Make it easier to opt out of bevy_audio (bevyengine#23126)

# Objective

- bevy_seedling is incompatible with `bevy_audio`
- all our profile features (`3d`, `2d`, `ui`) enable the `audio`
collection feature
- That means that a seedling user needs to do this to land at the
current `default`:
```toml
[dependencies]
bevy = { version = "0.18.0", default-features = false, features = [
  # 2d
  "2d_bevy_render",
  "default_app",
  "picking",
  "scene",

  # 3d
  "3d_bevy_render",

  # ui
  "ui_api",
  "ui_bevy_render",

  # default_platform
  "android-game-activity",
  "bevy_gilrs",
  "bevy_winit",
  "default_font",
  "multi_threaded",
  "std",
  "sysinfo_plugin",
  "wayland",
  "webgl2",
  "x11",
] }
```

## Solution

- Disable the `audio` collection features for all profile features and
instead enable it by default
- The new seedling config now looks like this:
```toml
[dependencies]
bevy = { version = "0.18.0", default-features = false, features = [
  "2d",
  "3d",
  "ui",
] }
```

## Testing

- None, CI should take care of this

Author: janhohenheim

Cargo.toml

@@ -131,7 +131,7 @@ unsafe_op_in_unsafe_fn = "warn"
 unused_qualifications = "warn"
 
 [features]
-default = ["2d", "3d", "ui"]
+default = ["2d", "3d", "ui", "audio"]
 
 # PROFILE: The default 2D Bevy experience. This includes the core Bevy framework, 2D functionality, Bevy UI, scenes, audio, and picking.
 2d = [
@@ -140,7 +140,6 @@ default = ["2d", "3d", "ui"]
   "2d_bevy_render",
   "ui",
   "scene",
-  "audio",
   "picking",
 ]
 
@@ -151,7 +150,6 @@ default = ["2d", "3d", "ui"]
   "3d_bevy_render",
   "ui",
   "scene",
-  "audio",
   "picking",
 ]
 
@@ -162,7 +160,6 @@ ui = [
   "ui_api",
   "ui_bevy_render",
   "scene",
-  "audio",
   "picking",
   "bevy_ui_widgets",
 ]

docs/cargo_features.md

@@ -21,7 +21,7 @@ bevy = { version = "0.18", default-features = false, features = ["2d"] }
 
 |Profile|Description|
 |-|-|
-|default|The full default Bevy experience. This is a combination of the following profiles: 2d, 3d, ui|
+|default|The full default Bevy experience. This is a combination of the following profiles: 2d, 3d, ui, audio|
 |2d|The default 2D Bevy experience. This includes the core Bevy framework, 2D functionality, Bevy UI, scenes, audio, and picking.|
 |3d|The default 3D Bevy experience. This includes the core Bevy framework, 3D functionality, Bevy UI, scenes, audio, and picking.|
 |ui|The default Bevy UI experience.  This includes the core Bevy framework, Bevy UI, scenes, audio, and picking.|

release-content/migration-guides/audio_feature.md

@@ -0,0 +1,27 @@
+---
+title: "`audio` feature is now no longer implied by the `3d`, `2d`, or `ui` features"
+pull_requests: [23126]
+---
+
+Our `default` features used to be
+
+- 2d
+- 3d
+- ui
+
+where each of these features enabled the `audio` feature among others.
+
+Since Cargo doesn't allow selectively disabling features, if you wished to disable `bevy_audio`, either because you don't need audio or because you use an alternative such as bevy_seedling, you had a problem with that.
+You needed to essentially enable all features enabled by the above *except* `audio`, leading to a big `features` soup. To avoid this,
+`audio` is now no longer enabled by the above features, but instead enabled by default, bumping our default features to:
+
+- 2d
+- 3d
+- ui
+- audio
+
+Now what does this mean for you?
+
+- If you used all default features before, nothing changes for you.
+- If you want to opt out of using `bevy_audio`, it is now as simple as disabling default features, and manually opting into `3d`, `2d`, and/or `ui`.
+- If you already opted into non-default features and want to continue using `bevy_audio`, you will now have to add the `audio` feature.