unikernel/duniverse/ppxlib/test/503_migrations/preserve-effect-downward/run.t

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+We should be able to migrate the effect syntax downward and back. This allows users
+to run ppx-es on 5.3 on source files that use the effect syntax until we update our
+internal AST to fully support it.
+
+We have a custom driver that will force migration of the AST down to 5.2 and back to
+the compiler's version and print it as source code using the compiler's printer,
+regardless of ppxlib's internal AST version.
+
+If we run the driver on the following source file:
+
+  $ cat > test.ml << EOF
+  > let handler f =
+  > match f () with
+  > | x -> x
+  > | effect Random_bits, k -> Effect.Deep.continue k (Random.bits ())
+  > EOF
+
+it should successfully roundtrip to 5.2 and print the source code unchanged:
+
+  $ ./driver.exe test.ml --use-compiler-pp
+  let handler f =
+    match f () with
+    | x -> x
+    | effect Random_bits,  k -> Effect.Deep.continue k (Random.bits ())