unikernel/duniverse/ppxlib/test/driver/compiler-pp/driver.ml

@@ -0,0 +1,34 @@
+open Ppxlib
+
+let existential ~loc =
+  let lident = { loc; txt = Longident.parse "Constructor" } in
+  let pattern =
+    {
+      ppat_loc = loc;
+      ppat_loc_stack = [];
+      ppat_attributes = [];
+      ppat_desc =
+        Ppat_construct (lident, Some ([ { loc; txt = "a" } ], [%pat? _]));
+    }
+  in
+  [%stri let f x = match x with [%p pattern] -> ()]
+
+let named_existential =
+  Context_free.Rule.extension
+    (Extension.V3.declare "named_existentials" Extension.Context.structure_item
+       Ast_pattern.(pstr nil)
+       (fun ~ctxt ->
+         let loc = Expansion_context.Extension.extension_point_loc ctxt in
+         existential ~loc))
+
+let () =
+  Driver.V2.register_transformation ~rules:[ named_existential ]
+    "named_existentials"
+
+let str_type_decl =
+  Deriving.Generator.V2.make_noarg (fun ~ctxt _type_decl ->
+      let loc = Expansion_context.Deriver.derived_item_loc ctxt in
+      [ existential ~loc ])
+
+let _ = Deriving.add ~str_type_decl "named_existentials"
+let () = Driver.standalone ()

unikernel/duniverse/ppxlib/test/driver/compiler-pp/dune

@@ -0,0 +1,16 @@
+(executable
+ (name driver)
+ (enabled_if
+  (and
+   (>= %{ocaml_version} "4.09")
+   (< %{ocaml_version} "4.13")))
+ (libraries ppxlib)
+ (preprocess
+  (pps ppxlib.metaquot)))
+
+(cram
+ (enabled_if
+  (and
+   (>= %{ocaml_version} "4.09")
+   (< %{ocaml_version} "4.13")))
+ (deps driver.exe))

unikernel/duniverse/ppxlib/test/driver/compiler-pp/run.t

@@ -0,0 +1,65 @@
+The --use-compiler-pp flag can be used when using the driver's source code
+output, either directly when generating a .corrected file or to force
+printing the AST as source using the installed compiler's printer.
+
+Our driver has a deriver and an extension that produces a pattern-matching with
+named existentials.
+
+This feature has been introduced in 4.13 so the syntax is unsupported before that.
+
+If we run the driver in source output mode, without the `--use-compiler-pp` flag,
+it will successfully print out the source using the 4.13 syntax. If we're running
+on an older compiler, like we are for this test, that can be troublesome.
+
+If instead we use the flag, this will force the migration thus causing an error as
+named existentials can't be migrated down to 4.12.
+
+Let's consider the following file:
+
+  $ cat > test.ml << EOF
+  > [%%named_existentials]
+  > EOF
+
+Running the driver will generate a function with a single pattern matching in it:
+
+  $ ./driver.exe test.ml
+  let f x = match x with | Constructor (type a) _ -> ()
+
+Now if we run it with `--use-compiler-pp`, we should get the migration error:
+
+  $ ./driver.exe --use-compiler-pp test.ml
+  File "test.ml", line 1, characters 0-22:
+  1 | [%%named_existentials]
+      ^^^^^^^^^^^^^^^^^^^^^^
+  Error: migration error: existentials in pattern-matching is not supported before OCaml 4.13
+  [1]
+
+This should also work for correction based code gen:
+
+  $ cat > test_inline.ml << EOF
+  > type t = int
+  > [@@deriving_inline named_existentials]
+  > [@@@end]
+  > EOF
+
+If we run the driver without `--use-compiler-pp`:
+
+  $ ./driver.exe test_inline.ml -diff-cmd -
+  type t = int[@@deriving_inline named_existentials]
+  [@@@end ]
+  $ cat test_inline.ml.ppx-corrected
+  type t = int
+  [@@deriving_inline named_existentials]
+  let _ = fun (_ : t) -> ()
+  let f x = match x with | Constructor (type a) _ -> ()
+  let _ = f
+  [@@@end]
+
+and with the flag:
+
+  $ ./driver.exe test_inline.ml -diff-cmd - --use-compiler-pp
+  File "test_inline.ml", lines 1-2, characters 0-38:
+  1 | type t = int
+  2 | [@@deriving_inline named_existentials]
+  Error: migration error: existentials in pattern-matching is not supported before OCaml 4.13
+  [1]