[libcxx-commits] [PATCH] D65900: [libc++] Use [[nodiscard]] for lock_guard, as an extension

[Louis Dionne via Phabricator via libcxx-commits]

ldionne created this revision.
ldionne added reviewers: mclow.lists, EricWF.
Herald added subscribers: libcxx-commits, dexonsmith, jkorous, christof.
Herald added a project: libc++.

D64914 <https://reviews.llvm.org/D64914> added support for applying [[nodiscard]] to constructors. This
commit uses that capability to flag incorrect uses of std::lock_guard
where one forgets to actually create a variable for the lock_guard.

rdar://45790820


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D65900

Files:
  libcxx/include/__config
  libcxx/include/__mutex_base
  libcxx/test/libcxx/diagnostics/nodiscard_extensions.fail.cpp
  libcxx/test/libcxx/thread/thread.lock/thread.lock.guard/nodiscard.fail.cpp


Index: libcxx/test/libcxx/thread/thread.lock/thread.lock.guard/nodiscard.fail.cpp
===================================================================
--- /dev/null
+++ libcxx/test/libcxx/thread/thread.lock/thread.lock.guard/nodiscard.fail.cpp
@@ -0,0 +1,32 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: libcpp-has-no-threads
+// UNSUPPORTED: clang-6, clang-6, clang-8, clang-9
+// UNSUPPORTED: apple-clang-9, apple-clang-10, apple-clang-11
+
+// <mutex>
+
+// template <class Mutex> class lock_guard;
+
+// [[nodiscard]] explicit lock_guard(mutex_type& m);
+// [[nodiscard]] lock_guard(mutex_type& m, adopt_lock_t);
+
+// Test that we properly apply [[nodiscard]] to lock_guard's constructors,
+// which is a libc++ extension.
+
+// MODULES_DEFINES: _LIBCPP_ENABLE_NODISCARD
+#define _LIBCPP_ENABLE_NODISCARD
+#include <mutex>
+
+int main(int, char**) {
+    std::mutex m;
+    std::lock_guard<std::mutex>{m}; // expected-error{{ignoring temporary created by a constructor declared with 'nodiscard' attribute}}
+    std::lock_guard<std::mutex>{m, std::adopt_lock}; // expected-error{{ignoring temporary created by a constructor declared with 'nodiscard' attribute}}
+    return 0;
+}
Index: libcxx/test/libcxx/diagnostics/nodiscard_extensions.fail.cpp
===================================================================
--- libcxx/test/libcxx/diagnostics/nodiscard_extensions.fail.cpp
+++ libcxx/test/libcxx/diagnostics/nodiscard_extensions.fail.cpp
@@ -16,7 +16,7 @@
 // guides from existing ctors, needed by default_searcher() below.
 // UNSUPPORTED: apple-clang-9
 
-// Test that entities declared [[nodiscard]] as at extension by libc++, are
+// Test that entities declared [[nodiscard]] as an 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
Index: libcxx/include/__mutex_base
===================================================================
--- libcxx/include/__mutex_base
+++ libcxx/include/__mutex_base
@@ -94,10 +94,11 @@
     mutex_type& __m_;
 public:
 
-    _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_NODISCARD_CTOR_EXT _LIBCPP_INLINE_VISIBILITY
     explicit lock_guard(mutex_type& __m) _LIBCPP_THREAD_SAFETY_ANNOTATION(acquire_capability(__m))
         : __m_(__m) {__m_.lock();}
-    _LIBCPP_INLINE_VISIBILITY
+
+    _LIBCPP_NODISCARD_CTOR_EXT _LIBCPP_INLINE_VISIBILITY
     lock_guard(mutex_type& __m, adopt_lock_t) _LIBCPP_THREAD_SAFETY_ANNOTATION(requires_capability(__m))
         : __m_(__m) {}
     _LIBCPP_INLINE_VISIBILITY
Index: libcxx/include/__config
===================================================================
--- libcxx/include/__config
+++ libcxx/include/__config
@@ -1010,6 +1010,19 @@
 #  define _LIBCPP_NODISCARD_AFTER_CXX17
 #endif
 
+#if __has_cpp_attribute(nodiscard) >= 201907L
+   // Attribute to use to mark a constructor as [[nodiscard]] when the
+   // Standard specifies the constructor as [[nodiscard]]
+#  define _LIBCPP_NODISCARD_CTOR _LIBCPP_NODISCARD
+
+   // Attribute to use to mark a constructor as [[nodiscard]] as a
+   // libc++ extension.
+#  define _LIBCPP_NODISCARD_CTOR_EXT _LIBCPP_NODISCARD_EXT
+#else
+#  define _LIBCPP_NODISCARD_CTOR     // compiler doesn't support P1771
+#  define _LIBCPP_NODISCARD_CTOR_EXT // compiler doesn't support P1771
+#endif
+
 #if _LIBCPP_STD_VER > 14 && defined(__cpp_inline_variables) && (__cpp_inline_variables >= 201606L)
 #  define _LIBCPP_INLINE_VAR inline
 #else


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