SystemBuilder construction without world access

[ecoskey]

Objective

• Enable constructing systems from param builders without world access
• Required for SystemRunner param and macro syntax #21811, which requires constructing systems from builders without world access

Solution

• Add BuilderSystem, a wrapper which defers state construction until the first run
• in addition to (...params).build_state(&mut world).build_system(|| {}), now you can also call (...params).build_system(|| {}) directly

Testing

• compiles

(will add more tests while fixing review nits)

[pablo-lua]

Linked PR merged, no more blocks

[pablo-lua]

I'm certain this is useful, but is it possible to elaborate on some use cases or examples maybe? (Not necessarily on the code, more like in the pr description)

[ecoskey]

I'm certain this is useful, but is it possible to elaborate on some use cases or examples maybe? (Not necessarily on the code, more like in the pr description)

good point. It's mostly motivated by #21811, since it requires constructing systems in-place without world access. I'll update the pr description 🙂

[kfc35]

I could not find any issues

[alice-i-cecile]

I like the changes, but I would like a test for the core claim "you can now construct SystemBuilder without world access". It's helpful as a reviewer for now, but much more important in the future to ensure that this doesn't accidentally break :)

I would also not complain about more docs on SystemParamBuilder about the intended usage / patterns :) Not blocking though.

[alice-i-cecile]

Let me know when CI passes :)