fix: make ignored rules fallback

Fixes ocaml#8703

This is a lighter change than the original in ocaml#8518.

Signed-off-by: Etienne Millon <me@emillon.org>

doc/changes/8518.md

@@ -1,2 +1,3 @@
-- Ignore internal promote rules when `--ignore-promoted-rules` is set (#8518,
-  fix #8417, @rgrinberg)
+- Turn internal promote rules into fallback rules when
+  `--ignore-promoted-rules` is set (#8518, #...., fix #8417, fix #8703,
+  @rgrinberg, @emillon)

src/dune_rules/super_context.ml

@@ -259,31 +259,29 @@ let extend_action t ~dir action =
 ;;
 
 let make_rule t ?mode ?loc ~dir { Action_builder.With_targets.build; targets } =
-  match mode with
-  | Some mode when Rule_mode_decoder.is_ignored mode ~until_clean:`Keep -> None
-  | _ ->
-    let build = extend_action t build ~dir in
-    Some
-      (Rule.make
-         ?mode
-         ~info:(Rule.Info.of_loc_opt loc)
-         ~context:(Some (Context.build_context (Env_tree.context t)))
-         ~targets
-         build)
+  let (mode : Rule.Mode.t option) =
+    match mode with
+    | Some mode when Rule_mode_decoder.is_ignored mode ~until_clean:`Keep -> Some Fallback
+    | _ -> mode
+  in
+  let build = extend_action t build ~dir in
+  Rule.make
+    ?mode
+    ~info:(Rule.Info.of_loc_opt loc)
+    ~context:(Some (Context.build_context (Env_tree.context t)))
+    ~targets
+    build
 ;;
 
 let add_rule t ?mode ?loc ~dir build =
-  match make_rule t ?mode ?loc ~dir build with
-  | None -> Memo.return ()
-  | Some rule -> Rules.Produce.rule rule
+  let rule = make_rule t ?mode ?loc ~dir build in
+  Rules.Produce.rule rule
 ;;
 
 let add_rule_get_targets t ?mode ?loc ~dir build =
-  match make_rule t ?mode ?loc ~dir build with
-  | None -> Memo.return None
-  | Some rule ->
-    let+ () = Rules.Produce.rule rule in
-    Some rule.targets
+  let rule = make_rule t ?mode ?loc ~dir build in
+  let+ () = Rules.Produce.rule rule in
+  Some rule.targets
 ;;
 
 let add_rules t ?loc ~dir builds = Memo.parallel_iter builds ~f:(add_rule ?loc t ~dir)

test/blackbox-tests/test-cases/missing-opam-generated-file.t

@@ -16,6 +16,3 @@ will overwrite `pkg.opam`. Now it does not update the file.
   $ touch opam
 
   $ dune build -p pkg @install
-  Error: No rule found for pkg.opam
-  -> required by alias install
-  [1]