIBCPP_DISABLE_NODISCARD_AFTER_CXX17**:
+  This macro can be used to disable diagnostics emitted from functions marked
+  ``[[nodiscard]]`` in dialects after C++17.  See :ref:`Extended Applications of [[nodiscard]] <nodiscard extension>`
+  for more information.
+
+
+Libc++ Extensions
+=================
+
+This section documents various extensions provided by libc++, how they're
+provided, and any information regarding how to use them.
+
+.. _nodiscard extension:
+
+Extended applications of ``[[nodiscard]]``
+------------------------------------------
+
+The ``[[nodiscard]]`` attribute is intended to help users find bugs where
+function return values are ignored when they shouldn't be. After C++17 the
+C++ standard has started to declared such library functions as ``[[nodiscard]]``.
+However, this application is limited and applies only to dialects after C++17.
+Users who want help diagnosing misuses of STL functions may desire a more
+liberal application of ``[[nodiscard]]``.
+
+For this reason libc++ provides an extension that does just that! The
+extension must be enabled by defining ``_LIBCPP_ENABLE_NODISCARD``. The extended
+applications of ``[[nodiscard]]`` takes two forms:
+
+1. Backporting ``[[nodiscard]]`` to entities declared as such by the
+   standard in newer dialects, but not in the present one.
+
+2. Extended applications of ``[[nodiscard]]``, at the libraries discretion,
+   applied to entities never declared as such by the standard.
+
+Users may also opt-out of additional applications ``[[nodiscard]]`` using
+additional macros.
+
+Applications of the first form, which backport ``[[nodiscard]]`` from a newer
+dialect may be disabled using macros specific to the dialect it was added. For
+example ``_LIBCPP_DISABLE_NODISCARD_AFTER_CXX17``.
+
+Applications of the second form, which are pure extensions, may be disabled
+by defining ``_LIBCPP_DISABLE_NODISCARD_EXT``.
+
+
+Entities declared with ``_LIBCPP_NODISCARD_EXT``
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+This section lists all extended applications of ``[[nodiscard]]`` to entities
+which no dialect declares as such (See the second form described above).
+
+* ``get_temporary_buffer``

libcxx/include/__config

@@ -1049,8 +1049,30 @@ template <unsigned> struct __static_assert_check {};
 #  define _LIBCPP_CONSTEXPR_AFTER_CXX17
 #endif
 
-#if __has_cpp_attribute(nodiscard) && _LIBCPP_STD_VER > 17 && !defined(_LIBCPP_DISABLE_NODISCARD_AFTER_CXX17)
-#  define _LIBCPP_NODISCARD_AFTER_CXX17 [[nodiscard]]
+// The _LIBCPP_NODISCARD_ATTRIBUTE should only be used to define other
+// NODISCARD macros to the correct attribute.
+#if __has_cpp_attribute(nodiscard) || defined(_LIBCPP_COMPILER_MSVC)
+#  define _LIBCPP_NODISCARD_ATTRIBUTE [[nodiscard]]
+#elif defined(_LIBCPP_COMPILER_CLANG) && !defined(_LIBCPP_CXX03_LANG)
+#  define _LIBCPP_NODISCARD_ATTRIBUTE [[clang::warn_unused_result]]
+#else
+// We can't use GCC's [[gnu::warn_unused_result]] and
+// __attribute__((warn_unused_result)), because GCC does not silence them via
+// (void) cast.
+#  define _LIBCPP_NODISCARD_ATTRIBUTE
+#endif
+
+// _LIBCPP_NODISCARD_EXT may be used to apply [[nodiscard]] to entities not
+// specified as such as an extension.
+#if defined(_LIBCPP_ENABLE_NODISCARD) && !defined(_LIBCPP_DISABLE_NODISCARD_EXT)
+#  define _LIBCPP_NODISCARD_EXT _LIBCPP_NODISCARD_ATTRIBUTE
+#else
+#  define _LIBCPP_NODISCARD_EXT
+#endif
+
+#if !defined(_LIBCPP_DISABLE_NODISCARD_AFTER_CXX17) && \
+    (_LIBCPP_STD_VER > 17 || defined(_LIBCPP_ENABLE_NODISCARD))
+#  define _LIBCPP_NODISCARD_AFTER_CXX17 _LIBCPP_NODISCARD_ATTRIBUTE
 #else
 #  define _LIBCPP_NODISCARD_AFTER_CXX17
 #endif

libcxx/include/memory

@@ -2004,7 +2004,7 @@ public:
 };
 
