Add flag --empty-sourcemap

This patch implements the `--empty-sourcemap` flag. It is used to
generate an empty sourcemap, which is cheap. Sometimes application
authors would like the cheapness of not building sourcemap files to
speed up development iterations, but their build pipelines do not handle
the case where `.map` files are not produced at all (think, e.g., custom
Dune rules depending on `.bc.map` files). In that cases, they would like
to still generate the empty `.bc.map` files.

Co-authored-by: enoumy <enoumy@gmail.com>

Author: 2 people

CHANGES.md

@@ -23,6 +23,7 @@
 * Ppx: explicitly disallow polymorphic method (#1897)
 * Ppx: allow "function" in object literals (#1897)
 * Lib: make the Wasm version of Json.output work with native ints and JavaScript objects (#1872)
+* Compiler: add the `--empty-sourcemap` flag
 * Compiler: static evaluation of more primitives (#1912)
 * Compiler: faster compilation by stopping sooner when optimizations become unproductive (#1939)
 * Compiler: improve debug/sourcemap location of closures (#1947)

compiler/bin-js_of_ocaml/cmd_arg.ml

@@ -54,7 +54,7 @@ type t =
   { common : Jsoo_cmdline.Arg.t
   ; (* compile option *)
     profile : Driver.profile option
-  ; source_map : (string option * Source_map.Standard.t) option
+  ; source_map : Source_map.Encoding_spec.t option
   ; runtime_files : string list
   ; no_runtime : bool
   ; include_runtime : bool
@@ -158,6 +158,10 @@ let options =
     let doc = "Do not inline sources in source map." in
     Arg.(value & flag & info [ "source-map-no-source" ] ~doc)
   in
+  let sourcemap_empty =
+    let doc = "Always generate empty source maps." in
+    Arg.(value & flag & info [ "empty-sourcemap"; "empty-source-map" ] ~doc)
+  in
   let sourcemap_root =
     let doc = "root dir for source map." in
     Arg.(value & opt (some string) None & info [ "source-map-root" ] ~doc)
@@ -296,6 +300,7 @@ let options =
       sourcemap
       sourcemap_inline_in_js
       sourcemap_don't_inline_content
+      sourcemap_empty
       sourcemap_root
       target_env
       output_file
@@ -330,12 +335,19 @@ let options =
           | `Name file, _ -> Some file, Some (chop_extension file ^ ".map")
           | `Stdout, _ -> None, None
         in
-        Some
-          ( sm_output_file
-          , { (Source_map.Standard.empty ~inline_source_content) with
-              file
-            ; sourceroot = sourcemap_root
-            } )
+        let source_map =
+          { (Source_map.Standard.empty ~inline_source_content) with
+            file
+          ; sourceroot = sourcemap_root
+          }
+        in
+        let spec =
+          { Source_map.Encoding_spec.output_file = sm_output_file
+          ; source_map
+          ; keep_empty = sourcemap_empty
+          }
+        in
+        Some spec
       else None
     in
     let params : (string * string) list = List.flatten set_param in
@@ -391,6 +403,7 @@ let options =
       $ sourcemap
       $ sourcemap_inline_in_js
       $ sourcemap_don't_inline_content
+      $ sourcemap_empty
       $ sourcemap_root
       $ target_env
       $ output_file
@@ -437,6 +450,10 @@ let options_runtime_only =
     let doc = "Do not inline sources in source map." in
     Arg.(value & flag & info [ "source-map-no-source" ] ~doc)
   in
+  let sourcemap_empty =
+    let doc = "Always generate empty source maps." in
+    Arg.(value & flag & info [ "empty-sourcemap"; "empty-source-map" ] ~doc)
+  in
   let sourcemap_root =
     let doc = "root dir for source map." in
     Arg.(value & opt (some string) None & info [ "source-map-root" ] ~doc)
@@ -548,6 +565,7 @@ let options_runtime_only =
       sourcemap
       sourcemap_inline_in_js
       sourcemap_don't_inline_content
+      sourcemap_empty
       sourcemap_root
       target_env
       output_file
@@ -570,12 +588,19 @@ let options_runtime_only =
           | `Name file, _ -> Some file, Some (chop_extension file ^ ".map")
           | `Stdout, _ -> None, None
         in
-        Some
-          ( sm_output_file
-          , { (Source_map.Standard.empty ~inline_source_content) with
-              file
-            ; sourceroot = sourcemap_root
-            } )
+        let source_map =
+          { (Source_map.Standard.empty ~inline_source_content) with
+            file
+          ; sourceroot = sourcemap_root
+          }
+        in
+        let spec =
+          { Source_map.Encoding_spec.output_file = sm_output_file
+          ; source_map
+          ; keep_empty = sourcemap_empty
+          }
+        in
+        Some spec
       else None
     in
     let params : (string * string) list = List.flatten set_param in
@@ -626,6 +651,7 @@ let options_runtime_only =
       $ sourcemap
       $ sourcemap_inline_in_js
       $ sourcemap_don't_inline_content
+      $ sourcemap_empty
       $ sourcemap_root
       $ target_env
       $ output_file

