readVec

        fn readVec(io_r: *Io.Reader, data: [][]u8) Io.Reader.Error!usize {
            const r: *Reader = @alignCast(@fieldParentPtr("interface", io_r));
            const io = r.io;
            var iovecs_buffer: [max_iovecs_len][]u8 = undefined;
            const dest_n, const data_size = try io_r.writableVector(&iovecs_buffer, data);
            const dest = iovecs_buffer[0..dest_n];
            assert(dest[0].len > 0);
            const n = r.stream.read(io, dest) catch |err| {
                r.err = err;
                return error.ReadFailed;
            };
            if (n == 0) {
                return error.EndOfStream;
            }
            if (n > data_size) {
                r.interface.end += n - data_size;
                return data_size;
            }
            return n;
        }