Add missing In_channel function on some OCaml versions

Author: OlivierNicole

compiler/lib/stdlib.ml

@@ -1227,9 +1227,17 @@ module Fun = struct
 end
 
 module In_channel = struct
-  include In_channel
+  let stdlib_input_line = input_line
+  [@@if ocaml_version >= (4, 14, 0)]
+
+  module In_channel = In_channel
   [@@if ocaml_version >= (4, 14, 0)]
 
+  module In_channel = struct end
+  [@@if ocaml_version < (4, 14, 0)]
+
+  include In_channel
+
   (* Read up to [len] bytes into [buf], starting at [ofs]. Return total bytes
      read. *)
   let read_upto ic buf ofs len =
@@ -1325,6 +1333,18 @@ module In_channel = struct
     end
     [@@if ocaml_version < (4, 14, 0)]
 
+    (* [In_channel.input_lines] only exists in the stdlib since 5.1. We would
+       also like to use tail modulo cons (introduced in 4.14.0) to implement it
+       here for versions between 4.14.0 and 5.1.0, because it gives better
+       performance. Here we reimplement it for all versions of OCaml due to a
+       limitation of [ppx_optcompt_light], namely, that it doesn't support
+       version ranges. *)
+  let [@tail_mod_cons] rec input_lines ic =
+    match stdlib_input_line ic with
+    | line -> line :: input_lines ic
+    | exception End_of_file -> []
+  [@@if ocaml_version >= (4, 14, 0)]
+
   let input_lines ic =
     let rec aux acc =
       match input_line ic with