Scary Rust Compiler Error

0.log

% rustc --version
rustc 1.10.0 (cfcb716cf 2016-07-03)

% cargo run
   Compiling scary-compiler-message v0.1.0 (file:///Users/jac/scary-compiler-message)
src/main.rs:27:36: 27:40 error: no method named `iter` found for type `(specs::Storage<Foo, std::sync::RwLockReadGuard<'_, specs::Allocator>, std::sync::RwLockReadGuard<'_, specs::MaskedStorage<Foo>>>, specs::Storage<Bar, std::sync::RwLockReadGuard<'_, specs::Allocator>, std::sync::RwLockReadGuard<'_, specs::MaskedStorage<Bar>>>)` in the current scope
src/main.rs:27     for (foo, bar) in (foos, bars).iter() {
                                                  ^~~~
src/main.rs:27:36: 27:40 note: the method `iter` exists but the following trait bounds were not satisfied: `specs::Storage<Foo, std::sync::RwLockReadGuard<'_, specs::Allocator>, std::sync::RwLockReadGuard<'_, specs::MaskedStorage<Foo>>> : specs::Join`, `specs::Storage<Bar, std::sync::RwLockReadGuard<'_, specs::Allocator>, std::sync::RwLockReadGuard<'_, specs::MaskedStorage<Bar>>> : specs::Join`, `specs::Storage<Foo, std::sync::RwLockReadGuard<'_, specs::Allocator>, std::sync::RwLockReadGuard<'_, specs::MaskedStorage<Foo>>> : specs::Join`, `specs::Storage<Bar, std::sync::RwLockReadGuard<'_, specs::Allocator>, std::sync::RwLockReadGuard<'_, specs::MaskedStorage<Bar>>> : specs::Join`
error: aborting due to previous error
error: Could not compile `scary-compiler-message`.

To learn more, run the command again with --verbose.

Cargo.toml

[package]
name = "scary-compiler-message"
version = "0.1.0"

[dependencies]
specs = "0.7.0"

main.rs

extern crate specs;
use specs::Join;

#[derive(Clone, Default)]
struct Foo;
impl specs::Component for Foo {
    type Storage = specs::NullStorage<Foo>;
}

#[derive(Clone, Default)]
struct Bar;
impl specs::Component for Bar {
    type Storage = specs::NullStorage<Bar>;
}

fn main() {
    let mut world = specs::World::new();
    world.register::<Foo>();
    world.register::<Bar>();
    
    let entity = world.create_now().with(Foo).with(Bar).build();

    let foos = world.read::<Foo>();
    let bars = world.read::<Bar>();

    // This should be (&foos, &bars).iter()
    // But the compiler error does not point the user towards a solution.
    for (foo, bar) in (foos, bars).iter() {
        println!("Hi");
    }
}