# Objective
- Required by #16622 due to differing implementations of `System` by
`FunctionSystem` and `ExclusiveFunctionSystem`.
- Optimize the memory usage of instances of `apply_deferred` in system
schedules.
## Solution
By changing `apply_deferred` from being an ordinary system that ends up
as an `ExclusiveFunctionSystem`, and instead into a ZST struct that
implements `System` manually, we save ~320 bytes per instance of
`apply_deferred` in any schedule.
## Testing
- All current tests pass.
---
## Migration Guide
- If you were previously calling the special `apply_deferred` system via
`apply_deferred(world)`, don't.
# Objective
- Contributes to #15460
## Solution
- Removed `petgraph` as a dependency from the `bevy_ecs` crate.
- Replaced `TarjanScc` and `GraphMap` with specialised in-tree
alternatives.
## Testing
- Ran CI locally.
- Added new unit tests to check ordering invariants.
- Confirmed `petgraph` is no longer present in `cargo tree -p bevy_ecs`
## Migration Guide
The `Dag::graph` method no longer returns a `petgraph` `DiGraph` and
instead returns the new `DiGraph` type within `bevy_ecs`. Edge and node
iteration methods are provided so conversion to the `petgraph` type
should be trivial if required.
## Notes
- `indexmap` was already in the dependency graph for `bevy_ecs`, so its
inclusion here makes no difference to compilation time for Bevy.
- The implementation for `Graph` is heavily inspired from the `petgraph`
original, with specialisations added to simplify and improve the type.
- `petgraph` does have public plans for `no_std` support, however there
is no timeframe on if or when that functionality will be available.
Moving to an in-house solution in the interim allows Bevy to continue
developing its `no_std` offerings and further explore alternate graphing
options.
---------
Co-authored-by: Lixou <82600264+DasLixou@users.noreply.github.com>
Co-authored-by: vero <11307157+atlv24@users.noreply.github.com>
