perf(library/init/lean/frontend): do not hold on to outputs (syntax trees etc.) by default

library/init/lean/frontend.lean

@@ -24,12 +24,13 @@ do t ← parser.mk_token_trie $
      trailing_term_parsers := term.builtin_trailing_parsers,
    }
 
-meta def run_frontend (filename input : string) (print_msg : message → except_t string io unit) :
+meta def run_frontend (filename input : string) (print_msg : message → except_t string io unit) (collect_outputs : bool) :
   except_t string io (list module_parser_output) := do
   parser_cfg ← monad_except.lift_except $ mk_config filename input,
   let expander_cfg : expander_config := {filename := filename, input := input, transformers := builtin_transformers},
   let parser_k := parser.run parser_cfg input (λ st _, module.parser st),
   let elab_k := elaborator.run {filename := filename, input := input, initial_parser_cfg := parser_cfg},
+  let add_output (out : module_parser_output) outs := if collect_outputs then out::outs else [],
   io.prim.iterate_eio (parser_k, elab_k, parser_cfg, expander_cfg, ([] : list module_parser_output)) $ λ ⟨parser_k, elab_k, parser_cfg, expander_cfg, outs⟩, do
     let pos := match outs with out::_ := out.pos | [] := default _,
     r ← monad_lift $ profileit_pure "parsing" pos $ λ _, parser_k.resume parser_cfg,
@@ -63,14 +64,14 @@ meta def run_frontend (filename input : string) (print_msg : message → except_
             pos := ⟨1, 0⟩,
             text := "parser cache hit rate: " ++ to_string out.cache.hit ++ "/" ++
               to_string (out.cache.hit + out.cache.miss)},-/
-          pure $ sum.inr (out::outs).reverse
+          pure $ sum.inr (add_output out outs).reverse
         }
         | coroutine_result_core.yielded elab_out elab_k := do {
           elab_out.messages.to_list.mfor print_msg,
-          pure (sum.inl (parser_k, elab_k, elab_out.parser_cfg, elab_out.expander_cfg, out :: outs))
+          pure (sum.inl (parser_k, elab_k, elab_out.parser_cfg, elab_out.expander_cfg, add_output out outs))
         }
       }
-      | except.error e := print_msg e *> pure (sum.inl (parser_k, elab_k, parser_cfg, expander_cfg, out :: outs))
+      | except.error e := print_msg e *> pure (sum.inl (parser_k, elab_k, parser_cfg, expander_cfg, add_output out outs))
     }
 
 @[export lean_process_file]
@@ -87,7 +88,7 @@ meta def process_file (f s : string) (json : bool) : io bool := do
         ", \"caption\": " ++ repr msg.caption ++
         ", \"text\": " ++ repr msg.text ++ "}"
     else io.println msg.to_string,
-  ex ← (run_frontend f s print_msg).run,
+  ex ← (run_frontend f s print_msg ff).run,
   match ex with
   | except.ok _    := pure tt
   | except.error e := do