Make.Mapping_fileSourceval create :
?report_file_sizes:
((Optint.Int63.t * Optint.Int63.t * Optint.Int63.t) -> unit) ->
root:string ->
generation:int ->
register_entries:
(register_entry:(off:Optint.Int63.t -> len:int -> unit) -> unit) ->
unit ->
(t, Errs.t) result