file_watcher feature fails to build for wasm32 because notify-debounder-full is only for native

[simbleau]

Bevy version

[dependencies]
bevy = { version = "0.12", default-features = false, features = [
  "bevy_asset",
  "file_watcher",
  # The rest don't matter
] }

What went wrong

we updated to using notify-debouncer-full-0.3.1 in bevy-asset 0.12:

$ cargo tree
│       │   ├── bevy_asset v0.12.0
│       │   │   ├── notify-debouncer-full v0.3.1

attempting to build for wasm32 will yield this lovely error:

error[E0432]: unresolved import `file_id::get_file_id`
   --> /Users/simbleau/.cargo/registry/src/index.crates.io-6f17d22bba15001f/notify-debouncer-full-0.3.1/src/cache.rs:6:15
    |
6   | use file_id::{get_file_id, FileId};
    |               ^^^^^^^^^^^ no `get_file_id` in the root
    |
note: found an item that was configured out
   --> /Users/simbleau/.cargo/registry/src/index.crates.io-6f17d22bba15001f/file-id-0.2.1/src/lib.rs:111:8
    |
111 | pub fn get_file_id(path: impl AsRef<Path>) -> io::Result<FileId> {
    |        ^^^^^^^^^^^
    = note: the item is gated behind the `unix` feature
note: found an item that was configured out
   --> /Users/simbleau/.cargo/registry/src/index.crates.io-6f17d22bba15001f/file-id-0.2.1/src/lib.rs:121:8
    |
121 | pub fn get_file_id(path: impl AsRef<Path>) -> io::Result<FileId> {
    |        ^^^^^^^^^^^
    = note: the item is gated behind the `windows` feature

For more information about this error, try `rustc --explain E0432`.
error: could not compile `notify-debouncer-full` (lib) due to previous error

The key phrase here is: note: found an item that was configured out

this is because there is no wasm32 family function for get_file_id:
https://github.com/notify-rs/notify/blob/c4d047078549c2fec0bbfada29653097776b8b2d/file-id/src/lib.rs#L110

Solution

file_watcher should only bring in notify-debouncer-full on non wasm targets, and do nothing for wasm targets, just as it did before.

[simbleau]

For those needing a fast solution:

• Remove the file_watcher feature. It used to work on wasm-family systems (because it did nothing to watch for files on wasm) but those following the migration guide will find the issue above.