nvdec: Make use of [[nodiscard]] where applicable

Prevents bugs from occurring where the results of a function are
accidentally discarded

src/video_core/command_classes/codecs/codec.h

@@ -42,11 +42,11 @@ public:
     void Decode();
 
     /// Returns most recently decoded frame
-    AVFrame* GetCurrentFrame();
-    const AVFrame* GetCurrentFrame() const;
+    [[nodiscard]] AVFrame* GetCurrentFrame();
+    [[nodiscard]] const AVFrame* GetCurrentFrame() const;
 
     /// Returns the value of current_codec
-    NvdecCommon::VideoCodec GetCurrentCodec() const;
+    [[nodiscard]] NvdecCommon::VideoCodec GetCurrentCodec() const;
 
 private:
     bool initialized{};

src/video_core/command_classes/codecs/h264.cpp

@@ -43,7 +43,8 @@ H264::H264(GPU& gpu_) : gpu(gpu_) {}
 
 H264::~H264() = default;
 
-std::vector<u8>& H264::ComposeFrameHeader(NvdecCommon::NvdecRegisters& state, bool is_first_frame) {
+const std::vector<u8>& H264::ComposeFrameHeader(NvdecCommon::NvdecRegisters& state,
+                                                bool is_first_frame) {
     H264DecoderContext context{};
     gpu.MemoryManager().ReadBlock(state.picture_info_offset, &context, sizeof(H264DecoderContext));
 

src/video_core/command_classes/codecs/h264.h

@@ -51,14 +51,14 @@ public:
     void WriteScalingList(const std::vector<u8>& list, s32 start, s32 count);
 
     /// Return the bitstream as a vector.
-    std::vector<u8>& GetByteArray();
-    const std::vector<u8>& GetByteArray() const;
+    [[nodiscard]] std::vector<u8>& GetByteArray();
+    [[nodiscard]] const std::vector<u8>& GetByteArray() const;
 
 private:
     void WriteBits(s32 value, s32 bit_count);
     void WriteExpGolombCodedInt(s32 value);
     void WriteExpGolombCodedUInt(u32 value);
-    s32 GetFreeBufferBits();
+    [[nodiscard]] s32 GetFreeBufferBits();
     void Flush();
 
     s32 buffer_size{8};
@@ -74,8 +74,8 @@ public:
     ~H264();
 
     /// Compose the H264 header of the frame for FFmpeg decoding
-    std::vector<u8>& ComposeFrameHeader(NvdecCommon::NvdecRegisters& state,
-                                        bool is_first_frame = false);
+    [[nodiscard]] const std::vector<u8>& ComposeFrameHeader(NvdecCommon::NvdecRegisters& state,
+                                                            bool is_first_frame = false);
 
 private:
     struct H264ParameterSet {

src/video_core/command_classes/codecs/vp9.cpp

@@ -854,7 +854,7 @@ VpxBitStreamWriter VP9::ComposeUncompressedHeader() {
     return uncomp_writer;
 }
 
-std::vector<u8>& VP9::ComposeFrameHeader(NvdecCommon::NvdecRegisters& state) {
+const std::vector<u8>& VP9::ComposeFrameHeader(NvdecCommon::NvdecRegisters& state) {
     std::vector<u8> bitstream;
     {
         Vp9FrameContainer curr_frame = GetCurrentFrame(state);

src/video_core/command_classes/codecs/vp9.h

@@ -119,7 +119,7 @@ public:
 
     /// Composes the VP9 frame from the GPU state information. Based on the official VP9 spec
     /// documentation
-    std::vector<u8>& ComposeFrameHeader(NvdecCommon::NvdecRegisters& state);
+    [[nodiscard]] const std::vector<u8>& ComposeFrameHeader(NvdecCommon::NvdecRegisters& state);
 
     /// Returns true if the most recent frame was a hidden frame.
     [[nodiscard]] bool WasFrameHidden() const {

src/video_core/command_classes/codecs/vp9_types.h

@@ -231,9 +231,8 @@ struct PictureInfo {
     u32 surface_params{};
     INSERT_PADDING_WORDS(3);
 
-    Vp9PictureInfo Convert() const {
-
-        return Vp9PictureInfo{
+    [[nodiscard]] Vp9PictureInfo Convert() const {
+        return {
             .is_key_frame = (vp9_flags & FrameFlags::IsKeyFrame) != 0,
             .intra_only = (vp9_flags & FrameFlags::IntraOnly) != 0,
             .last_frame_was_key = (vp9_flags & FrameFlags::LastFrameIsKeyFrame) != 0,

src/video_core/command_classes/nvdec.h

@@ -26,8 +26,8 @@ public:
     void ProcessMethod(Method method, const std::vector<u32>& arguments);
 
     /// Return most recently decoded frame
-    AVFrame* GetFrame();
-    const AVFrame* GetFrame() const;
+    [[nodiscard]] AVFrame* GetFrame();
+    [[nodiscard]] const AVFrame* GetFrame() const;
 
 private:
     /// Invoke codec to decode a frame