test: regression guard for ppx_runtime_libraries cmi-dep coverage

test/blackbox-tests/test-cases/per-module-lib-deps/ppx-runtime-libraries.t

@@ -0,0 +1,83 @@
+A library declared as [(kind ppx_rewriter) (ppx_runtime_libraries
+baz)] declares libraries the consumer needs at compile time
+*after* preprocessing. The runtime libraries' modules are
+referenced by ppx-rewritten output, so ocamldep on the consumer's
+source cannot reason about which of them are referenced — the
+next ppx invocation may use any module of [baz].
+
+The compile rule for any consumer of such a ppx must therefore
+have a dep that covers every cmi in the runtime library's objdir.
+A glob over the objdir is the natural shape; this test pins that
+shape, guarding against any future inter-library dep handling
+that narrows the recorded deps for ppx_runtime_libraries-induced
+libraries.
+
+  $ make_dune_project 3.24
+
+[baz] is a regular unwrapped library that the ppx's expansion
+would reference at runtime.
+
+  $ mkdir baz
+  $ cat > baz/dune <<EOF
+  > (library (name baz) (wrapped false))
+  > EOF
+  $ cat > baz/baz_mod.ml <<EOF
+  > let value = 0
+  > EOF
+
+[bar] is a [ppx_rewriter] declaring [baz] as a runtime library.
+The [ppx.driver] entry lets dune treat [bar] as a [pps] target
+without ppxlib; the driver itself is a no-op stub that satisfies
+dune's invocation contract enough for rule generation.
+
+  $ mkdir bar
+  $ cat > bar/dune <<EOF
+  > (library
+  >  (name bar)
+  >  (kind ppx_rewriter)
+  >  (ppx_runtime_libraries baz)
+  >  (ppx.driver (main Bar.main)))
+  > EOF
+  $ cat > bar/bar.ml <<EOF
+  > let main () =
+  >   let out = ref "" in
+  >   let args =
+  >     [ ("-o", Arg.Set_string out, "")
+  >     ; ("--impl", Arg.Set_string (ref ""), "")
+  >     ; ("--as-ppx", Arg.Set (ref false), "")
+  >     ; ("--cookie", Arg.Set (ref false), "")
+  >     ]
+  >   in
+  >   let anon _ = () in
+  >   Arg.parse (Arg.align args) anon "";
+  >   close_out (open_out !out)
+  > EOF
+
+[foo] uses [bar] as a preprocessor. [foo.ml] does not mention
+[Baz_mod] textually — the ppx's runtime references would only
+appear post-pp. Without [(libraries ...)], [foo]'s
+[requires_compile] is populated entirely by [add_pp_runtime_deps]
+in [src/dune_rules/lib.ml]: [bar] (the pps) plus its runtime
+closure ([baz]).
+
+  $ mkdir foo
+  $ cat > foo/dune <<EOF
+  > (library (name foo) (wrapped false) (preprocess (pps bar)))
+  > EOF
+  $ cat > foo/foo.ml <<EOF
+  > let _ = ()
+  > EOF
+
+The compile rule's recorded deps for [foo.cmi] must cover every
+cmi in [baz/.baz.objs/byte/]. Without that, a change to any cmi
+in [baz] would not invalidate [foo.cmi]'s rule and an incremental
+rebuild could silently use a stale [foo.cmi]. The [-I
+baz/.baz.objs/byte] flag is already on the compile command line
+via [requires_compile] (so the initial build succeeds regardless),
+but only the rule's recorded deps drive incremental correctness.
+
+  $ dune rules --root . --format=json --deps _build/default/foo/.foo.objs/byte/foo.cmi |
+  > jq -r 'include "dune"; .[] | depsGlobs
+  >   | select(.dir | endswith("baz/.baz.objs/byte"))
+  >   | "\(.dir) \(.predicate)"' | sort -u
+  _build/default/baz/.baz.objs/byte *.cmi