test: lock in incremental-rebuild for root_module-aliased deps

Add a regression test asserting that a consumer using
[(root_module ...)] to alias a dependency rebuilds when the
dependency's [.mli] changes, and does not rebuild when only the
dependency's [.ml] changes. The property holds today via dune's
existing glob-over-objdir mechanism; the test guards it as a
specification against future inter-library-dependency-tracking
changes (notably the per-module filter being developed in

Signed-off-by: Robin Bate Boerop <me@robinbb.com>
ocaml#14116).

Author: robinbb

test/blackbox-tests/test-cases/root-module/incremental-rebuild.t

@@ -0,0 +1,53 @@
+A consumer library that uses [(root_module ...)] to alias one of
+its dependencies is rebuilt when the dependency's interface
+changes, and not rebuilt when only the dependency's
+implementation changes. This locks in the incremental-rebuild
+property for [Root]-aliased dependencies that any future change
+to dune's inter-library-dependency tracking must preserve.
+
+  $ cat > dune-project <<EOF
+  > (lang dune 3.0)
+  > EOF
+
+  $ mkdir lib1 lib2
+  $ cat > lib1/dune <<EOF
+  > (library (name lib1))
+  > EOF
+  $ cat > lib1/lib1.ml <<EOF
+  > let extra = 0
+  > let greeting = "hello-" ^ string_of_int extra
+  > EOF
+  $ cat > lib1/lib1.mli <<EOF
+  > val greeting : string
+  > EOF
+
+  $ cat > lib2/dune <<EOF
+  > (library (name lib2) (libraries lib1) (root_module root))
+  > EOF
+  $ cat > lib2/lib2.ml <<EOF
+  > let () = print_endline Root.Lib1.greeting
+  > EOF
+
+  $ dune build @check
+
+Editing only [lib1.mli] (the [.ml] is unchanged) changes
+[lib1.cmi] and must invalidate [lib2]:
+
+  $ cat > lib1/lib1.mli <<EOF
+  > val greeting : string
+  > val extra : int
+  > EOF
+  $ dune build @check
+  $ dune trace cat | jq -s 'include "dune"; [.[] | targetsMatchingFilter(test("\\.lib2\\.objs/"))] | length > 0'
+  true
+
+Editing only [lib1.ml] (no [.mli] change) leaves [lib1.cmi]
+untouched, so [lib2] is not rebuilt:
+
+  $ cat > lib1/lib1.ml <<EOF
+  > let extra = 1
+  > let greeting = "hello-" ^ string_of_int extra
+  > EOF
+  $ dune build @check
+  $ dune trace cat | jq -s 'include "dune"; [.[] | targetsMatchingFilter(test("\\.lib2\\.objs/"))] | length'
+  0