test: observational baseline for per-module include flags (#4572)

test/blackbox-tests/test-cases/per-module-lib-deps/per-module-include-flags.t

@@ -0,0 +1,51 @@
+Observational baseline: a consumer module's compile command
+currently carries [-I] flags for every library in the cctx-wide
+[(libraries ...)] closure, regardless of which libraries the
+module's source actually references.
+
+[consumer_lib] declares two library dependencies, [dep_lib] and
+[unrelated_lib], but its only module references just [Dep_module].
+A future per-module filter could observe via ocamldep that the
+module references nothing from [unrelated_lib] and drop that
+library's objdir from the [-I] path. This test records today's
+[-I] contents so that a future filter improvement can flip the
+asserted array to a single entry.
+
+  $ cat > dune-project <<EOF
+  > (lang dune 3.0)
+  > EOF
+
+  $ cat > dune <<EOF
+  > (library (name dep_lib) (wrapped false) (modules dep_module))
+  > (library (name unrelated_lib) (wrapped false) (modules unrelated_module))
+  > (library
+  >  (name consumer_lib)
+  >  (wrapped false)
+  >  (modules consumer_module)
+  >  (libraries dep_lib unrelated_lib))
+  > EOF
+
+  $ cat > dep_module.ml <<EOF
+  > let v = 1
+  > EOF
+  $ cat > unrelated_module.ml <<EOF
+  > let _ = ()
+  > EOF
+  $ cat > consumer_module.ml <<EOF
+  > let _ = Dep_module.v
+  > EOF
+
+  $ dune build @check
+
+Inspect the [-I] flags on [consumer_module]'s [.cmo] compile rule.
+Filter to objdir-shaped paths so we don't print the consumer's
+own [.consumer_lib.objs/byte] entry — that's always present and
+not what this test is about. Today both [dep_lib]'s objdir and
+[unrelated_lib]'s objdir appear:
+
+  $ dune rules --root . --format=json _build/default/.consumer_lib.objs/byte/consumer_module.cmo \
+  > | jq 'include "dune"; .[] | [ruleActionFlagValues("-I") | select(test("\\.dep_lib\\.objs|\\.unrelated_lib\\.objs"))]'
+  [
+    ".dep_lib.objs/byte",
+    ".unrelated_lib.objs/byte"
+  ]