libstdc++: Add [[nodiscard]] to lock types

Adding this attribute means users get a warning when they accidentally
create a temporary lock instead of creating an automatic variable with
block scope.

For std::lock_guard both constructors have side effects (they both take
a mutex and so both cause it to be unlocked at the end of the full
expression when a temporary is constructed). Ideally we would just put
the attribute on the class instead of the constructors, but that doesn't
work with GCC (PR c++/85973).

For std::unique_lock the default constructor and std::defer_lock_t
constructor do not cause any locking or unlocking, so do not need to
give a warning. It might still be a mistake to create a temporary using
those constructors, but it's harmless and seems unlikely anyway. For a
lock object created with one of those constructors you would expect the
lock object to be referred to later in the function, and that would not
even compile if it was constructed as an unnamed temporary.

std::scoped_lock gets the same treatment as std::lock_guard, except that
the explicit specialization for zero lockables has no side effects so
doesn't need to warn.

libstdc++-v3/ChangeLog:

	* include/bits/std_mutex.h (lock_guard): Add [[nodiscard]]
	attribute to constructors.
	* include/bits/unique_lock.h (unique_lock): Likewise.
	* include/std/mutex (scoped_lock, scoped_lock<Mutex>): Likewise.
	* testsuite/30_threads/lock_guard/cons/nodiscard.cc: New test.
	* testsuite/30_threads/scoped_lock/cons/nodiscard.cc: New test.
	* testsuite/30_threads/unique_lock/cons/nodiscard.cc: New test.

6 files changed, 101 insertions, 0 deletions

diff --git a/libstdc++-v3/include/bits/std_mutex.h b/libstdc++-v3/include/bits/std_mutex.h
index 4693055..9ac8c76 100644
--- a/libstdc++-v3/include/bits/std_mutex.h
+++ b/libstdc++-v3/include/bits/std_mutex.h
@@ -245,9 +245,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     public:
       typedef _Mutex mutex_type;
 
+      [[__nodiscard__]]
       explicit lock_guard(mutex_type& __m) : _M_device(__m)
       { _M_device.lock(); }
 
+      [[__nodiscard__]]
       lock_guard(mutex_type& __m, adopt_lock_t) noexcept : _M_device(__m)
       { } // calling thread owns mutex
 
diff --git a/libstdc++-v3/include/bits/unique_lock.h b/libstdc++-v3/include/bits/unique_lock.h
index c28e645..07474d2 100644
--- a/libstdc++-v3/include/bits/unique_lock.h
+++ b/libstdc++-v3/include/bits/unique_lock.h
@@ -66,6 +66,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       : _M_device(0), _M_owns(false)
       { }
 
+      [[__nodiscard__]]
       explicit unique_lock(mutex_type& __m)
       : _M_device(std::__addressof(__m)), _M_owns(false)
       {
@@ -77,10 +78,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       : _M_device(std::__addressof(__m)), _M_owns(false)
       { }
 
+      [[__nodiscard__]]
       unique_lock(mutex_type& __m, try_to_lock_t)
       : _M_device(std::__addressof(__m)), _M_owns(_M_device->try_lock())
       { }
 
+      [[__nodiscard__]]
       unique_lock(mutex_type& __m, adopt_lock_t) noexcept
       : _M_device(std::__addressof(__m)), _M_owns(true)
       {
@@ -88,6 +91,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       }
 
       template<typename _Clock, typename _Duration>
+	[[__nodiscard__]]
 	unique_lock(mutex_type& __m,
 		    const chrono::time_point<_Clock, _Duration>& __atime)
 	: _M_device(std::__addressof(__m)),
@@ -95,6 +99,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	{ }
 
       template<typename _Rep, typename _Period>
+	[[__nodiscard__]]
 	unique_lock(mutex_type& __m,
 		    const chrono::duration<_Rep, _Period>& __rtime)
 	: _M_device(std::__addressof(__m)),
diff --git a/libstdc++-v3/include/std/mutex b/libstdc++-v3/include/std/mutex
index bd3a1cb..9d22ce8 100644
--- a/libstdc++-v3/include/std/mutex
+++ b/libstdc++-v3/include/std/mutex
@@ -744,9 +744,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     class scoped_lock
     {
     public:
+
+      [[nodiscard]]
       explicit scoped_lock(_MutexTypes&... __m) : _M_devices(std::tie(__m...))
       { std::lock(__m...); }
 
+      [[nodiscard]]
       explicit scoped_lock(adopt_lock_t, _MutexTypes&... __m) noexcept
       : _M_devices(std::tie(__m...))
       { } // calling thread owns mutex
@@ -779,9 +782,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     public:
       using mutex_type = _Mutex;
 
+      [[nodiscard]]
       explicit scoped_lock(mutex_type& __m) : _M_device(__m)
       { _M_device.lock(); }
 
