test(melange): add a test showing dependent re-compilation (ocaml#10501)

Signed-off-by: Antonio Nuno Monteiro <[email protected]>

test/blackbox-tests/test-cases/melange/recompile-dependents.t

@@ -0,0 +1,52 @@
+Test Melange dependent recompilation
+
+Set up and build a Melange library and `melange.emit`
+
+  $ cat > dune-project <<EOF
+  > (lang dune 3.13)
+  > (using melange 0.1)
+  > EOF
+
+  $ mkdir lib
+  $ cat > lib/dune <<EOF
+  > (library
+  >  (name foo)
+  >  (modes melange))
+  > EOF
+
+  $ cat > lib/foo.ml <<EOF
+  > let x () = "hello"
+  > EOF
+  $ cat > lib/foo.mli <<EOF
+  > val x : unit -> string
+  > EOF
+
+  $ cat > dune <<EOF
+  > (melange.emit
+  >  (target out)
+  >  (libraries foo))
+  > EOF
+  $ cat > x.ml <<EOF
+  > let () = Js.log (Foo.x ())
+  > EOF
+
+  $ dune build
+  $ node ./_build/default/out/x.js
+  hello
+
+Now change `foo.ml`, but keep `foo.mli` intact
+
+  $ cat > lib/foo.ml <<EOF
+  > let x () = "hi"
+  > EOF
+
+Build again, noting that x.ml could have been skipped?
+
+  $ dune build --display short
+          melc lib/.foo.objs/melange/foo.{cmj,cmt}
+          melc .out.mobjs/melange/melange__X.{cmi,cmj,cmt}
+          melc out/lib/foo.js
+          melc out/x.js
+
+  $ node ./_build/default/out/x.js
+  hi