Convert to fallible system in IntoSystemConfigs

[hymm]

Objective

• Fallible systems #16589 added an enum to switch between fallible and infallible system. This branching should be unnecessary if we wrap infallible systems in a function to return Ok(()).

Solution

• Create a wrapper system for System<(), ()>s that returns Ok on the call to run and run_unsafe. The wrapper should compile out, but I haven't checked.
• I removed the impl IntoSystemConfigs for BoxedSystem<(), ()> as I couldn't figure out a way to keep the impl without double boxing.

Testing

• ran many_foxes example to check if it still runs.

Migration Guide

• IntoSystemConfigs has been removed for BoxedSystem<(), ()>. Either use InfallibleSystemWrapper before boxing or make your system return bevy::ecs::prelude::Result.

[alice-i-cecile]

@Neo-Zhixing how do you feel about this implementation instead?

[alice-i-cecile]

I prefer this design: I think it's clearer and reducing the branching is good.

[NthTensor]

Seems great! Have you tested logging and panics to see if this mangles the source tracking, as was the issue in #8705?

[hymm]

Have you tested logging and panics to see if this mangles the source tracking, as was the issue in #8705?

I'll check, but it should be fine. This should just add one more line to the backtrace.

[NthTensor]

This is clearly a much better approach. Thanks for tidying up after me :).

[hymm]

Panics seem ok.

thread '<unnamed>' panicked at examples/stress_tests/many_foxes.rs:116:5:
testing panics backtraces
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
Encountered a panic in system `many_foxes::setup`!
Encountered a panic in system `bevy_app::main_schedule::Main::run_main`!
error: process didn't exit successfully: `target\debug\examples\many_foxes.exe` (exit code: 101)

[Neo-Zhixing]

Oops I'm a bit late to the discussions, but this is clearly a much better approach.