test: regression guard for menhir mock-compile + sibling-module incremental rebuild on lib-cmi change

test/blackbox-tests/test-cases/per-module-lib-deps/menhir-incremental-lib-cmi.t

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+Regression guard for menhir mock-compile and sibling module
+incremental rebuild when a sibling library's interface changes.
+
+When a library declares a menhir grammar whose semantic action
+references types or values from a [(libraries ...)] dep, dune emits
+two compile rules whose deps cover that library's cmis: menhir's
+[--infer] mock-compile (which runs [ocamlc -i] on a generated mock
+module to type-check the grammar's semantic actions) and the regular
+sibling-module compile that also references the dep. Both rules
+must invalidate when the dep's interface changes.
+
+This test pins the property. The menhir mock-compile path
+([menhir_rules.ml]) derives a sandboxed cctx via
+[Compilation_context.set_sandbox]; if a future change to the
+dep-graph machinery weakens cross-rule invalidation along that path
+(e.g., by sharing cached dep-info builders across cctxs in a
+sandbox-dependent way), this test surfaces the regression.
+
+  $ cat > dune-project <<EOF
+  > (lang dune 3.23)
+  > (using menhir 3.0)
+  > EOF
+
+[dep] is a sibling library exposing a value the parent library's
+menhir grammar will reference.
+
+  $ mkdir dep
+  $ cat > dep/dune <<EOF
+  > (library (name dep))
+  > EOF
+  $ cat > dep/dep.ml <<EOF
+  > let value = 42
+  > EOF
+  $ cat > dep/dep.mli <<EOF
+  > val value : int
+  > EOF
+
+[parser]: a library with a menhir grammar whose semantic action
+references [Dep.value], plus a sibling module [helper] that also
+references [Dep.value].
+
+  $ mkdir parser
+  $ cat > parser/dune <<EOF
+  > (library
+  >  (name parser)
+  >  (libraries dep))
+  > (menhir (modules grammar))
+  > EOF
+  $ cat > parser/grammar.mly <<EOF
+  > %token EOF
+  > %start <int> main
+  > %%
+  > main: EOF { Dep.value }
+  > EOF
+  $ cat > parser/helper.ml <<EOF
+  > let echo () = Dep.value
+  > EOF
+  $ cat > parser/helper.mli <<EOF
+  > val echo : unit -> int
+  > EOF
+
+Initial build.
+
+  $ dune build @check 2>&1 | head -20
+
+The mock-compile invocation is recognisable by [-i] in its argv;
+its target is [grammar__mock.mli.inferred]. The presence of this
+event positively asserts the [--infer] code path ran:
+
+  $ dune trace cat | jq -s 'include "dune"; [.[] | progMatching("ocamlc") | select(.process_args | index("-i") != null) | .target_files]'
+  [
+    [
+      "_build/default/parser/grammar__mock.mli.inferred"
+    ]
+  ]
+
+The mock's [-I] path includes [dep]'s objdir, so the grammar's
+semantic action type-checks against [dep]:
+
+  $ dune trace cat | jq -s 'include "dune"; [.[] | progMatching("ocamlc") | select(.process_args | index("-i") != null) | .process_args | [.[] | select(startswith("dep/"))]]'
+  [
+    [
+      "dep/.dep.objs/byte"
+    ]
+  ]
+
+Modify [dep]'s interface (add a value). Both consumers — the
+menhir mock-compile and [helper]'s [.cmo] — must invalidate.
+
+  $ cat > dep/dep.mli <<EOF
+  > val value : int
+  > val extra : string
+  > EOF
+  $ cat > dep/dep.ml <<EOF
+  > let value = 42
+  > let extra = "x"
+  > EOF
+
+Second build. Assert independently that each of the two
+invariants holds: the menhir mock-compile rebuilt, and the helper
+[.cmo] rebuilt. The test stays robust whether other helper
+artefacts (cmi/cmti) also rebuild — that is governed by
+cctx-wide-vs-per-module dep filtering and isn't the property
+under guard here.
+
+  $ dune build @check 2>&1 | head -20
+  $ dune trace cat | jq -s 'include "dune"; [.[] | targetsMatchingFilter(test("grammar__mock\\.mli\\.inferred"))]'
+  [
+    {
+      "target_files": [
+        "_build/default/parser/grammar__mock.mli.inferred"
+      ]
+    }
+  ]
+  $ dune trace cat | jq -s 'include "dune"; [.[] | targetsMatchingFilter(test("/parser__Helper\\.cmo$"))]'
+  [
+    {
+      "target_files": [
+        "_build/default/parser/.parser.objs/byte/parser__Helper.cmo",
+        "_build/default/parser/.parser.objs/byte/parser__Helper.cmt"
+      ]
+    }
+  ]