[libc++] Use [[nodiscard]] for lock_guard, as an extension
Summary:
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
Reviewers: mclow.lists, EricWF
Subscribers: christof, jkorous, dexonsmith, libcxx-commits, Quuxplusone, lebedev.ri
Tags: #libc
Differential Revision: https://reviews.llvm.org/D65900
git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@368664 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/docs/UsingLibcxx.rst b/docs/UsingLibcxx.rst
index 56c98af..05721bf 100644
--- a/docs/UsingLibcxx.rst
+++ b/docs/UsingLibcxx.rst
@@ -345,3 +345,4 @@
* ``search``
* ``unique``
* ``upper_bound``
+* ``lock_guard``'s constructors
diff --git a/include/__mutex_base b/include/__mutex_base
index f828bea..a85ded8 100644
--- a/include/__mutex_base
+++ b/include/__mutex_base
@@ -94,10 +94,11 @@
mutex_type& __m_;
public:
- _LIBCPP_INLINE_VISIBILITY
+ _LIBCPP_NODISCARD_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_EXT _LIBCPP_INLINE_VISIBILITY
lock_guard(mutex_type& __m, adopt_lock_t) _LIBCPP_THREAD_SAFETY_ANNOTATION(requires_capability(__m))
: __m_(__m) {}
_LIBCPP_INLINE_VISIBILITY
diff --git a/test/libcxx/diagnostics/nodiscard_extensions.fail.cpp b/test/libcxx/diagnostics/nodiscard_extensions.fail.cpp
index 10582b9..43cc080 100644
--- a/test/libcxx/diagnostics/nodiscard_extensions.fail.cpp
+++ b/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
diff --git a/test/libcxx/thread/thread.lock/thread.lock.guard/nodiscard.fail.cpp b/test/libcxx/thread/thread.lock/thread.lock.guard/nodiscard.fail.cpp
new file mode 100644
index 0000000..bac255c
--- /dev/null
+++ b/test/libcxx/thread/thread.lock/thread.lock.guard/nodiscard.fail.cpp
@@ -0,0 +1,34 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+
+// [[nodiscard]] on constructors isn't supported by all compilers
+// 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;
+}
