test: show package install layout regression with MDX

test/blackbox-tests/test-cases/package-materialization/mdx-rule.t

@@ -0,0 +1,71 @@
+Show regression from #14373 where local packages need explicit dependency
+declarations when used with MDX.
+
+  $ cat >dune-project <<EOF
+  > (lang dune 3.24)
+  > (package (name iter_mdx_regression))
+  > EOF
+
+  $ mkdir src
+
+  $ cat >src/dune <<EOF
+  > (library (public_name iter_mdx_regression))
+  > EOF
+
+  $ cat >src/iter_mdx_regression.ml <<EOF
+  > let answer = 42
+  > EOF
+
+  $ cat >README.md <<'EOF'
+  > ```ocaml
+  > # #require "iter_mdx_regression";;
+  > # Iter_mdx_regression.answer;;
+  > - : int = 42
+  > ```
+  > EOF
+
+This is the shape of the rule that used to work accidentally because
+workspace packages were visible through the shared install area.
+
+  $ cat >dune <<'EOF'
+  > (rule
+  >  (alias runtest)
+  >  (deps
+  >   (:dep README.md))
+  >  (action
+  >   (progn
+  >    (run ocaml-mdx test %{dep})
+  >    (diff? %{dep} %{dep}.corrected))))
+  > EOF
+
+  $ dune runtest
+  File "README.md", line 1, characters 0-0:
+  --- README.md
+  +++ README.md.corrected
+  @@ -1,5 +1,7 @@
+   ```ocaml
+   # #require "iter_mdx_regression";;
+  +No such package: iter_mdx_regression
+   # Iter_mdx_regression.answer;;
+  -- : int = 42
+  +Line 1, characters 1-20:
+  +Error: Unbound module Iter_mdx_regression
+   ```
+  [1]
+
+Declaring the package dependency gives the action an OCAMLPATH where
+ocaml-mdx can find the local package.
+
+  $ cat >dune <<'EOF'
+  > (rule
+  >  (alias runtest)
+  >  (deps
+  >   (package iter_mdx_regression)
+  >   (:dep README.md))
+  >  (action
+  >   (progn
+  >    (run ocaml-mdx test %{dep})
+  >    (diff? %{dep} %{dep}.corrected))))
+  > EOF
+
+  $ dune runtest