 template <class _Tp>
-_LIBCPP_NO_CFI
+_LIBCPP_NODISCARD_EXT _LIBCPP_NO_CFI
 pair<_Tp*, ptrdiff_t>
 get_temporary_buffer(ptrdiff_t __n) _NOEXCEPT
 {

libcxx/test/libcxx/diagnostics/enable_nodiscard.fail.cpp

@@ -0,0 +1,33 @@
+// -*- C++ -*-
+//===----------------------------------------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+// Test that _LIBCPP_NODISCARD_EXT and _LIBCPP_NODISCARD_AFTER_CXX17 are defined
+// to the appropriate warning-generating attribute when _LIBCPP_ENABLE_NODISCARD
+// is explicitly provided.
+
+// UNSUPPORTED: c++98, c++03
+
+// GCC 7 is the first version to introduce [[nodiscard]]
+// UNSUPPORTED: gcc-4.9, gcc-5, gcc-6
+
+// MODULES_DEFINES: _LIBCPP_ENABLE_NODISCARD
+#define _LIBCPP_ENABLE_NODISCARD
+
+#include <__config>
+
+_LIBCPP_NODISCARD_EXT int foo() { return 42; }
+_LIBCPP_NODISCARD_AFTER_CXX17 int bar() { return 42; }
+
+int main() {
+  foo(); // expected-error-re {{ignoring return value of function declared with {{'nodiscard'|warn_unused_result}} attribute}}
+  bar(); // expected-error-re {{ignoring return value of function declared with {{'nodiscard'|warn_unused_result}} attribute}}
+  (void)foo(); // OK. void casts disable the diagnostic.
+  (void)bar();
+}

libcxx/test/libcxx/diagnostics/enable_nodiscard_disable_after_cxx17.fail.cpp

@@ -0,0 +1,33 @@
+// -*- C++ -*-
+//===----------------------------------------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++98, c++03
+
+// GCC 7 is the first version to introduce [[nodiscard]]
+// UNSUPPORTED: gcc-4.9, gcc-5, gcc-6
+
+// Test that _LIBCPP_DISABLE_NODISCARD_EXT only disables _LIBCPP_NODISCARD_EXT
+// and not _LIBCPP_NODISCARD_AFTER_CXX17.
+
+// MODULES_DEFINES: _LIBCPP_ENABLE_NODISCARD
+// MODULES_DEFINES: _LIBCPP_DISABLE_NODISCARD_AFTER_CXX17
+#define _LIBCPP_ENABLE_NODISCARD
+#define _LIBCPP_DISABLE_NODISCARD_AFTER_CXX17
+#include <__config>
+
+
+_LIBCPP_NODISCARD_EXT int foo() { return 42; }
+_LIBCPP_NODISCARD_AFTER_CXX17 int bar() { return 42; }
+
+int main() {
+  foo(); // expected-error-re {{ignoring return value of function declared with {{'nodiscard'|warn_unused_result}} attribute}}
+  bar(); // OK.
+  (void)foo(); // OK.
+}

libcxx/test/libcxx/diagnostics/enable_nodiscard_disable_nodiscard_ext.fail.cpp

@@ -0,0 +1,31 @@
+// -*- C++ -*-
+//===----------------------------------------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++98, c++03
+
+// GCC 7 is the first version to introduce [[nodiscard]]
+// UNSUPPORTED: gcc-4.9, gcc-5, gcc-6
+
+
+// MODULES_DEFINES: _LIBCPP_ENABLE_NODISCARD
+// MODULES_DEFINES: _LIBCPP_DISABLE_NODISCARD_EXT
+#define _LIBCPP_ENABLE_NODISCARD
+#define _LIBCPP_DISABLE_NODISCARD_EXT
+#include <__config>
+
+
+_LIBCPP_NODISCARD_EXT int foo() { return 42; }
+_LIBCPP_NODISCARD_AFTER_CXX17 int bar() { return 42; }
+
+int main() {
+  bar(); // expected-error-re {{ignoring return value of function declared with {{'nodiscard'|warn_unused_result}} attribute}}
+  foo(); // OK.
+  (void)bar(); // OK.
+}

libcxx/test/libcxx/diagnostics/nodiscard.pass.cpp

@@ -8,18 +8,13 @@
 //
 //===----------------------------------------------------------------------===//
 
