Steps to trigger some build issues related to deriving_inline

Author: mbarbin

lib/vcs/src/dune

@@ -41,6 +41,7 @@
  (preprocess
   (pps
    -unused-code-warnings=force
+   ppx_compare
    ppx_enumerate
    ppx_sexp_conv
    ppx_sexp_value)))

lib/vcs/src/graph.ml

@@ -60,6 +60,63 @@ module Node_kind = struct
 
     include (
     struct
+      (* CR mbarbin: Here is a sequence that produces some issue for me:
+
+         1, Starting from a build in watch mode: `dune build @all @lint -w`
+
+         2. After everything stabilizes, I get the "Success, waiting for
+            filesystem changes..." output from dune.
+
+         Then I try the following:
+
+         3. Replace the `@@deriving` by a `@@deriving_inline`, and add an additional
+            line after it: `[@@@deriving.end]`.
+
+         4. Save the file.
+
+         This triggers a rebuild, and a promotion error that shows the code to be inserted.
+
+         5. I run: `dune promote`.
+
+         This creates the following error:
+
+         {v
+          Error: ppxlib: the corrected code doesn't round-trip.
+          This is probably a bug in the OCaml printer:
+          <no differences produced by diff>
+          diff: /tmp/build_2c6e2f_dune/ppxlib6f4ca1: No such file or directory
+          diff: /tmp/build_2c6e2f_dune/ppxlibaf673d: No such file or directory
+          Had 2 errors, waiting for filesystem changes...
+         v}
+
+         6. If I save the file again, the code gets reformatted but the error
+            does not go away.
+
+         {v
+          Error: ppxlib: the corrected code doesn't round-trip.
+          This is probably a bug in the OCaml printer:
+          <no differences produced by diff>
+          Had 1 error, waiting for filesystem changes...
+         v}
+
+         7. If I kill the build, and restart it the error is now different (looks worse):
+
+         {v
+Error: ppxlib: the corrected code doesn't round-trip.
+This is probably a bug in the OCaml printer:
+<no differences produced by diff>
+Uncaught exception:
+
+  (Failure
+   "Both files, /tmp/build_f345a9_dune/ppxlib7c84c6 and /tmp/build_f345a9_dune/ppxlibe07aed, do not exist")
+
+Raised at Stdlib.failwith in file "stdlib.ml", line 29, characters 17-33
+Called from Dune__exe__Compare.compare_main in file "bin/compare.ml", line 129, characters 7-77
+Called from Dune__exe__Compare.main in file "bin/compare.ml", line 174, characters 13-38
+Called from Command.For_unix.run.(fun) in file "command/src/command.ml", lines 3388-3399, characters 8-31
+Called from Base__Exn.handle_uncaught_aux in file "src/exn.ml", line 126, characters 6-10
+         v}
+      *)
       type nonrec t = t =
         | Root of { rev : Rev.t }
         | Commit of
@@ -73,8 +130,29 @@ module Node_kind = struct
             }
       [@@deriving_inline equal]
 
-      [%%ocaml.error
-        "Ppxlib.Deriving: 'equal' is not a supported type deriving generator"]
+      let equal =
+        (fun a__016_ b__017_ ->
+           if Stdlib.( == ) a__016_ b__017_
+           then true
+           else (
+             match a__016_, b__017_ with
+             | Root _a__018_, Root _b__019_ -> Rev.equal _a__018_.rev _b__019_.rev
+             | Root _, _ -> false
+             | _, Root _ -> false
+             | Commit _a__020_, Commit _b__021_ ->
+               Stdlib.( && )
+                 (Rev.equal _a__020_.rev _b__021_.rev)
+                 (Node.equal _a__020_.parent _b__021_.parent)
+             | Commit _, _ -> false
+             | _, Commit _ -> false
+             | Merge _a__022_, Merge _b__023_ ->
+               Stdlib.( && )
+                 (Rev.equal _a__022_.rev _b__023_.rev)
+                 (Stdlib.( && )
+                    (Node.equal _a__022_.parent1 _b__023_.parent1)
+                    (Node.equal _a__022_.parent2 _b__023_.parent2)))
+         : t -> t -> bool)
+      ;;
 
       [@@@deriving.end]
     end :