compiler/bin-js_of_ocaml/cmd_arg.mli

@@ -23,7 +23,7 @@ type t =
   { common : Jsoo_cmdline.Arg.t
   ; (* compile option *)
     profile : Driver.profile option
-  ; source_map : (string option * Source_map.Standard.t) option
+  ; source_map : Source_map.Encoding_spec.t option
   ; runtime_files : string list
   ; no_runtime : bool
   ; include_runtime : bool

compiler/bin-js_of_ocaml/compile.ml

@@ -37,33 +37,33 @@ let jsoo_header formatter build_info =
   Pretty_print.string formatter (Printf.sprintf "%s\n" Global_constant.header);
   Pretty_print.string formatter (Build_info.to_string build_info)
 
-type source_map_output =
-  | No_sourcemap
-  | Inline
-  | File of string
+let source_map_enabled : Source_map.Encoding_spec.t option -> bool = function
+  | None -> false
+  | Some _ -> true
 
-let source_map_enabled = function
-  | No_sourcemap -> false
-  | Inline | File _ -> true
-
-let output_gen ~standalone ~custom_header ~build_info ~source_map output_file f =
+let output_gen
+    ~standalone
+    ~custom_header
+    ~build_info
+    ~(source_map : Source_map.Encoding_spec.t option)
+    output_file
+    f =
   let f chan k =
     let fmt = Pretty_print.to_out_channel chan in
     Driver.configure fmt;
     if standalone then header ~custom_header fmt;
     if Config.Flag.header () then jsoo_header fmt build_info;
     let sm = f ~standalone ~source_map (k, fmt) in
     match source_map, sm with
-    | No_sourcemap, _ | _, None -> ()
-    | ((Inline | File _) as output), Some sm ->
+    | None, _ | _, None -> ()
+    | Some { output_file = output; source_map = _; keep_empty = _ }, Some sm ->
         if Debug.find "invariant" () then Source_map.invariant sm;
         let urlData =
           match output with
-          | No_sourcemap -> assert false
-          | Inline ->
+          | None ->
               let data = Source_map.to_string sm in
               "data:application/json;base64," ^ Base64.encode_exn data
-          | File output_file ->
+          | Some output_file ->
               Source_map.to_file sm output_file;
               Filename.basename output_file
         in
@@ -156,12 +156,8 @@ let run
     ; include_runtime
     ; effects
     } =
-  let source_map_base = Option.map ~f:snd source_map in
-  let source_map =
-    match source_map with
-    | None -> No_sourcemap
-    | Some (None, _) -> Inline
-    | Some (Some file, _) -> File file
+  let source_map_base =
+    Option.map ~f:(fun spec -> spec.Source_map.Encoding_spec.source_map) source_map
   in
   let include_cmis = toplevel && not no_cmis in
   let custom_header = common.Jsoo_cmdline.Arg.custom_header in
@@ -223,9 +219,9 @@ let run
   Linker.check_deps ();
   if times () then Format.eprintf "  parsing js: %a@." Timer.print t1;
   if times () then Format.eprintf "Start parsing...@.";
-  let need_debug = source_map_enabled source_map || Config.Flag.debuginfo () in
+  let need_debug = Option.is_some source_map || Config.Flag.debuginfo () in
   let check_debug (one : Parse_bytecode.one) =
-    if source_map_enabled source_map && Parse_bytecode.Debug.is_empty one.debug
+    if Option.is_some source_map && Parse_bytecode.Debug.is_empty one.debug
     then
       warn
         "Warning: '--source-map' is enabled but the bytecode program was compiled with \
@@ -254,7 +250,7 @@ let run
       (one : Parse_bytecode.one)
       ~check_sourcemap
       ~standalone
-      ~source_map
+      ~(source_map : Source_map.Encoding_spec.t option)
       ~link
       output_file =
     if check_sourcemap then check_debug one;

