Return Future from trait impl: can't call and_then (on trait object)

help
1

I am relatively new to Rust and work on a library that works with data that is loaded by a method that is implemented by the library user. This method returns a Future (still futures 0.1 though for now). I have the following trait and an implementation for it:

// Implemented in library
#[derive(Clone, Debug)]
pub struct DataLoaderResult {
    path: String,
    byterange: Range<u64>,
    content: Vec<u8>,
    compress: String,
    raw: bool,
}

pub trait DataLoader {
    fn get(&self, path: String, progress: Option<bool>, tuples: Vec<(String, u64, u64)>, num: usize)
        -> Pin<Box<dyn Future<Item = HashMap<String, Result<DataLoaderResult, Error>>, Error = Error>>>;
}

// Implemented by lib user
impl DataLoader for HTTPDataLoader {
    fn get(&self, path: String, progress: Option<bool>, tuples: Vec<(String, u64, u64)>, num: usize)
        -> Pin<Box<dyn Future<Item = HashMap<String, Result<DataLoaderResult, Error>>, Error = Error>>> {
        Box::pin(HashMap::new());
    }
}

// This impl is stored as a field in a struct:
pub struct CacheService<'a> {
    pub data_loader: &'a dyn DataLoader,
}

The trait method returns a Future and I would like to call it in a method of CacheService above and then call and_then() on it, i.e.:

let load_data = self.data_loader.get(...);
load_data.and_then(|data| {
  // do something
}

However, if I do this, I get this error:

error: the `and_then` method cannot be invoked on a trait object
    --> /home/tom/dev/rust-test/src/lib.rs:1315:36
     |
1315 |         data.and_then(|remote_fragments| {
     |                     ^^^^^^^^
     |
    ::: /home/tom/.cargo/registry/src/index.crates.io-6f17d22bba15001f/futures-0.1.31/src/future/mod.rs:529:21
     |
529  |               Self: Sized,
     |                     ----- this has a `Sized` requirement

What is going on here? I would really appreciate some insight and possible solutions. It looks like that I have to use a trait object (because there are possibly different implementations of get(), but maybe that's a misconception.

2

Pinning wasn't a thing in futures v0.1. This compiles in the playground:

use futures_0_1_31::future::{ok, Future};
use std::collections::HashMap;

#[derive(Clone, Debug)]
struct DataLoaderResult;

struct HTTPDataLoader;

struct MyError;

trait DataLoader {
    fn get(&self) -> Box<dyn Future<Item = HashMap<String, DataLoaderResult>, Error = MyError>>;
}

// Implemented by lib user
impl DataLoader for HTTPDataLoader {
    fn get(&self) -> Box<dyn Future<Item = HashMap<String, DataLoaderResult>, Error = MyError>> {
        Box::new(ok(HashMap::new()))
    }
}

// This impl is stored as a field in a struct:
struct CacheService<'a> {
    pub data_loader: &'a dyn DataLoader,
}

fn main() {
    let loader: &dyn DataLoader = &HTTPDataLoader;
    let cache = CacheService {
        data_loader: loader,
    };

    let load_data = cache.data_loader.get();

    let _ = load_data.and_then(|_data| {
        // do something
        ok(())
    });
}
3

Thanks, this fixes it indeed! I must have confused the pinning with information on the core futures.