I'm split on this. FileId is just 256 bytes and implements Copy. So maybe returning Option<FileId> would be easiest. But I'm willing to merge it, if you think it's an improvement.
Please update the change-log as well.

I changed the impl from Deref<Target=FileId> to AsRef<FileId>. The AsRef trait is more correct here.

I see no downside to allow a impl return. They are zero cost at runtime.
Using impl AsRef<FileId> allows the cache to return a owned value, a reference, a wrapper type or even a mutex guard.
In my codebase the FileId is wrapped in a new type to add some functionality, that was the motivation for this change.

As far as I see the change is backwards compatible.
The trait will still allow implementations that return Option<&FileId> for the file id.
Clippy will just show a refining_impl_trait warning.

Please update the change-log as well.

I am unfamiliar with the process. Should I just add the change at the top of the .md without a version / date?

I am unfamiliar with the process. Should I just add the change at the top of the .md without a version / date?

Yes. Add it under debouncer-full 0.6.0 (unreleased).

Done. I opted for a simpler description for the change in the changelog. Note that file-id is also touched.

Thanks.