Exclusive systems accessing World in their signature cannot be chained.

[AlexOkafor]

Bevy version

https://github.com/bevyengine/bevy/tree/v0.7.0
83c6ffb

What you did

When trying to setup fallible systems, I had a case where one system needed exclusive access to the world, but could also error. Upon chaining it with an error handler I would get a compile error. Other exclusive systems that didn't refer to the world seemed to work. I'm new to rust so I couldn't exactly make sense of what the type errors was wanting from me. Below is a minimal repo case of the compile error:

// main.rs
use bevy::prelude::*;

struct DummyEvent;

fn exclusive_1(mut _writer: EventWriter<DummyEvent>) -> std::io::Result<()> {
    Ok(())
}

fn exclusive_2(_world: &mut World) -> std::io::Result<()> {
    Ok(())
}

fn error_handler(In(result): In<std::io::Result<()>>) {
    //noop
}

fn main() {
    App::new()
        .add_plugins(DefaultPlugins)
        .add_event::<DummyEvent>()
        .add_startup_system(exclusive_1.chain(error_handler).exclusive_system())
        // compile error when using a different signature system
        .add_startup_system(exclusive_2.chain(error_handler).exclusive_system())
        .run();
}

What went wrong

I was expecting systems that have the signature:
fn exclusive_2(_world: &mut World) -> std::io::Result<()> can be chained as exclusive systems, but the compiler suggests there are some trait bounds not being satisfied.

Additional information

Compiler error below:

error[E0599]: the method `chain` exists for fn item `for<'r> fn(&'r mut bevy::prelude::World) -> Result<(), std::io::Error> {exclusive_2}`, but its trait bounds were not satisfied
  --> src\main.rs:23:41
   |
23 |         .add_startup_system(exclusive_2.chain(error_handler).exclusive_system())
   |                                         ^^^^^ method cannot be called on `for<'r> fn(&'r mut bevy::prelude::World) -> Result<(), std::io::Error> {exclusive_2}` due to unsatisfied trait bounds
   |
   = note: `exclusive_2` is a function, perhaps you wish to call it
   = note: the following trait bounds were not satisfied:
           `for<'r> fn(&'r mut bevy::prelude::World) -> Result<(), std::io::Error> {exclusive_2}: IntoSystem<(), _, _>`
           which is required by `for<'r> fn(&'r mut bevy::prelude::World) -> Result<(), std::io::Error> {exclusive_2}: bevy::prelude::IntoChainSystem<_, _, _, _, _>`
           `&for<'r> fn(&'r mut bevy::prelude::World) -> Result<(), std::io::Error> {exclusive_2}: IntoSystem<(), _, _>`
           which is required by `&for<'r> fn(&'r mut bevy::prelude::World) -> Result<(), std::io::Error> {exclusive_2}: bevy::prelude::IntoChainSystem<_, _, _, _, _>`
           `&mut for<'r> fn(&'r mut bevy::prelude::World) -> Result<(), std::io::Error> {exclusive_2}: IntoSystem<(), _, _>`
           which is required by `&mut for<'r> fn(&'r mut bevy::prelude::World) -> Result<(), std::io::Error> {exclusive_2}: bevy::prelude::IntoChainSystem<_, _, _, _, _>`
           `for<'r> fn(&'r mut bevy::prelude::World) -> Result<(), std::io::Error> {exclusive_2}: Iterator`
           which is required by `&mut for<'r> fn(&'r mut bevy::prelude::World) -> Result<(), std::io::Error> {exclusive_2}: Iterator`

For more information about this error, try `rustc --explain E0599`.
error: could not compile `chain-repo` due to previous error

[DJMcNab]

This would be resolved by #4166

I think that exclusive_2.exclusive_system().chain(whatever) might work?

[AlexOkafor]

Thanks for the heads up on that PR! Just tried what you suggested and unfortunately won't work as-is (0.7.0).