The current implementation of $rename treats as an error any rename operation with a source path that is "non-viable": .e.g., attempting to rename "a.b" in a document that contains

. The check for that includes a TODO noting that we could instead treat this as a no-op, as we do for any $rename with a non-existent source path.

If we do want to make that change, now would be a good time, as we are making backwards-incompatible changes to update functionality.