type-level abstractions for scroll

something.rs

#![feature(try_from)] // for "anything that can be converted into usize"

extern crate scroll; // if you're not using the edition 2018 preview

use scroll::ctx::TryFromCtx;
use std::convert::TryInto;
use scroll::Pread;

struct LengthData<Data, Length>(Data, std::marker::PhantomData<Length>);

impl <'a, Data, Length> TryFromCtx<'a> for LengthData<Data, Length>
    where Data: TryFromCtx<'a, Size = usize> + 'a,
          Length: TryInto<usize, Error = std::num::TryFromIntError>
                + TryFromCtx<'a, Size = usize> + 'a,
          scroll::Error: std::convert::From<<Data as scroll::ctx::TryFromCtx<'a>>::Error>
                       + std::convert::From<<Length as scroll::ctx::TryFromCtx<'a>>::Error>,
          <Data as TryFromCtx<'a>>::Error: std::convert::From<scroll::Error>,
          <Length as TryFromCtx<'a>>::Error: std::convert::From<scroll::Error> {

    type Error = scroll::Error;
    type Size = usize;
    fn try_from_ctx(src: &'a [u8], _ctx: ()) -> Result<(Self, Self::Size), Self::Error> {
        let read = &mut 0usize;
        let length: usize = src.gread::<Length>(read)?.try_into().unwrap();

        Ok((LengthData((&src[*read..*read+length]).pread(0)?, std::marker::PhantomData), *read))
    }
}

// Reads an entire buffer as a UTF-16 string. maybe use StrCtx from scroll in the future?
struct UTF16<Endian>(String, std::marker::PhantomData<Endian>);

impl<'a, Endian> TryFromCtx<'a> for UTF16<Endian> where Endian: byteorder::ByteOrder + 'a {
    type Error = scroll::Error;
    type Size = usize;
    fn try_from_ctx(src: &'a [u8], _ctx: ()) -> Result<(Self, Self::Size), Self::Error> {
        if src.len() % 2 != 0 {
            return Err(scroll::Error::Custom("Length of utf-16 string is not a multiple of 2!".to_owned()));
        }

        let mut data = vec![0u16; src.len()/2];
        Endian::read_u16_into(src, &mut data);
        Ok((UTF16(String::from_utf16_lossy(&data).into(), std::marker::PhantomData), src.len()))
    }
}

impl<Length, Endian> From<LengthData<UTF16<Endian>, Length>> for String {
    fn from(src: LengthData<UTF16<Endian>, Length>) -> Self {
        (src.0).0
    }
}