test: observational baseline for transitive-unreferenced-module (#4572)

test/blackbox-tests/test-cases/per-module-lib-deps/transitive-unreferenced-module.t

@@ -0,0 +1,74 @@
+A consumer transitively depends upon a library through one module of an
+intermediate library, never naming a sibling module of the transitive lib in
+source. The current but undesirable behaviour is that the consumer rebuilds
+when any module of the transitively depended upon library changes, even
+unreferenced ones.
+
+[intermediate_lib] and [main] each include an unused dummy module so neither
+stanza is single-module — single-module stanzas take a fast path that would
+mask the behaviour being baselined here.
+
+  $ cat > dune-project <<EOF
+  > (lang dune 3.23)
+  > EOF
+
+  $ cat > dune <<EOF
+  > (library (name dep_lib) (wrapped false) (modules reached_module unreached_module))
+  > (library (name intermediate_lib) (wrapped false) (modules intermediate_module intermediate_dummy) (libraries dep_lib))
+  > (executable (name main) (modules main main_dummy) (libraries intermediate_lib))
+  > EOF
+
+  $ cat > reached_module.ml <<EOF
+  > let v = 42
+  > EOF
+  $ cat > reached_module.mli <<EOF
+  > val v : int
+  > EOF
+  $ cat > unreached_module.ml <<EOF
+  > let v = 43
+  > EOF
+  $ cat > unreached_module.mli <<EOF
+  > val v : int
+  > EOF
+  $ cat > intermediate_module.ml <<EOF
+  > let v = Reached_module.v
+  > EOF
+  $ cat > intermediate_dummy.ml <<EOF
+  > let _ = ()
+  > EOF
+  $ cat > main.ml <<EOF
+  > let _ = Intermediate_module.v
+  > EOF
+  $ cat > main_dummy.ml <<EOF
+  > let _ = ()
+  > EOF
+
+  $ dune build ./main.exe
+
+Edit [unreached_module] (both [.mli] and [.ml]) — a module no source in this
+test references — and record the rebuild list for [main]:
+
+  $ cat > unreached_module.mli <<EOF
+  > val v : int
+  > val extra : int
+  > EOF
+  $ cat > unreached_module.ml <<EOF
+  > let v = 43
+  > let extra = 99
+  > EOF
+  $ dune build ./main.exe
+  $ dune trace cat | jq -s 'include "dune"; [.[] | targetsMatchingFilter(test("dune__exe__Main\\."))]'
+  [
+    {
+      "target_files": [
+        "_build/default/.main.eobjs/byte/dune__exe__Main.cmi",
+        "_build/default/.main.eobjs/byte/dune__exe__Main.cmti"
+      ]
+    },
+    {
+      "target_files": [
+        "_build/default/.main.eobjs/native/dune__exe__Main.cmx",
+        "_build/default/.main.eobjs/native/dune__exe__Main.o"
+      ]
+    }
+  ]