test: staged_pps and sandboxing (#6747)

regression test for #6644

Signed-off-by: Rudi Grinberg <[email protected]>

test/blackbox-tests/test-cases/github6644.t

@@ -0,0 +1,33 @@
+staged_pps should work even if sandboxing is enabled
+
+Regression test for #6644
+
+  $ cat >dune-project <<EOF
+  > (lang dune 3.6)
+  > (package
+  >  (name foo))
+  > EOF
+
+  $ cat >dune <<EOF
+  > (library
+  >  (name driver1)
+  >  (public_name foo.driver)
+  >  (modules ())
+  >  (ppx.driver (main "(fun () -> assert false)")))
+  > (library
+  >  (name dummy_rewriter)
+  >  (modules ())
+  >  (libraries driver1)
+  >  (kind ppx_rewriter))
+  > (library
+  >  (name foo)
+  >  (modules foo)
+  >  (preprocess (staged_pps dummy_rewriter)))
+  > EOF
+  $ touch foo.ml
+
+  $ dune build foo.cma
+  File ".foo.objs/byte/_unknown_", line 1, characters 0-0:
+  Error: This rule forbids all sandboxing modes (but it also requires
+  sandboxing)
+  [1]