Reproduction for rust-lang/rfcs#1409

Cargo.toml

[package]
name = "repro"
version = "0.1.0"
authors = ["Jonathan Reem <[email protected]>"]

[dependencies]
scoped-pool = "*"
scopeguard = "*"

lib.rs

#[macro_use]
extern crate scopeguard; // for defer!, https://github.com/bluss/scopeguard
extern crate scoped_pool; // https://github.com/reem/rust-scoped-pool

use scoped_pool::{Scope, Pool};

#[derive(Default)]
pub struct Fs {
     _blah: Vec<u8> // details
}

impl Fs {
    fn init<'fs>(&'fs self, _: &Scope<'fs>) {
        // Pass off (self, scope) to an internal type which spawns a bunch of worker
        // threads that operate on self - this function does not block.
    }

    // Starts a threadpool, creates Fs, passes it to F, takes care to clean up on panic etc.
    pub fn run<F, R>(threads: usize, fun: F) -> R
    where F: for<'fs> FnOnce(&'fs Fs, &Scope<'fs>) -> R { // Problematic clause
         let pool = Pool::new(threads);
         defer!(pool.shutdown());

         let fs = &Fs::default();
         pool.scoped(move |scope| {
              fs.init(scope); // Start threadpool.
              defer!(fs.shutdown());
              fun(fs, scope)
         })
     }

     // Stops worker threads started by init
     fn shutdown(&self) { }
}

#[cfg(test)]
mod tests {
    use super::Fs;
    use std::sync::Mutex;

    #[test]
    fn it_doesnt_work() {
        let m = &::std::sync::Mutex::new(());

        Fs::run(12, |fs, scope| {
            scope.execute(move || {
                let _l = m.lock().unwrap();
            });
        });
    }
}