test(top-module): reproduce over-rebuild on unreferenced lib's .mli change

test/blackbox-tests/test-cases/top-module/over-rebuild-from-intf-only-dep.t

@@ -0,0 +1,104 @@
+Reproduction: `dune ocaml top-module` over-rebuilds when an
+unreferenced library's interface changes. The test passes while
+the bug is present; promote when fixed.
+
+`dune ocaml top-module` synthesises a compile for `m.ml` with the
+`.mli` dropped, so only `m.ml`'s actual references should drive
+the cmi-deps. Instead, the cmi-deps come from the parent library's
+full `(libraries ...)` closure, so editing any of those deps'
+interfaces triggers a rebuild — even libraries referenced ONLY
+from the (dropped) `.mli`.
+
+Code under test: `src/dune_rules/top_module.ml`.
+
+  $ make_dune_project 3.24
+
+`dep_for_intf` is referenced from `m.mli` only. `m.ml` never
+mentions `Dep_for_intf`.
+
+  $ mkdir dep_for_intf
+  $ cat > dep_for_intf/dune <<EOF
+  > (library (name dep_for_intf))
+  > EOF
+  $ cat > dep_for_intf/dep_for_intf.ml <<EOF
+  > type t = int
+  > let zero = 0
+  > EOF
+  $ cat > dep_for_intf/dep_for_intf.mli <<EOF
+  > type t = int
+  > EOF
+
+`dep_for_impl` is referenced from `m.ml` only. `m.mli` never
+mentions `Dep_for_impl`.
+
+  $ mkdir dep_for_impl
+  $ cat > dep_for_impl/dune <<EOF
+  > (library (name dep_for_impl))
+  > EOF
+  $ cat > dep_for_impl/dep_for_impl.ml <<EOF
+  > let value = 7
+  > let extra = "x"
+  > EOF
+  $ cat > dep_for_impl/dep_for_impl.mli <<EOF
+  > val value : int
+  > EOF
+
+`mylib` declares both libraries in `(libraries ...)` but module
+`m` splits them: `.ml` uses only `Dep_for_impl`; `.mli` uses only
+`Dep_for_intf`.
+
+  $ mkdir mylib
+  $ cat > mylib/dune <<EOF
+  > (library
+  >  (name mylib)
+  >  (libraries dep_for_intf dep_for_impl))
+  > EOF
+  $ cat > mylib/m.mli <<EOF
+  > val tag : Dep_for_intf.t
+  > EOF
+  $ cat > mylib/m.ml <<EOF
+  > let tag = Dep_for_impl.value
+  > EOF
+
+Initial regular build, then `dune ocaml top-module mylib/m.ml`:
+
+  $ dune build @check
+  $ dune ocaml top-module mylib/m.ml > /dev/null 2>&1
+
+Control: edit `dep_for_impl`'s `.mli` to expose `extra`. `m.ml`
+references `Dep_for_impl.value`, so its top-module compile depends
+on `dep_for_impl.cmi`. Rebuild expected. The trace contains the
+`mylib__M.cmo` build action.
+
+  $ cat > dep_for_impl/dep_for_impl.mli <<EOF
+  > val value : int
+  > val extra : string
+  > EOF
+  $ dune ocaml top-module mylib/m.ml --trace-file=_build/trace-impl.csexp > /dev/null 2>&1
+  $ dune trace cat --trace-file=_build/trace-impl.csexp | jq -s 'include "dune"; [.[] | targetsMatchingFilter(test("\\.topmod/mylib/m\\.ml/mylib__M\\.cmo$"))]'
+  [
+    {
+      "target_files": [
+        "_build/default/.topmod/mylib/m.ml/mylib__M.cmo"
+      ]
+    }
+  ]
+
+Probe: edit `dep_for_intf`'s `.mli` to expose `zero`. `m.ml` never
+references `Dep_for_intf` — the only reference was in the
+discarded `m.mli`. The top-module compile rebuilds anyway; this is
+the over-invalidation.
+
+  $ cat > dep_for_intf/dep_for_intf.mli <<EOF
+  > type t = int
+  > val zero : t
+  > EOF
+  $ dune ocaml top-module mylib/m.ml --trace-file=_build/trace-intf.csexp > /dev/null 2>&1
+  $ dune trace cat --trace-file=_build/trace-intf.csexp | jq -s 'include "dune"; [.[] | targetsMatchingFilter(test("\\.topmod/mylib/m\\.ml/mylib__M\\.cmo$"))]'
+  [
+    {
+      "target_files": [
+        "_build/default/.topmod/mylib/m.ml/mylib__M.cmo"
+      ]
+    }
+  ]