`back_to_the_buffer` cannot easily be used recursively

[tom-a-wagner]

As the title says, back_to_the_buffer can not easily be used from within another back_to_the_buffer call, which makes it hard to serialize some nested formats.

The default/recommended way to use back_to_the_buffer would be to use gen(<serializer>) inside the gen closure like in the docs.

back_to_the_buffer(
    4,
    move |buf| gen(string("test"), buf), // < this line
    move |buf, len| gen_simple(be_u32(len as u32), buf)
)

However, this wraps buf in a second WriteContext, so that the serializer is called with a WriteContext<WriteContext<W: Write + BackToTheBuffer>>.
With this, we can no longer call back_to_the_buffer again, since it requires a WriteContext<W: Write + BackToTheBuffer>.

It took me quite a while to come up with this solution: (adapted to the doc example)

back_to_the_buffer(
    4,
    move |buf| {
        let begin = buf.position;
        (string("test")(buf)).map(|wctx| {
            let pos = wctx.position;
            (wctx, pos - begin)
        })
    },
    move |buf, len| gen_simple(be_u32(len as u32), buf)
)

Now back_to_the_buffer could be called again in the stringcombinator.

Could this be fixed/made easier in a nice way?
If not, maybe this example could at least be added to the docs so that others don't run into the same wall as I did.

Thanks!

[str4d]

The above could be turned into a helper function, which would bring things back towards the docs:

fn gen_bttb<W: Write, F: SerializeFn<W>>(
    f: F,
    w: WriteContext<W>,
) -> Result<(WriteContext<W>, u64), GenError> {
    let begin = w.position;
    f(w).map(|wctx| {
        let pos = wctx.position;
        (wctx, pos - begin)
    })
}

back_to_the_buffer(
    4,
    move |buf| gen_bttb(string("test"), buf),
    move |buf, len| gen_simple(be_u32(len as u32), buf)
)