"error: reached the recursion limit during monomorphization (selection ambiguity)" with chunks

[conradev]

Compiling the following code fails with:

error: reached the recursion limit during monomorphization (selection ambiguity)

extern crate futures; // 0.1.10

use futures::{stream, Stream, Future};
use futures::sync::oneshot;

fn main() {
    let (tx, rx) = oneshot::channel(); // future to fetch a collection of objects
    tx.complete((0..200).collect());

    rx.map(|l: Vec<usize>| stream::iter(l.into_iter().map(|i| Ok(i))))
        .flatten_stream()
        .chunks(50)
        .map(|i| Ok(i)) // do work with batch of objects
        .buffer_unordered(5)
        .collect();
}

rustc 1.15.0 (10893a9a3 2017-01-19)

The idea here is to batch process a bunch of objects. If I remove the chunks(50) line, the code successfully compiles 😮

[dwrensha]

Possibly related to #367.

[conradev]

Some more info:

• This happens all the way back to 0.1.3, when Chunks was introduced.
• I resolved the issue for now by waiting synchronously for the collection of objects, and then making the stream, removing the need for the map and flatten_stream.

[alexcrichton]

I wonder if this may be a bug in the compiler? We've caught some before through this library. It'd be helpful to reduce the library to the bare bones though to see what's necessary to trigger the bug.

[alexcrichton]

Ok I've reduced it at rust-lang/rust#40003

[goffrie]

I suppose this issue can be closed now?