+      [[nodiscard]]
       explicit scoped_lock(adopt_lock_t, mutex_type& __m) noexcept
       : _M_device(__m)
       { } // calling thread owns mutex
diff --git a/libstdc++-v3/testsuite/30_threads/lock_guard/cons/nodiscard.cc b/libstdc++-v3/testsuite/30_threads/lock_guard/cons/nodiscard.cc
new file mode 100644
index 0000000..50137f9
--- /dev/null
+++ b/libstdc++-v3/testsuite/30_threads/lock_guard/cons/nodiscard.cc
@@ -0,0 +1,20 @@
+// { dg-do compile { target c++11 } }
+
+#include <mutex>
+
+struct Mutex
+{
+  void lock();
+  void unlock();
+  bool try_lock();
+};
+
+using namespace std;
+
+void
+test_nodiscard(Mutex& m)
+{
+  lock_guard<Mutex>{m}; // { dg-warning "ignoring return value" }
+  lock_guard<Mutex>{m, adopt_lock}; // { dg-warning "ignoring return value" }
+  lock_guard<Mutex>(m, adopt_lock); // { dg-warning "ignoring return value" }
+}
diff --git a/libstdc++-v3/testsuite/30_threads/scoped_lock/cons/nodiscard.cc b/libstdc++-v3/testsuite/30_threads/scoped_lock/cons/nodiscard.cc
new file mode 100644
index 0000000..ca673e2
--- /dev/null
+++ b/libstdc++-v3/testsuite/30_threads/scoped_lock/cons/nodiscard.cc
@@ -0,0 +1,29 @@
+// { dg-do compile { target c++17 } }
+// { dg-require-gthreads "" }
+// { dg-additional-options "-pthread" { target pthread } }
+
+#include <mutex>
+
+struct Mutex
+{
+  void lock();
+  void unlock();
+  bool try_lock();
+};
+
+using namespace std;
+
+void
+test_nodiscard(Mutex& m, mutex& m2)
+{
+  scoped_lock<>{}; // no warning
+  scoped_lock<>(); // no warning
+
+  scoped_lock{m}; // { dg-warning "ignoring return value" }
+  scoped_lock{adopt_lock, m}; // { dg-warning "ignoring return value" }
+  scoped_lock(adopt_lock, m); // { dg-warning "ignoring return value" }
+  scoped_lock(m, m2); // { dg-warning "ignoring return value" }
+  scoped_lock{m, m2}; // { dg-warning "ignoring return value" }
+  scoped_lock{adopt_lock, m, m2}; // { dg-warning "ignoring return value" }
+  scoped_lock(adopt_lock, m, m2); // { dg-warning "ignoring return value" }
+}
diff --git a/libstdc++-v3/testsuite/30_threads/unique_lock/cons/nodiscard.cc b/libstdc++-v3/testsuite/30_threads/unique_lock/cons/nodiscard.cc
new file mode 100644
index 0000000..79e347e
--- /dev/null
+++ b/libstdc++-v3/testsuite/30_threads/unique_lock/cons/nodiscard.cc
@@ -0,0 +1,40 @@
+// { dg-do compile { target c++11 } }
+
+#include <mutex>
+#include <chrono>
+
+using namespace std;
+
+struct Mutex
+{
+  void lock();
+  void unlock();
+  bool try_lock();
+
+  bool try_lock_for(chrono::seconds);
+  bool try_lock_until(chrono::system_clock::time_point);
+};
+
+
+void
+test_nodiscard(Mutex& m)
+{
+  unique_lock<Mutex>(); // no warning
+  unique_lock<Mutex>{}; // no warning
+  unique_lock<Mutex>(m, defer_lock); // no warning
+  unique_lock<Mutex>{m, defer_lock}; // no warning
+
+  unique_lock<Mutex>{m}; // { dg-warning "ignoring return value" }
+  unique_lock<Mutex>{m, try_to_lock}; // { dg-warning "ignoring return value" }
+  unique_lock<Mutex>(m, try_to_lock); // { dg-warning "ignoring return value" }
+  unique_lock<Mutex>{m, adopt_lock}; // { dg-warning "ignoring return value" }
+  unique_lock<Mutex>(m, adopt_lock); // { dg-warning "ignoring return value" }
+
+  chrono::seconds reltime(1);
+  unique_lock<Mutex>{m, reltime}; // { dg-warning "ignoring return value" }
+  unique_lock<Mutex>(m, reltime); // { dg-warning "ignoring return value" }
+  chrono::system_clock::time_point abstime(reltime);
+  unique_lock<Mutex>{m, abstime}; // { dg-warning "ignoring return value" }
+  unique_lock<Mutex>(m, abstime); // { dg-warning "ignoring return value" }
+}
+