test: regression guard for cross-lib reads of pre-pp source

test/blackbox-tests/test-cases/per-module-lib-deps/cross-lib-walk-pre-pp-source.t

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+A consumer of a library that uses [(preprocess (action ...))] must
+still build when the dep's pre-preprocessing source is not valid
+OCaml on its own. Concretely: if any code path runs ocamldep on the
+raw, pre-[Module.pped] form of the dep's [.ml] (the [Module.t] from
+[Dir_contents.modules_of_local_lib], whose [Module.source] points
+at the unprocessed [.ml] and whose [Module.pp_flags] is [None]),
+ocamldep rejects the source with a syntax error and the build
+fails.
+
+The neighbour test [cross-lib-action-preprocess.t] covers the same
+scenario with a pass-through preprocessor and plain-OCaml sources,
+so a code path that runs ocamldep on the pre-pp form goes
+undetected. This test plugs that gap: [pp.exe] strips [#]-prefixed
+lines, and [dep/foo.ml] starts with a [#if]/[#endif] block.
+Ocamldep on the post-pp output is fine; ocamldep on the pre-pp
+input is a syntax error.
+
+  $ cat > dune-project <<EOF
+  > (lang dune 3.0)
+  > EOF
+
+A trivial preprocessor that drops every line starting with [#].
+This mimics cppo's behaviour without needing an external tool: the
+[.ml] holds [#if]/[#endif]-style markers; the preprocessor's output
+is plain OCaml.
+
+  $ mkdir pp
+  $ cat > pp/dune <<EOF
+  > (executable (name pp))
+  > EOF
+  $ cat > pp/pp.ml <<'EOF'
+  > let () =
+  >   let ic = open_in_bin Sys.argv.(1) in
+  >   try
+  >     while true do
+  >       let line = input_line ic in
+  >       if String.length line = 0 || line.[0] <> '#'
+  >       then print_endline line
+  >     done
+  >   with End_of_file -> ()
+  > EOF
+
+[dep] is an unwrapped, multi-module library whose modules are
+preprocessed. [foo.ml] has a [#]-prefixed directive that ocamldep
+cannot parse but the preprocessor strips. [bar.ml] is the second
+module: keeping the lib multi-module ensures any future per-module
+code path that short-circuits single-module stanzas doesn't mask
+the regression this test guards against.
+
+  $ mkdir dep
+  $ cat > dep/dune <<EOF
+  > (library
+  >  (name dep)
+  >  (wrapped false)
+  >  (preprocess (action (run %{exe:../pp/pp.exe} %{input-file}))))
+  > EOF
+  $ cat > dep/foo.ml <<'EOF'
+  > #if FAKE_DIRECTIVE
+  > let v = 1
+  > #endif
+  > EOF
+  $ cat > dep/bar.ml <<EOF
+  > let helper = 42
+  > EOF
+
+[consumer] references [Foo] from [dep]. Any cross-library
+dependency-tracking code path that resolves [Foo]'s deps against
+the dep's pre-pp source would fail here:
+
+  $ mkdir consumer
+  $ cat > consumer/dune <<EOF
+  > (library (name consumer) (wrapped false) (libraries dep))
+  > EOF
+  $ cat > consumer/c.ml <<EOF
+  > let _ = Foo.v
+  > EOF
+  $ cat > consumer/d.ml <<EOF
+  > let _ = ()
+  > EOF
+
+The build must succeed. If a future change reintroduces a cross-
+library code path that runs ocamldep on the pre-[Module.pped] form
+of a foreign-lib module without re-applying that lib's preprocessor,
+this test will fail with a syntax error from [dep/foo.ml].
+
+  $ dune build @check