[clang-tidy] use nodiscard

Introduced in C++17. It replaces gcc's warn_unused_result.

Found with modernize-use-nodiscard.

Signed-off-by: Rosen Penev <rosenp@gmail.com>

src/output/plugins/FifoOutputPlugin.cxx

@@ -64,7 +64,7 @@ private:
 	void Open(AudioFormat &audio_format) override;
 	void Close() noexcept override;
 
-	std::chrono::steady_clock::duration Delay() const noexcept override;
+	[[nodiscard]] std::chrono::steady_clock::duration Delay() const noexcept override;
 	size_t Play(const void *chunk, size_t size) override;
 	void Cancel() noexcept override;
 };

src/output/plugins/NullOutputPlugin.cxx

@@ -47,7 +47,7 @@ private:
 			delete timer;
 	}
 
-	std::chrono::steady_clock::duration Delay() const noexcept override {
+	[[nodiscard]] std::chrono::steady_clock::duration Delay() const noexcept override {
 		return sync && timer->IsStarted()
 			? timer->GetDelay()
 			: std::chrono::steady_clock::duration::zero();

src/output/plugins/OpenALOutputPlugin.cxx

@@ -56,7 +56,7 @@ private:
 	void Open(AudioFormat &audio_format) override;
 	void Close() noexcept override;
 
-	gcc_pure
+	[[nodiscard]] gcc_pure
 	std::chrono::steady_clock::duration Delay() const noexcept override {
 		return filled < NUM_BUFFERS || HasProcessed()
 			? std::chrono::steady_clock::duration::zero()
@@ -71,19 +71,19 @@ private:
 	void Cancel() noexcept override;
 
 private:
-	gcc_pure
+	[[nodiscard]] gcc_pure
 	ALint GetSourceI(ALenum param) const noexcept {
 		ALint value;
 		alGetSourcei(source, param, &value);
 		return value;
 	}
 
-	gcc_pure
+	[[nodiscard]] gcc_pure
 	bool HasProcessed() const noexcept {
 		return GetSourceI(AL_BUFFERS_PROCESSED) > 0;
 	}
 
-	gcc_pure
+	[[nodiscard]] gcc_pure
 	bool IsPlaying() const noexcept {
 		return GetSourceI(AL_SOURCE_STATE) == AL_PLAYING;
 	}

src/output/plugins/PulseOutputPlugin.cxx

@@ -99,7 +99,7 @@ public:
 	void Open(AudioFormat &audio_format) override;
 	void Close() noexcept override;
 
-	std::chrono::steady_clock::duration Delay() const noexcept override;
+	[[nodiscard]] std::chrono::steady_clock::duration Delay() const noexcept override;
 	size_t Play(const void *chunk, size_t size) override;
 	void Cancel() noexcept override;
 	bool Pause() override;

src/output/plugins/RecorderOutputPlugin.cxx

@@ -88,7 +88,7 @@ private:
 	size_t Play(const void *chunk, size_t size) override;
 
 private:
-	gcc_pure
+	[[nodiscard]] gcc_pure
 	bool HasDynamicPath() const noexcept {
 		return !format_path.empty();
 	}

src/output/plugins/ShoutOutputPlugin.cxx

@@ -57,7 +57,7 @@ struct ShoutOutput final : AudioOutput {
 	void Open(AudioFormat &audio_format) override;
 	void Close() noexcept override;
 
-	std::chrono::steady_clock::duration Delay() const noexcept override;
+	[[nodiscard]] std::chrono::steady_clock::duration Delay() const noexcept override;
 	void SendTag(const Tag &tag) override;
 	size_t Play(const void *chunk, size_t size) override;
 	void Cancel() noexcept override;