fix(playback): avoid unnecessary boxing (#60)

Add a helper trait to upcast trait object.
see also: https://www.zhihu.com/question/643804984/answer/3393253605

anni-playback/src/decoder/decoder.rs

@@ -362,7 +362,7 @@ impl Decoder {
         buffer_signal: Arc<AtomicBool>,
     ) -> anyhow::Result<Playback> {
         let duration_hint = source.duration_hint();
-        let mss = MediaSourceStream::new(Box::new(source), Default::default());
+        let mss = MediaSourceStream::new(source.into(), Default::default());
         let format_options = FormatOptions {
             enable_gapless: true,
             ..Default::default()

anni-playback/src/sources/mod.rs

@@ -27,7 +27,7 @@ struct Receiver {
     receiver: std::sync::mpsc::Receiver<(usize, Vec<u8>)>,
 }
 
-pub trait AnniSource: MediaSource {
+pub trait AnniSource: MediaSource + IntoBoxedMediaSource {
     /// The duration of underlying source in seconds.
     fn duration_hint(&self) -> Option<u64> {
         None
@@ -46,6 +46,23 @@ impl MediaSource for Box<dyn AnniSource> {
 
 impl AnniSource for std::fs::File {}
 
+// helper trait to do upcasting
+pub trait IntoBoxedMediaSource {
+    fn into_media_source(self: Box<Self>) -> Box<dyn MediaSource>;
+}
+
+impl<T: MediaSource + 'static> IntoBoxedMediaSource for T {
+    fn into_media_source(self: Box<Self>) -> Box<dyn MediaSource> {
+        self
+    }
+}
+
+impl From<Box<dyn AnniSource>> for Box<dyn MediaSource> {
+    fn from(value: Box<dyn AnniSource>) -> Self {
+        value.into_media_source()
+    }
+}
+
 // Specialization is not well-supported so far (even the unstable feature is unstable ww).
 // Therefore, we do not provide the default implementation below.
 // Users can use a newtype pattern if needed.