add support for non-'a lifetimes when deriving SystemParam

[ahouts]

Objective

Fixes #2587

Solution

Check if the struct definition contains a lifetime. If it does, use that instead of 'a.

[alice-i-cecile]

Hey, thanks for fixing this! It was a very silly little bug.

[mockersf]

Maybe not compatible with #2605

[alice-i-cecile]

IMO we should merge this ASAP, then rebase #2605 onto it. This change is much simpler, and I tend to find that order is much easier to fix things in.

[cart]

I think first we should have a conversation about how deriving will work with the new lifetimes. I can think of a couple of paths forward:

1. Use lifetime orders to map the x and y from struct MyParam<'x, 'y> to SystemParam<'w, 's>. If only one lifetime exists, assume it is w. If you have a struct that only needs an s lifetime, you must define both and then declare PhantomData for w.
2. Only allow w and s lifetimes. Detect w-only, s-only, and w + s cases (to avoid the need for PhantomData) and handle them appropriately. Hard-code w and s in the SystemParam trait impl.

I'm honestly more in favor of (2). It requires using specific lifetimes, but it doesn't have weird implicit ordering requirements and seems more flexible / compatible with other non-w/s lifetimes. No matter what, making this work nicely is going outside the bounds of normal expectations. We'll need to make compromises somewhere.

[alice-i-cecile]

I'm happy with 2 if the behavior is explicitly documented :)

[ahouts]

Another option is to require an annotation on each lifetime. It would be more verbose, but at least it doesn't depend on seemingly innocuous things like lifetime name or ordering.

#[derive(SystemParam)]
struct MyParam<#[bevy_ecs(world)] 'w, #[bevy_ecs(state)] 's]> {
    foo: Res<'w, usize>,
    bar: Res<'s, usize>,
}

Or if you don't mind adding annotations that might conflict with other macros:

#[derive(SystemParam)]
struct MyParam<#[world] 'w, #[state] 's]> {
    foo: Res<'w, usize>,
    bar: Res<'s, usize>,
}

[ahouts]

Regardless of what is chosen, this PR as written is no longer applicable.