test: observational baseline for per-module rebuild counts

test/blackbox-tests/test-cases/per-module-lib-deps/unwrapped-tight-deps.t

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+Baseline: consumer-module rebuild targets when individual modules
+of an unwrapped dependency library change.
+
+This is an observational test. It records the rebuild targets for
+the consumer module [consumer] when each of three entry modules of
+the dependency library [dep_lib] has its interface edited.
+[consumer] references only [Referenced_dep] from [dep_lib];
+[Unread_dep_a] and [Unread_dep_b] are present but unreferenced.
+
+On current main, editing any of the three rebuilds [consumer]
+because library-level dependency filtering (and, within that,
+per-module tightening) is not yet in place: the consumer is
+conservatively rebuilt whenever any entry module's cmi changes.
+Work on https://github.com/ocaml/dune/issues/4572 is expected to
+tighten this, at which point editing [Unread_dep_a] or
+[Unread_dep_b] leaves [consumer] untouched and the emitted target
+list becomes empty.
+
+See: https://github.com/ocaml/dune/issues/4572
+
+  $ cat > dune-project <<EOF
+  > (lang dune 3.23)
+  > EOF
+
+[dep_lib] is an unwrapped library with three entry modules, each
+with an explicit interface so signature changes propagate through
+.cmi files:
+
+  $ mkdir dep_lib
+  $ cat > dep_lib/dune <<EOF
+  > (library (name dep_lib) (wrapped false))
+  > EOF
+  $ cat > dep_lib/unread_dep_a.ml <<EOF
+  > let v = 1
+  > EOF
+  $ cat > dep_lib/unread_dep_a.mli <<EOF
+  > val v : int
+  > EOF
+  $ cat > dep_lib/referenced_dep.ml <<EOF
+  > let v = 2
+  > EOF
+  $ cat > dep_lib/referenced_dep.mli <<EOF
+  > val v : int
+  > EOF
+  $ cat > dep_lib/unread_dep_b.ml <<EOF
+  > let v = 3
+  > EOF
+  $ cat > dep_lib/unread_dep_b.mli <<EOF
+  > val v : int
+  > EOF
+
+[consumer_lib] has two modules. The module of interest, [consumer],
+references only [Referenced_dep] from [dep_lib]. A second, unused
+module [filler] is present only to keep [consumer_lib] a multi-
+module stanza: dune skips ocamldep for single-module stanzas with
+no library deps as an optimisation, and the per-module-lib-deps
+filter depends on ocamldep output. Including [filler] isolates
+this test from the skip-ocamldep optimisation so the rebuild
+targets for [consumer] reflect only the per-module filter's work:
+
+  $ mkdir consumer_lib
+  $ cat > consumer_lib/dune <<EOF
+  > (library (name consumer_lib) (wrapped false) (libraries dep_lib))
+  > EOF
+  $ cat > consumer_lib/consumer.ml <<EOF
+  > let w = Referenced_dep.v
+  > EOF
+  $ cat > consumer_lib/filler.ml <<EOF
+  > let _ = ()
+  > EOF
+
+  $ dune build @check
+
+Edit [Unread_dep_a]'s interface — a module [consumer] does not
+reference — and record the rebuild targets for [consumer]:
+
+  $ cat > dep_lib/unread_dep_a.mli <<EOF
+  > val v : int
+  > val extra : unit -> int
+  > EOF
+  $ cat > dep_lib/unread_dep_a.ml <<EOF
+  > let v = 1
+  > let extra () = 7
+  > EOF
+  $ dune build @check
+  $ dune trace cat | jq -s 'include "dune"; [.[] | targetsMatchingFilter(test("consumer_lib/\\.consumer_lib\\.objs/byte/consumer\\."))]'
+  [
+    {
+      "target_files": [
+        "_build/default/consumer_lib/.consumer_lib.objs/byte/consumer.cmi",
+        "_build/default/consumer_lib/.consumer_lib.objs/byte/consumer.cmo",
+        "_build/default/consumer_lib/.consumer_lib.objs/byte/consumer.cmt"
+      ]
+    }
+  ]
+
+Same for [Unread_dep_b]:
+
+  $ cat > dep_lib/unread_dep_b.mli <<EOF
+  > val v : int
+  > val other : string
+  > EOF
+  $ cat > dep_lib/unread_dep_b.ml <<EOF
+  > let v = 3
+  > let other = "hi"
+  > EOF
+  $ dune build @check
+  $ dune trace cat | jq -s 'include "dune"; [.[] | targetsMatchingFilter(test("consumer_lib/\\.consumer_lib\\.objs/byte/consumer\\."))]'
+  [
+    {
+      "target_files": [
+        "_build/default/consumer_lib/.consumer_lib.objs/byte/consumer.cmi",
+        "_build/default/consumer_lib/.consumer_lib.objs/byte/consumer.cmo",
+        "_build/default/consumer_lib/.consumer_lib.objs/byte/consumer.cmt"
+      ]
+    }
+  ]
+
+Edit [Referenced_dep]'s interface — the one module [consumer] does
+reference — and record the rebuild targets ([consumer] must
+rebuild):
+
+  $ cat > dep_lib/referenced_dep.mli <<EOF
+  > val v : int
+  > val new_fn : int -> int
+  > EOF
+  $ cat > dep_lib/referenced_dep.ml <<EOF
+  > let v = 2
+  > let new_fn x = x + 1
+  > EOF
+  $ dune build @check
+  $ dune trace cat | jq -s 'include "dune"; [.[] | targetsMatchingFilter(test("consumer_lib/\\.consumer_lib\\.objs/byte/consumer\\."))]'
+  [
+    {
+      "target_files": [
+        "_build/default/consumer_lib/.consumer_lib.objs/byte/consumer.cmi",
+        "_build/default/consumer_lib/.consumer_lib.objs/byte/consumer.cmo",
+        "_build/default/consumer_lib/.consumer_lib.objs/byte/consumer.cmt"
+      ]
+    }
+  ]