// My media source implementation:
struct DatasetMediaSource<'a> {
    r: hdf5::ByteReader<'a>, // Implements io::Read and io::Seek, but does not have a static life time
    size: u64,
}
impl<'a> DatasetMediaSource<'a> {
    fn new(ds: &'a hdf5::Dataset) -> Result<DatasetMediaSource<'a>> {
        Ok(DatasetMediaSource {
            r: ds.as_byte_reader()?,
            size: ds.size() as u64,
        })
    }
}
impl<'a> Read for DatasetMediaSource<'a> {
    fn read(&mut self, buf: &mut [u8]) -> std::io::Result<usize> {
        self.r.read(buf)
    }
}
impl<'a> Seek for DatasetMediaSource<'a> {
    fn seek(&mut self, pos: SeekFrom) -> std::io::Result<u64> {
        self.r.seek(pos)
    }
}
impl<'a> MediaSource for DatasetMediaSource<'a> {
    fn is_seekable(&self) -> bool {
        true
    }

    fn byte_len(&self) -> Option<u64> {
        Some(self.size)
    }
}

// media_source_stream.rs
pub struct MediaSourceStream {
    /// The source reader.
    inner: Box<dyn MediaSource>, // same as: Box<dyn MediaSource + 'static>
   /// ...

let source = Box::new(DatasetMediaSource::new(&ds)?); // type: Box<dyn MediaSource + 'a>
                                              ^^^ borrowed value does not live long enough