test: (modules_without_implementation) expansion (#10280)

Signed-off-by: Antonio Nuno Monteiro <anmonteiro@gmail.com>

test/blackbox-tests/test-cases/modules-without-implementation-expansion.t

@@ -0,0 +1,87 @@
+Test the ability of `(modules_without_implementation)` to contain dynamic
+forms such as `(:include)` and variables such as `"%{read-lines:}"`.
+
+  $ cat >dune-project <<EOF
+  > (lang dune 3.11)
+  > EOF
+
+As we will see later in the test, it is imperative that build dependencies
+needed to evaluate the `(modules)` field not live in the same directory as the
+containing stanza. We will put them in a subdirectory:
+
+  $ mkdir -p gen
+
+We define a rule that creates a file (in sexp syntax, to be passed to
+`(:include)`) containing a single name:
+
+  $ cat >gen/dune <<EOF
+  > (rule (with-stdout-to lst (echo mod)))
+  > EOF
+
+The unit `mod.mli` is present in the working tree, `lib.ml` uses it:
+
+  $ cat >mod.mli <<EOF
+  > type t = | A | B
+  > EOF
+
+  $ cat >main.ml <<EOF
+  > let f (x: Mod.t) = match x with A -> "a" | B -> "b"
+  > let () = print_endline (f A)
+  > EOF
+
+We declare a `library` where the list of modules_without_implementation is read
+from the (generated)
+
+file `gen/lst`:
+
+  $ cat >dune <<EOF
+  > (executable
+  >  (name main)
+  >  (modes byte)
+  >  (modules main (:include gen/lst))
+  >  (modules_without_implementation (:include gen/lst)))
+  > EOF
+
+Let's check that it fails in the current version of Dune:
+
+  $ dune exec ./main.exe
+  File "dune", line 5, characters 1-52:
+  5 |  (modules_without_implementation (:include gen/lst)))
+       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+  Error: the ability to specify non-constant module lists is only available
+  since version 3.13 of the dune language. Please update your dune-project file
+  to have (lang dune 3.13).
+  [1]
+
+Update the version...
+
+  $ cat >dune-project <<EOF
+  > (lang dune 3.13)
+  > EOF
+
+... and it works!
+
+  $ dune exec ./main.exe
+  a
+
+Let's do some examples using libraries:
+
+  $ cat >dune <<EOF
+  > (library
+  >  (name lib)
+  >  (modes byte)
+  >  (modules (:include gen/lst))
+  >  (modules_without_implementation (:include gen/lst)))
+  > EOF
+  $ dune build lib.cma
+
+We can also use special forms such as `%{read-lines:}`:
+
+  $ cat >dune <<EOF
+  > (library
+  >  (name lib)
+  >  (modes byte)
+  >  (modules %{read-lines:gen/lst})
+  >  (modules_without_implementation %{read-lines:gen/lst}))
+  > EOF
+