add comment, expand the test

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

Author: anmonteiro

src/dune_rules/module_compilation.ml

@@ -325,7 +325,10 @@ let build_cm
             ; A "-c"
             ; Command.Ml_kind.flag ml_kind
             ; Dep src
-            ; Hidden_deps (Dep.Set.of_files (Option.to_list original))
+            ; (* We add a hidden dependency on the original, pre-PPX source
+                 file, which the compiler wants to find to display error
+                 location snippets. *)
+              Hidden_deps (Dep.Set.of_files (Option.to_list original))
             ; other_targets
             ]
       >>| Action.Full.add_sandbox sandbox))

test/blackbox-tests/test-cases/melange/ppx-preview.t

@@ -15,15 +15,8 @@ Show PPX snippet preview is shown in Dune
   > let x: nope = "hello"
   > EOF
 
-  $ cat > dune <<EOF
-  > (melange.emit
-  >  (target output)
-  >  (libraries the_lib)
-  >  (emit_stdlib false))
-  > EOF
-
   $ export DUNE_SANDBOX=symlink
-  $ dune build @melange
+  $ dune build @all
   File "lib/the_lib.ml", line 1, characters 7-11:
   1 | let x: nope = "hello"
              ^^^^
@@ -33,10 +26,30 @@ Show PPX snippet preview is shown in Dune
 Works if the sandbox is disabled
 
   $ export DUNE_SANDBOX=none
-  $ dune build @melange
+  $ dune build @all
   File "lib/the_lib.ml", line 1, characters 7-11:
   1 | let x: nope = "hello"
              ^^^^
   Error: Unbound type constructor nope
   [1]
 
+  $ cat > lib/the_lib.mli <<EOF
+  > val x: nope
+  > EOF
+
+  $ export DUNE_SANDBOX=symlink
+  $ dune build @all
+  File "lib/the_lib.mli", line 1, characters 7-11:
+  1 | val x: nope
+             ^^^^
+  Error: Unbound type constructor nope
+  [1]
+
+  $ export DUNE_SANDBOX=none
+  $ dune build @all
+  File "lib/the_lib.mli", line 1, characters 7-11:
+  1 | val x: nope
+             ^^^^
+  Error: Unbound type constructor nope
+  [1]
+