opt-out multi-threaded feature flag

[flisky]

Objective

Fixes #9113

Solution

disable multi-threaded default feature

Migration Guide

The multi-threaded feature in bevy_ecs and bevy_tasks is no longer enabled by default. However, this remains a default feature for the umbrella bevy crate. If you depend on bevy_ecs or bevy_tasks directly, you should consider enabling this to allow systems to run in parallel.

[github-actions]

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[alice-i-cecile]

Hmm. Why make this a non-default feature rather than making sure users can set a single top-level feature flag?

Enabling multithreading seems like a defensible default behavior for bevy_ecs by itself to me.

[flisky]

Sorry, I don't quite get the making sure users can set a single top-level feature flag.

if we enable default multi-threaded for bevy_ecs, other bevy crates depends on bevy_ecs needs bevy_ecs = { ... default-features = false, features = ["bevy_reflect"]}, and (optional) expose multi-threaded by themselves, which I think it's maintain burden. I think most users use the umbrella bevy crate, which enables multi-threaded by default.

[github-actions]

It looks like your PR is a breaking change, but you didn't provide a migration guide.

Could you add some context on what users should update when this change get released in a new version of Bevy?
It will be used to help writing the migration guide for the version. Putting it after a ## Migration Guide will help it get automatically picked up by our tooling.

[alice-i-cecile]

Okay, I'm content with that justification. @james7132 and @mockersf will likely have opinions too on the best way to do this.

[james7132]

This definitely worsens the experience for people using bevy_ecs by itself, but I don't think we should negatively impact the experience of using it as a dependency for everyone else by requiring them to disable default features every time they use it.

We may need to explicitly document this as it does seem like most consumers of bevy_ecs, by default, want multi threaded execution.

This is also a breaking change and needs a migration guide.

[alice-i-cecile]

@flisky can you please add a migration guide section to the PR description and then I'll merge this. Let us know if you'd like a hand with that!

[flisky]

@alice-i-cecile thanks for the reminding, and any suggestion is appreciated.

[alice-i-cecile]

The multithreaded feature inbevy_ecs and bevy_tasks is no longer enabled by default. However, this remains a default feature for the umbrella bevy crate. If you depend on bevy_ecs or bevy_tasks directly, you should consider enabling this to allow systems to run in parallel.

Improved wording for you!

[flisky]

@alice-i-cecile Thank you!

[james7132]

This should still be explicitly documented, either in Cargo.toml for bevy_ecs or in some README, but I don't think we should block this PR on this.