abstract type for Tracing_ stub

src/private/tracing_.dummy.ml

@@ -1,5 +1,7 @@
+type span = unit
+
 let enabled () = false
-let dummy_span = 0L
+let dummy_span = ()
 let enter_span _name = dummy_span
 let exit_span = ignore
 let set_thread_name = ignore

src/private/tracing_.mli

@@ -1,6 +1,8 @@
-val dummy_span : int64
-val enter_span : string -> int64
-val exit_span : int64 -> unit
-val with_span : string -> (int64 -> 'a) -> 'a
+type span
+
+val dummy_span : span
+val enter_span : string -> span
+val exit_span : span -> unit
+val with_span : string -> (span -> 'a) -> 'a
 val enabled : unit -> bool
 val set_thread_name : string -> unit

src/private/tracing_.real.ml

@@ -1,11 +1,13 @@
 module Trace = Trace_core
 
+type span = Trace.span
+
 let enabled = Trace.enabled
-let dummy_span = Int64.min_int
+let dummy_span = Trace_core.Collector.dummy_span
 let dummy_file_ = "<unknown file>"
 let set_thread_name = Trace.set_thread_name
 
-let[@inline] enter_span name : int64 =
+let[@inline] enter_span name : span =
   if name = "" then
     dummy_span
   else