Show other occurences of ambiguous label in warning

src/model/location_.ml

@@ -25,3 +25,7 @@ let in_string s ~offset ~length s_span =
     start = point_in_string s offset s_span.start;
     end_ = point_in_string s (offset + length) s_span.start;
   }
+
+let pp_span_start fmt s =
+  Format.fprintf fmt "File \"%s\", line %d, character %d" s.file s.start.line
+    s.start.column

src/model/location_.mli

@@ -5,3 +5,5 @@ end
 val set_end_as_offset_from_start : int -> span -> span
 
 val in_string : string -> offset:int -> length:int -> span -> span
+
+val pp_span_start : Format.formatter -> span -> unit

src/xref2/link.ml

@@ -28,6 +28,15 @@ let synopsis_of_module env (m : Component.Module.t) =
       | Ok sg -> synopsis_from_comment (Component.extract_signature_doc sg)
       | Error _ -> None)
 
+let ambiguous_label_warning label_name labels =
+  let pp_label_loc fmt (`Label (_, x)) =
+    Location_.pp_span_start fmt (Location_.location x)
+  in
+  Lookup_failures.report_warning
+    "@[<2>Label '%s' is ambiguous. The other occurences are:@ %a@]" label_name
+    (Format.pp_print_list ~pp_sep:Format.pp_force_newline pp_label_loc)
+    labels
+
 (** Duplicate labels are disambiguated by adding a "_2" suffix to them. The
     order in which it is inserted into the env is used to decide which prefix to
     assign, the first is unchanged.
@@ -57,7 +66,7 @@ let ensure_label_unique env heading =
   | Error `Not_found -> heading (* Unexpected. *)
   | Error (`Ambiguous (hd, tl)) -> (
       if h.heading_label_explicit then
-        Lookup_failures.report_warning "Label '%s' is ambiguous." label_name;
+        ambiguous_label_warning label_name (hd :: tl);
       match dup_index (hd :: tl) with
       | 0 -> heading (* Don't update the first occurence. *)
       | index ->

test/xref2/v407_and_above/labels.t/run.t

@@ -6,9 +6,13 @@
   Hint:
     Define labels explicitly using the syntax '{1:explicit-label Heading text}'.
   File "test.odoc":
-  Label 'B' is ambiguous.
+  Label 'B' is ambiguous. The other occurences are:
+    File "test.mli", line 16, character 6
+    File "test.mli", line 3, character 4
   File "test.odoc":
-  Label 'B' is ambiguous.
+  Label 'B' is ambiguous. The other occurences are:
+    File "test.mli", line 10, character 6
+    File "test.mli", line 3, character 4
 
 Labels:
 Some are not in order because the 'doc' field appears after the rest in the output.