# Objective
This is a necessary precursor to #9122 (this was split from that PR to
reduce the amount of code to review all at once).
Moving `!Send` resource ownership to `App` will make it unambiguously
`!Send`. `SubApp` must be `Send`, so it can't wrap `App`.
## Solution
Refactor `App` and `SubApp` to not have a recursive relationship. Since
`SubApp` no longer wraps `App`, once `!Send` resources are moved out of
`World` and into `App`, `SubApp` will become unambiguously `Send`.
There could be less code duplication between `App` and `SubApp`, but
that would break `App` method chaining.
## Changelog
- `SubApp` no longer wraps `App`.
- `App` fields are no longer publicly accessible.
- `App` can no longer be converted into a `SubApp`.
- Various methods now return references to a `SubApp` instead of an
`App`.
## Migration Guide
- To construct a sub-app, use `SubApp::new()`. `App` can no longer
convert into `SubApp`.
- If you implemented a trait for `App`, you may want to implement it for
`SubApp` as well.
- If you're accessing `app.world` directly, you now have to use
`app.world()` and `app.world_mut()`.
- `App::sub_app` now returns `&SubApp`.
- `App::sub_app_mut` now returns `&mut SubApp`.
- `App::get_sub_app` now returns `Option<&SubApp>.`
- `App::get_sub_app_mut` now returns `Option<&mut SubApp>.`
# Objective
It's useful to have access to render pipeline statistics, since they
provide more information than FPS alone. For example, the number of
drawn triangles can be used to debug culling and LODs. The number of
fragment shader invocations can provide a more stable alternative metric
than GPU elapsed time.
See also: Render node GPU timing overlay #8067, which doesn't provide
pipeline statistics, but adds a nice overlay.
## Solution
Add `RenderDiagnosticsPlugin`, which enables collecting pipeline
statistics and CPU & GPU timings.
---
## Changelog
- Add `RenderDiagnosticsPlugin`
- Add `RenderContext::diagnostic_recorder` method
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Co-authored-by: Alice Cecile <alice.i.cecile@gmail.com>