compiler/bin-js_of_ocaml/link.ml

@@ -23,7 +23,7 @@ open Cmdliner
 
 type t =
   { common : Jsoo_cmdline.Arg.t
-  ; source_map : (string option * Source_map.Standard.t) option
+  ; source_map : Source_map.Encoding_spec.t option
   ; js_files : string list
   ; output_file : string option
   ; resolve_sourcemap_url : bool
@@ -51,6 +51,10 @@ let options =
     let doc = "Inline sourcemap in the generated JavaScript." in
     Arg.(value & flag & info [ "source-map-inline" ] ~doc)
   in
+  let sourcemap_empty =
+    let doc = "Always generate empty source maps." in
+    Arg.(value & flag & info [ "empty-sourcemap"; "empty-source-map" ] ~doc)
+  in
   let sourcemap_root =
     let doc = "root dir for source map." in
     Arg.(value & opt (some string) None & info [ "source-map-root" ] ~doc)
@@ -83,6 +87,7 @@ let options =
       no_sourcemap
       sourcemap
       sourcemap_inline_in_js
+      sourcemap_empty
       sourcemap_root
       output_file
       resolve_sourcemap_url
@@ -100,12 +105,19 @@ let options =
           | Some file -> Some file, Some (chop_extension file ^ ".map")
           | None -> None, None
         in
-        Some
-          ( sm_output_file
-          , { (Source_map.Standard.empty ~inline_source_content:true) with
-              file
-            ; sourceroot = sourcemap_root
-            } )
+        let source_map =
+          { (Source_map.Standard.empty ~inline_source_content:true) with
+            file
+          ; sourceroot = sourcemap_root
+          }
+        in
+        let spec =
+          { Source_map.Encoding_spec.output_file = sm_output_file
+          ; source_map
+          ; keep_empty = sourcemap_empty
+          }
+        in
+        Some spec
       else None
     in
     `Ok
@@ -126,6 +138,7 @@ let options =
       $ no_sourcemap
       $ sourcemap
       $ sourcemap_inline_in_js
+      $ sourcemap_empty
       $ sourcemap_root
       $ output_file
       $ resolve_sourcemap_url

compiler/lib/link_js.ml

@@ -224,7 +224,14 @@ end = struct
     build_info, units
 end
 
-let link ~output ~linkall ~mklib ~toplevel ~files ~resolve_sourcemap_url ~source_map =
+let link
+    ~output
+    ~linkall
+    ~mklib
+    ~toplevel
+    ~files
+    ~resolve_sourcemap_url
+    ~(source_map : Source_map.Encoding_spec.t option) =
   (* we currently don't do anything with [toplevel]. It could be used
      to conditionally include link_info ?*)
   ignore (toplevel : bool);
@@ -315,13 +322,12 @@ let link ~output ~linkall ~mklib ~toplevel ~files ~resolve_sourcemap_url ~source
         match Line_reader.peek ic with
         | None -> ()
         | Some line ->
-            (match
-               action
-                 ~resolve_sourcemap_url
-                 ~drop_source_map:(Option.is_none source_map)
-                 file
-                 line
-             with
+            let drop_source_map =
+              match source_map with
+              | None | Some { keep_empty = true; _ } -> true
+              | Some { keep_empty = false; _ } -> false
+            in
+            (match action ~resolve_sourcemap_url ~drop_source_map file line with
             | Keep -> copy ic oc
             | Build_info bi ->
                 skip ic;
@@ -437,7 +443,7 @@ let link ~output ~linkall ~mklib ~toplevel ~files ~resolve_sourcemap_url ~source
   let t = Timer.make () in
   match source_map with
   | None -> ()
-  | Some (file, init_sm) ->
+  | Some { output_file = file; source_map = init_sm; keep_empty = _ } ->
       let sections =
         List.rev_map !sm ~f:(fun (sm, reloc, _offset) ->
             let sm =

compiler/lib/link_js.mli

@@ -24,5 +24,5 @@ val link :
   -> toplevel:bool
   -> files:string list
   -> resolve_sourcemap_url:bool
-  -> source_map:(string option * Source_map.Standard.t) option
+  -> source_map:Source_map.Encoding_spec.t option
   -> unit

compiler/lib/source_map.ml

@@ -792,3 +792,12 @@ let find_in_js_file file =
       in
       Some (of_string content)
   | _ -> None
+
+module Encoding_spec = struct
+  type t =
+    { output_file : string option  (** Source map file ([None] means generate inline. *)
+    ; source_map : Standard.t  (** Source map to extend. *)
+    ; keep_empty : bool
+          (** Don't add anything to the source map (for js_of_ocaml's "empty sourcemap" option. *)
+    }
+end

compiler/lib/source_map.mli

@@ -156,3 +156,12 @@ type info =
   ; sources : string list
   ; names : string list
   }
+
+module Encoding_spec : sig
+  type t =
+    { output_file : string option  (** Source map file ([None] means generate inline. *)
+    ; source_map : Standard.t  (** Source map to extend. *)
+    ; keep_empty : bool
+          (** Don't add anything to the source map (for js_of_ocaml's "empty sourcemap" option. *)
+    }
+end