commit f3c6846ad9015de0adb44f519ff010ff0ed51180 (HEAD, origin/upgrade-ppxlib-0.18.0)
Author: Nathan Rebours <nathan.p.rebours@gmail.com>
Date:   Mon Oct 5 18:20:23 2020 +0200

    Make ppx_fields_conv compatible with ppxlib.0.18.0
    
    ppxlib.0.18.0 upgrades to the 4.11 AST which results in a change
    in string constants representation. This PR makes ppx_fields_conv
    compatible with the latest ppxlib.
    
    You might want for the actual release of ppxlib.0.18.0 before merging
    this!
    
    Signed-off-by: Nathan Rebours <nathan.p.rebours@gmail.com>

diff --git a/ppx_fields_conv.opam b/ppx_fields_conv.opam
index 91a9692..3e2d24a 100644
--- a/ppx_fields_conv.opam
+++ b/ppx_fields_conv.opam
@@ -15,7 +15,7 @@ depends: [
   "base"      {>= "v0.14" & < "v0.15"}
   "fieldslib" {>= "v0.14" & < "v0.15"}
   "dune"      {>= "2.0.0"}
-  "ppxlib"    {>= "0.14.0"}
+  "ppxlib"    {>= "0.18.0"}
 ]
 synopsis: "Generation of accessor and iteration functions for ocaml records"
 description: "
diff --git a/src/ppx_fields_conv.ml b/src/ppx_fields_conv.ml
index 130e82b..87752cb 100644
--- a/src/ppx_fields_conv.ml
+++ b/src/ppx_fields_conv.ml
@@ -34,9 +34,6 @@ let check_no_collision =
 
 module A = struct (* Additional AST construction helpers *)
 
-  let exp_string : (loc:Location.t -> string -> expression) = fun ~loc s ->
-    pexp_constant ~loc (Pconst_string (s,None))
-
   let pat_name : (loc:Location.t -> string -> pattern) = fun ~loc name ->
     ppat_var ~loc (Loc.make name ~loc)
 
@@ -440,7 +437,7 @@ module Gen_struct = struct
           [%expr
               Fieldslib.Field.Field { Fieldslib.Field.For_generated_code.
                 force_variance = (fun (_ : [%t perm]) -> ());
-                name = [%e A.exp_string ~loc name];
+                name = [%e estring ~loc name];
                 getter = [%e A.exp_name ~loc name];
                 setter = [%e setter_field];
                 fset = [%e fset];}]
@@ -725,7 +722,7 @@ module Gen_struct = struct
     let getter_and_setters, fields = gen_fields ~private_ ~loc labdecs in
     let create = creation_fun ~loc record_name labdecs in
     let simple_create = simple_creation_fun ~loc record_name labdecs in
-    let names = List.map (Inspect.field_names labdecs) ~f:(A.exp_string ~loc) in
+    let names = List.map (Inspect.field_names labdecs) ~f:(estring ~loc) in
     let fields_module =
       if String.equal record_name "t" then "Fields" else "Fields_of_" ^ record_name
     in