Revert "Revert "Classic mode: partially lift the restrictions on inli…

…ning functions containing closures"" (#2556)

Revert "Revert "Classic mode: partially lift the restrictions on inlining fun…"

This reverts commit ad876d2.

middle_end/flambda2/from_lambda/closure_conversion.ml

@@ -2145,19 +2145,22 @@ let close_one_function acc ~code_id ~external_env ~by_function_slot
   let contains_subfunctions = Acc.seen_a_function acc in
   let cost_metrics = Acc.cost_metrics acc in
   let inline : Inline_attribute.t =
-    (* We make a decision based on [fallback_inlining_heuristic] here to try to
-       mimic Closure's behaviour as closely as possible, particularly when there
-       are functions involving constant closures, which are not lifted during
-       Closure (but will prevent inlining) but will likely have been lifted by
-       our other check in [Inlining_cost] (thus preventing us seeing they were
-       originally there). Note that while Closure never marks as inlinable
-       functions in a set a recursive definitions with more than one function,
-       we do not try to reproduce this particular property and can mark as
-       inlinable such functions. *)
-    if contains_subfunctions
-       && Flambda_features.Expert.fallback_inlining_heuristic ()
-    then Never_inline
-    else Inline_attribute.from_lambda (Function_decl.inline decl)
+    match Inline_attribute.from_lambda (Function_decl.inline decl) with
+    | (Always_inline | Available_inline | Never_inline) as attr -> attr
+    | (Unroll _ | Default_inline) as attr ->
+      (* We make a decision based on [fallback_inlining_heuristic] here to try
+         to mimic Closure's behaviour as closely as possible, particularly when
+         there are functions involving constant closures, which are not lifted
+         during Closure (but will prevent inlining) but will likely have been
+         lifted by our other check in [Inlining_cost] (thus preventing us seeing
+         they were originally there). Note that while Closure never marks as
+         inlinable functions in a set a recursive definitions with more than one
+         function, we do not try to reproduce this particular property and can
+         mark as inlinable such functions. *)
+      if contains_subfunctions
+         && Flambda_features.Expert.fallback_inlining_heuristic ()
+      then (* CR vlaviron: Store reason *) Never_inline
+      else attr
   in
   let free_names_of_body = Acc.free_names acc in
   let params_and_body =
@@ -2263,7 +2266,9 @@ let close_one_function acc ~code_id ~external_env ~by_function_slot
     else meta
   in
   let acc = Acc.add_code ~code_id ~code acc in
-  let acc = Acc.with_seen_a_function acc true in
+  let acc =
+    if has_lifted_closure then acc else Acc.with_seen_a_function acc true
+  in
   ( acc,
     ( Function_slot.Map.add function_slot approx by_function_slot,
       function_code_ids ) )