Bevy 0.15.1 change to fallible systems breaks projects

[Schmarni-Dev]

Bevy version

0.15.1

What went wrong

I tried to update to bevy 0.15.1 for a project and it crashed on startup, because of a missing systemparam (in this case an Event)
i get that the default in bevy 0.15.0 for missing systemparams was bad, i think panics are the better default, i support changing the default, but please don't do it in a patch release! especially without giving apps a way to change the default without forking bevy

so i tried to downgrade to 0.15.0; but the bevy crate doesn't depend on the exact version for subcrates, so while i was able to downgrade bevy, bevy_ecs and co are still 0.15.1, breaking my current code without a way back (other than cargo patching all crates to the git commit of bevy 0.15.0, maybe adding all the crates as app dependencies would work too idk)

[Schmarni-Dev]

luckily in my case i just needed to add a singe plugin, but still bad

[BenjaminBrienen]

The suggestion to panic is a reasonable stop-gap, and should have been done in the initial PR.
originally posted by @alice-i-cecile in #16718

To me, this is a good reason to put a change which is technically breaking into a patch release. From the perspective of engine maintainers, the feature which was shipped had a fundamental problem and didn't work as intended.

[alice-i-cecile]

Agreed. From Cart's (and the community's) perspective, the existing solution was fundamentally broken, and reverting to 0.14 behavior was preferred. I don't think we should do anything further here, but I'll leave it open for a bit more discussion.

[fallible-algebra]

The reversion to 0.14 behaviour does feel to me like a significant pain point, especially in a patch release after a release where "skip system/observer if its parameter's constraints aren't met" was lauded as a new feature that was a jump up in usability. It's disappointing to have to shift the mental model of how to work on my project in future or when cargo next decides that I should be depending on 0.15.1 because it's "only a patch change".

[alice-i-cecile]

At this point I think the die is cast. I agree that this was messy and painful though, and we'll be careful to avoid this sort of thing in the future.