test: demonstrate pointless running of ocamldep on the root module

Signed-off-by: Rudi Grinberg <[email protected]>

<!-- ps-id: 0e596563-4448-4087-b051-1eef7a4c9848 -->

Author: rgrinberg

test/blackbox-tests/test-cases/root-module/root-module-ocamldep.t

@@ -0,0 +1,45 @@
+Demonstrate if we're running ocamldep for the root module. There should be no
+need to do so since this module cannot depend on any other module in the same
+compilation unit.
+
+  $ cat >dune-project <<EOF
+  > (lang dune 3.20)
+  > EOF
+
+  $ mkdir lib/
+  $ cat >lib/dune <<EOF
+  > (library
+  >  (name bar))
+  > EOF
+  $ touch lib/bar.ml
+
+  $ cat >dune <<EOF
+  > (library
+  >  (name foo)
+  >  (libraries bar)
+  >  (root_module root))
+  > EOF
+
+  $ dune build foo.cma
+
+  $ find _build/default/.foo.objs | grep -i root | sort -u
+  _build/default/.foo.objs/byte/foo__Root.cmi
+  _build/default/.foo.objs/byte/foo__Root.cmo
+  _build/default/.foo.objs/byte/foo__Root.cmt
+  _build/default/.foo.objs/foo__Root.impl.all-deps
+  _build/default/.foo.objs/foo__Root.impl.d
+
+Not only is running ocamldep wasteful, but it can also lead to cycles:
+
+  $ cat >bar.ml <<EOF
+  > module X = Root.Bar
+  > EOF
+
+  $ dune build foo.cma
+  Error: Dependency cycle between:
+     _build/default/.foo.objs/foo__Bar.impl.all-deps
+  -> _build/default/.foo.objs/foo__Root.impl.all-deps
+  -> _build/default/.foo.objs/foo__Bar.impl.all-deps
+  -> required by _build/default/.foo.objs/byte/foo__Bar.cmo
+  -> required by _build/default/foo.cma
+  [1]