-// Test that _LIBCPP_NODISCARD_AFTER_CXX17 works
-//	#define _LIBCPP_NODISCARD_AFTER_CXX17 [[nodiscard]]
+// Test that _LIBCPP_NODISCARD_EXT is not defined to [[nodiscard]] unless
+// explicitly enabled by _LIBCPP_ENABLE_NODISCARD
 
-// UNSUPPORTED: c++98, c++03, c++11, c++14
-
-// MODULES_DEFINES: _LIBCPP_DISABLE_NODISCARD_AFTER_CXX17
-#define _LIBCPP_DISABLE_NODISCARD_AFTER_CXX17
 #include <__config>
 
-_LIBCPP_NODISCARD_AFTER_CXX17 int foo() { return 6; }
+_LIBCPP_NODISCARD_EXT int foo() { return 42; }
 
-int main ()
-{
-	foo();	// no error here!
+int main() {
+  foo(); // OK.
 }

libcxx/test/libcxx/diagnostics/nodiscard_aftercxx17.fail.cpp

@@ -12,7 +12,6 @@
 //	#define _LIBCPP_NODISCARD_AFTER_CXX17 [[nodiscard]]
 
 // UNSUPPORTED: c++98, c++03, c++11, c++14, c++17
-// UNSUPPORTED: clang-3.3, clang-3.4, clang-3.5, clang-3.6, clang-3.7, clang-3.8
 
 #include <__config>
 

libcxx/test/libcxx/diagnostics/nodiscard_aftercxx17.pass.cpp

@@ -0,0 +1,23 @@
+// -*- C++ -*-
+//===----------------------------------------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+// Test that _LIBCPP_NODISCARD_AFTER_CXX17 is disabled whenever
+// _LIBCPP_DISABLE_NODISCARD_AFTER_CXX17 is defined by the user.
+
+// MODULES_DEFINES: _LIBCPP_DISABLE_NODISCARD_AFTER_CXX17
+#define _LIBCPP_DISABLE_NODISCARD_AFTER_CXX17
+#include <__config>
+
+_LIBCPP_NODISCARD_AFTER_CXX17 int foo() { return 6; }
+
+int main ()
+{
+	foo();	// no error here!
+}

libcxx/test/libcxx/diagnostics/nodiscard_extensions.fail.cpp

@@ -0,0 +1,32 @@
+// -*- C++ -*-
+//===----------------------------------------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++98, c++03
+
+// Test that entities declared [[nodiscard]] as at extension by libc++, are
+// only actually declared such when _LIBCPP_ENABLE_NODISCARD is specified.
+
+// All entities to which libc++ applies [[nodiscard]] as an extension should
+// be tested here and in nodiscard_extensions.pass.cpp. They should also
+// be listed in `UsingLibcxx.rst` in the documentation for the extension.
+
+// MODULES_DEFINES: _LIBCPP_ENABLE_NODISCARD
+#define _LIBCPP_ENABLE_NODISCARD
+
+#include <memory>
+
+#include "test_macros.h"
+
+int main() {
+  {
+    // expected-error-re@+1 {{ignoring return value of function declared with {{'nodiscard'|warn_unused_result}} attribute}}
+    std::get_temporary_buffer<int>(1);
+  }
+}

libcxx/test/libcxx/diagnostics/nodiscard_extensions.pass.cpp

@@ -0,0 +1,26 @@
+// -*- C++ -*-
+//===----------------------------------------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+// Test that entities declared [[nodiscard]] as at extension by libc++, are
+// only actually declared such when _LIBCPP_ENABLE_NODISCARD is specified.
+
+// All entities to which libc++ applies [[nodiscard]] as an extension should
+// be tested here and in nodiscard_extensions.fail.cpp. They should also
+// be listed in `UsingLibcxx.rst` in the documentation for the extension.
+
+#include <memory>
+
+#include "test_macros.h"
+
+int main() {
+  {
+    std::get_temporary_buffer<int>(1);
+  }
+}

libcxx/utils/libcxx/test/format.py

@@ -250,9 +250,8 @@ class LibcxxTestFormat(object):
             #
             # Therefore, we check if the test was expected to fail because of
             # nodiscard before enabling it
-            test_str = "ignoring return value of function declared with " \
-              + "'nodiscard' attribute"
-            if test_str in contents:
+            test_str_list = ['ignoring return value', 'nodiscard', 'NODISCARD']
+            if any(test_str in contents for test_str in test_str_list):
                 test_cxx.flags += ['-Werror=unused-result']
         cmd, out, err, rc = test_cxx.compile(source_path, out=os.devnull)
         expected_rc = 0 if use_verify else 1