default features from bevy_asset and bevy_ecs can actually be disabled (#3097)

# Objective

- `bevy_ecs` exposes as an optional feature `bevy_reflect`. Disabling it doesn't compile.
- `bevy_asset` exposes as an optional feature `filesystem_watcher`. Disabling it doesn't compile. It is also not possible to disable this feature from Bevy

## Solution

- Fix compilation errors when disabling the default features. Make it possible to disable the feature `filesystem_watcher` from Bevy

Cargo.toml

@@ -27,6 +27,7 @@ default = [
   "hdr",
   "mp3",
   "x11",
+  "filesystem_watcher"
 ]
 
 # Force dynamic linking, which improves iterative compile times
@@ -68,6 +69,9 @@ mp3 = ["bevy_internal/mp3"]
 vorbis = ["bevy_internal/vorbis"]
 wav = ["bevy_internal/wav"]
 
+# Enable watching file system for asset hot reload
+filesystem_watcher = ["bevy_internal/filesystem_watcher"]
+
 serialize = ["bevy_internal/serialize"]
 
 # Display server protocol support (X11 is enabled by default)

crates/bevy_asset/Cargo.toml

@@ -9,7 +9,7 @@ license = "MIT OR Apache-2.0"
 keywords = ["bevy"]
 
 [features]
-default = ["filesystem_watcher"]
+default = []
 filesystem_watcher = ["notify"]
 
 [dependencies]

crates/bevy_asset/src/io/file_asset_io.rs

@@ -1,15 +1,23 @@
-use crate::{filesystem_watcher::FilesystemWatcher, AssetIo, AssetIoError, AssetServer};
+#[cfg(feature = "filesystem_watcher")]
+use crate::{filesystem_watcher::FilesystemWatcher, AssetServer};
+use crate::{AssetIo, AssetIoError};
 use anyhow::Result;
+#[cfg(feature = "filesystem_watcher")]
 use bevy_ecs::system::Res;
-use bevy_utils::{BoxedFuture, HashSet};
+use bevy_utils::BoxedFuture;
+#[cfg(feature = "filesystem_watcher")]
+use bevy_utils::HashSet;
+#[cfg(feature = "filesystem_watcher")]
 use crossbeam_channel::TryRecvError;
 use fs::File;
-use io::Read;
+#[cfg(feature = "filesystem_watcher")]
 use parking_lot::RwLock;
+#[cfg(feature = "filesystem_watcher")]
+use std::sync::Arc;
 use std::{
-    env, fs, io,
+    env, fs,
+    io::Read,
     path::{Path, PathBuf},
-    sync::Arc,
 };
 
 pub struct FileAssetIo {
@@ -21,6 +29,7 @@ pub struct FileAssetIo {
 impl FileAssetIo {
     pub fn new<P: AsRef<Path>>(path: P) -> Self {
         FileAssetIo {
+            #[cfg(feature = "filesystem_watcher")]
             filesystem_watcher: Default::default(),
             root_path: Self::get_root_path().join(path.as_ref()),
         }

crates/bevy_core/Cargo.toml

@@ -11,9 +11,9 @@ keywords = ["bevy"]
 
 [dependencies]
 # bevy
-bevy_app = { path = "../bevy_app", version = "0.5.0" }
+bevy_app = { path = "../bevy_app", version = "0.5.0", features = ["bevy_reflect"] }
 bevy_derive = { path = "../bevy_derive", version = "0.5.0" }
-bevy_ecs = { path = "../bevy_ecs", version = "0.5.0" }
+bevy_ecs = { path = "../bevy_ecs", version = "0.5.0", features = ["bevy_reflect"] }
 bevy_math = { path = "../bevy_math", version = "0.5.0" }
 bevy_reflect = { path = "../bevy_reflect", version = "0.5.0", features = ["bevy"] }
 bevy_tasks = { path = "../bevy_tasks", version = "0.5.0" }

crates/bevy_ecs/src/change_detection.rs

@@ -1,6 +1,7 @@
 //! Types that detect when their internal data mutate.
 
 use crate::{component::ComponentTicks, system::Resource};
+#[cfg(feature = "bevy_reflect")]
 use bevy_reflect::Reflect;
 use std::ops::{Deref, DerefMut};
 
@@ -188,9 +189,11 @@ impl_into_inner!(Mut<'a, T>, T,);
 impl_debug!(Mut<'a, T>,);
 
 /// Unique mutable borrow of a Reflected component
+#[cfg(feature = "bevy_reflect")]
 pub struct ReflectMut<'a> {
     pub(crate) value: &'a mut dyn Reflect,
     pub(crate) ticks: Ticks<'a>,
 }
 
+#[cfg(feature = "bevy_reflect")]
 change_detection_impl!(ReflectMut<'a>, dyn Reflect,);

crates/bevy_internal/Cargo.toml

@@ -29,6 +29,9 @@ mp3 = ["bevy_audio/mp3"]
 vorbis = ["bevy_audio/vorbis"]
 wav = ["bevy_audio/wav"]
 
+# Enable watching file system for asset hot reload
+filesystem_watcher = ["bevy_asset/filesystem_watcher"]
+
 serialize = ["bevy_input/serialize"]
 
 # Display server protocol support (X11 is enabled by default)

crates/bevy_transform/Cargo.toml

@@ -11,7 +11,7 @@ keywords = ["bevy"]
 [dependencies]
 # bevy
 bevy_app = { path = "../bevy_app", version = "0.5.0" }
-bevy_ecs = { path = "../bevy_ecs", version = "0.5.0" }
+bevy_ecs = { path = "../bevy_ecs", version = "0.5.0", features = ["bevy_reflect"] }
 bevy_math = { path = "../bevy_math", version = "0.5.0" }
 bevy_reflect = { path = "../bevy_reflect", version = "0.5.0", features = ["bevy"] }
 bevy_utils = { path = "../bevy_utils", version = "0.5.0" }

docs/cargo_features.md

@@ -14,6 +14,7 @@
 |hdr|[HDR](https://en.wikipedia.org/wiki/High_dynamic_range) support.|
 |mp3|MP3 audio format support.|
 |x11|Make GUI applications use X11 protocol. You could enable wayland feature to override this.|
+|filesystem_watcher|Enable watching the file system for asset hot reload|
 
 ## Optional Features