Constant FS_SYNC_SINGLE 

pub const FS_SYNC_SINGLE: u16 = _; // 8u16

The sync root is a single FILE, not a directory: the mirror holds exactly one entry — the root itself — keyed by the empty relative path “” (the same key a directory sync gives its root). Combining with RECURSIVE is invalid, and a directory root answers the invalid-path error (docs/design/fs-watch.md “Single-file sync”). Content, inline_max, FS_FETCH, and the write family behave as for any other sync, addressing path “”.