Warnings: filter out warnings from @generated files

Summary: This has long been in discussion. I'm implementing the simplest solution: when filtering warnings in hh_client, look at the first 50 lines of the files for the "generated" string. I'm also caching the results of that lookup. I'm testing the performance (see test plan), and it looks acceptable.

Differential Revision: D66236754

fbshipit-source-id: 505c6c696aadd94bc2b246c7afc4cce691e9e562

hphp/hack/src/client/clientArgs.ml

@@ -727,9 +727,7 @@ rewrite to the function names to something like `foo_1` and `foo_2`.
         " hide warnings in files matching a regexp" );
       ( "-Wgenerated",
         Arg.Unit (fun () -> add_warning_switch Filter_errors.Generated_files_on),
-        " show warnings in generated files (the ones specified by .hhconfig flag warnings_generated_files). "
-        ^ "To ignore more files, use -Wignore-files, avoid modifying warnings_generated_files"
-      );
+        " show warnings in generated files" );
       Common_argspecs.watchman_debug_logging watchman_debug_logging;
       (* Please keep these sorted in the alphabetical order *)
     ]

hphp/hack/src/client_and_server/dune

@@ -241,6 +241,7 @@
  (libraries
   error_codes
   errors
+  re
   user_error
   utils_core))
 

hphp/hack/src/client_and_server/filter_errors.ml

@@ -8,6 +8,7 @@
 
 open Hh_prelude
 module Code = Error_codes.Warning
+module Hashtbl = Stdlib.Hashtbl
 
 type switch =
   | WAll
@@ -72,10 +73,16 @@ end = struct
     codes: Warning_set.t;
     ignore_files: Str.regexp list;
     generated_files: Str.regexp list;
+    filter_out_generated_files: bool;
   }
 
   let empty =
-    { codes = Warning_set.empty; ignore_files = []; generated_files = [] }
+    {
+      codes = Warning_set.empty;
+      ignore_files = [];
+      generated_files = [];
+      filter_out_generated_files = true;
+    }
 
   let add_code code filter =
     { filter with codes = Warning_set.add code filter.codes }
@@ -97,7 +104,8 @@ end = struct
     | Code_on code -> add_code code filter
     | Code_off code -> remove_code code filter
     | Ignored_files re -> add_ignored_files re filter
-    | Generated_files_on -> { filter with generated_files = [] }
+    | Generated_files_on ->
+      { filter with generated_files = []; filter_out_generated_files = false }
 
   let make ~default_all ~generated_files switches =
     let switches =
@@ -128,12 +136,55 @@ end = struct
   let pass_path ignore_files file_path =
     not (List.exists ignore_files ~f:(match_regexp file_path))
 
-  let pass { codes; ignore_files; generated_files } (code, file_path) =
-    match Code.of_enum code with
+  module IsNonGenerated = struct
+    let is_generated_cache : (path, bool) Hashtbl.t = Hashtbl.create 1000
+
+    let at_generated =
+      Re.str
+        ((* Writing like this to prevent CI from thinking this that file is generated...*)
+         "@"
+        ^ "generated")
+      |> Re.compile
+
+    let max_lines = 50
+
+    let is_generated path =
+      try
+        Stdlib.In_channel.with_open_text path @@ fun ic ->
+        let rec go i =
+          if i >= max_lines then
+            false
+          else
+            match Stdlib.In_channel.input_line ic with
+            | None -> false
+            | Some line -> Re.execp at_generated line || go (i + 1)
+        in
+        go 0
+      with
+      | Sys_error _ ->
+        (* File not found. This could be the case for some tests. *) false
+
+    let pass ~skip path =
+      if skip then
+        true
+      else
+        match Hashtbl.find_opt is_generated_cache path with
+        | Some is_generated -> not is_generated
+        | None ->
+          let is_generated = is_generated path in
+          Hashtbl.add is_generated_cache path is_generated;
+          not is_generated
+  end
+
+  let pass
+      { codes; ignore_files; generated_files; filter_out_generated_files }
+      (code, file_path) =
+    match Error_codes.Warning.of_enum code with
     | None -> (* This is not a warning, so don't filter out *) true
     | Some code ->
       pass_code codes code
       && pass_path (ignore_files @ generated_files) file_path
+      && IsNonGenerated.pass ~skip:(not filter_out_generated_files) file_path
 end
 
 let filter filter (error_list : Errors.finalized_error list) =