test: record consumer rebuild count when shadowed dep lib changes

test/blackbox-tests/test-cases/per-module-lib-deps/lib-vs-lib-name-collision.t

@@ -0,0 +1,75 @@
+Two unwrapped libraries expose an entry module of the same name.
+A consumer library lists both and references the name in its code.
+OCaml's [-I] path resolution picks the lib whose [-I] comes first
+on the command line (the order of declared libraries in
+[(libraries ...)]), so the consumer compiles against that lib's
+version of the module. The other lib's same-named module is
+shadowed — unreachable from consumer code.
+
+This test is observational: it records the rebuild-target count
+for a consumer module after editing the *shadowed* lib's
+same-named module. The expected count on current [main] is
+positive because cross-library dep tracking is library-level: the
+consumer depends on a glob over each declared library's public
+cmi directory, so any change to the shadowed lib's public cmis
+invalidates the consumer.
+
+A future per-module dependency filter (#4572) would not
+automatically improve this case: without qualified-path analysis
+the filter cannot know which lib's [Shared] the consumer actually
+resolves through, and must conservatively depend on both.
+
+See: https://github.com/ocaml/dune/issues/4572
+
+  $ cat > dune-project <<EOF
+  > (lang dune 3.23)
+  > EOF
+
+  $ mkdir active_lib
+  $ cat > active_lib/dune <<EOF
+  > (library (name active_lib) (wrapped false))
+  > EOF
+  $ cat > active_lib/shared.ml <<EOF
+  > let from_active = "a"
+  > EOF
+
+  $ mkdir shadowed_lib
+  $ cat > shadowed_lib/dune <<EOF
+  > (library (name shadowed_lib) (wrapped false))
+  > EOF
+  $ cat > shadowed_lib/shared.ml <<EOF
+  > let from_shadowed = "b"
+  > EOF
+
+[consumer_lib] lists [active_lib] first, so [-I active_lib/.objs]
+comes before [-I shadowed_lib/.objs] on the command line and OCaml
+resolves [Shared] to [active_lib/shared.cmi]:
+
+  $ mkdir consumer_lib
+  $ cat > consumer_lib/dune <<EOF
+  > (library
+  >  (name consumer_lib)
+  >  (wrapped false)
+  >  (libraries active_lib shadowed_lib))
+  > EOF
+  $ cat > consumer_lib/consumer.ml <<EOF
+  > let _ = Shared.from_active
+  > EOF
+  $ cat > consumer_lib/filler.ml <<EOF
+  > let _ = ()
+  > EOF
+
+  $ dune build @check
+
+Edit [shadowed_lib]'s [Shared] — which [consumer] does *not* resolve
+through, because [active_lib] wins under [-I] order. [consumer]
+still rebuilds because its dependency on [shadowed_lib]'s object
+directory is a glob:
+
+  $ cat > shadowed_lib/shared.ml <<EOF
+  > let from_shadowed = "b"
+  > let also_from_shadowed = 42
+  > EOF
+  $ dune build @check
+  $ dune trace cat | jq -s 'include "dune"; [.[] | targetsMatchingFilter(test("consumer_lib/\\.consumer_lib\\.objs/byte/consumer\\."))] | length > 0'
+  true