Function fs_sync_flags_valid 

pub fn fs_sync_flags_valid(flags: u16) -> bool

C2S_FS_SYNC flag-combination validity: SINGLE syncs exactly one file, so RECURSIVE contradicts it and the pair is rejected at validation (docs/design/fs-watch.md “Single-file sync”). The exclusion flags apply to enumeration, which SINGLE does none of, so they are rejected with it too rather than silently doing nothing.