Ensure encodings aren't inadvertently tampered with

test/encoding/properly-ignored-by-checks-and-traversal/dune

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+(executable
+ (name id_driver)
+ (modules id_driver)
+ (libraries ppxlib))
+
+(executable
+ (name traversal_driver)
+ (modules traversal_driver)
+ (libraries ppxlib))
+
+(cram
+ (package ppxlib)
+ (enabled_if
+  (>= %{ocaml_version} 5.4))
+ (deps id_driver.exe traversal_driver.exe))

test/encoding/properly-ignored-by-checks-and-traversal/id_driver.ml

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+let () = Ppxlib.Driver.standalone ()

test/encoding/properly-ignored-by-checks-and-traversal/run.t

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+Encodings generated by migrations should not be treated as regular nodes in a few
+specific places.
+
+Let's consider the following bit of code:
+
+  $ cat > test.ml << EOF
+  > let x = (~a:1, ~b:2)
+  > let y = [%some_ext 4]
+  > EOF
+
+Once migrated to our AST, this will contain two extension points, the
+`[%some_ext 4]` from the source code, and the encoded labeled tuple.
+
+The former should trigger the extension check while the latter should not:
+
+  $ ./id_driver.exe -check -check-on-extensions test.ml
+  [%%ocaml.error "Extension `some_ext' was not translated"]
+  let x =
+    [%ppxlib.migration.pexp_labeled_tuple_5_4
+      (((`Some `a), 1), ((`Some `b), 2))]
+  let y = [%some_ext 4]
+
+
+The following driver has a global transformation registered that turns all
+integer litterals `i` into the litteral `i+1`.
+By default traversal classes should not recurse into encodings' payload:
+
+  $ ./traversal_driver.exe test.ml
+  let x =
+    [%ppxlib.migration.pexp_labeled_tuple_5_4
+      (((`Some `a), 2), ((`Some `b), 3))]
+  let y = [%some_ext 5]
+

test/encoding/properly-ignored-by-checks-and-traversal/traversal_driver.ml

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+open Ppxlib
+
+let plus_one_str str =
+  let i = int_of_string str in
+  string_of_int (i + 1)
+
+let t =
+  object
+    inherit Ast_traverse.map as super
+
+    method! expression_desc ed =
+      match ed with
+      | Pexp_constant (Pconst_integer (s, None)) ->
+          Pexp_constant (Pconst_integer (plus_one_str s, None))
+      | _ -> super#expression_desc ed
+  end
+
+let impl str = t#structure str
+
+let () =
+  Driver.register_transformation ~impl "ppx_plus_one";
+  Driver.standalone ()