Proof of concept: one shot file compilation

compiler/bsb/bsb_package_specs.ml

@@ -130,15 +130,23 @@ let from_json suffix (x : Ext_json_types.t) : Spec_set.t =
   | Arr {content; _} -> from_array suffix content
   | _ -> Spec_set.singleton (from_json_single suffix x)
 
-let bs_package_output = "-bs-package-output"
-
 [@@@warning "+9"]
 
-let package_flag ({format; in_source; suffix} : Bsb_spec_set.spec) dir =
-  Ext_string.inter2 bs_package_output
-    (Ext_string.concat5 (string_of_format format) Ext_string.single_colon
-       (if in_source then dir else Bsb_config.top_prefix_of_format format // dir)
-       Ext_string.single_colon suffix)
+let package_flag ({format; in_source; suffix} : Bsb_spec_set.spec) _dir =
+  (* Generates three separate compiler flags that were split from the original
+     single "-bs-package-output module:path:suffix" flag.
+
+     For in-source builds, pass "." as the base directory. The compiler will
+     extract the source subdirectory from output_prefix to construct the correct path.
+     For out-of-source builds, pass the lib output directory (e.g., "lib/es6").
+  *)
+  let module_system_flag = "-bs-module-system " ^ string_of_format format in
+  let suffix_flag = "-bs-suffix " ^ suffix in
+  let output_path =
+    if in_source then "." else Bsb_config.top_prefix_of_format format
+  in
+  let output_flag = "-bs-package-output " ^ output_path in
+  module_system_flag ^ " " ^ suffix_flag ^ " " ^ output_flag
 
 (* FIXME: we should adapt it *)
 let package_flag_of_package_specs (package_specs : t) ~(dirname : string) :
@@ -179,7 +187,15 @@ let get_list_of_output_js (package_specs : t)
         Ext_namespace.change_ext_ns_suffix output_file_sans_extension
           spec.suffix
       in
-      (if spec.in_source then Bsb_config.rev_lib_bs_prefix basename
+      (if spec.in_source then
+         (* When dir=".", name_sans_extension can be "./Module" (from Filename.concat "." "Module").
+            Strip the "./" prefix to avoid generating "../.././Module.res.js" *)
+         let basename_clean =
+           if Ext_string.starts_with basename "./" then
+             String.sub basename 2 (String.length basename - 2)
+           else basename
+         in
+         Bsb_config.rev_lib_bs_prefix basename_clean
        else Bsb_config.lib_bs_prefix_of_format spec.format // basename)
       :: acc)
     package_specs.modules []

compiler/bsc/rescript_compiler_main.ml

@@ -259,10 +259,23 @@ let command_line_flags : (string * Bsc_args.spec * string) array =
       string_call ignore,
       "*internal* Set jsx mode, this is no longer used and is a no-op." );
     ("-bs-jsx-preserve", set Js_config.jsx_preserve, "*internal* Preserve jsx");
+    ( "-bs-module-system",
+      string_call (fun s ->
+          Js_config.default_module_system :=
+            match Js_packages_info.module_system_of_string s with
+            | Some ms -> ms
+            | None ->
+              Bsc_args.bad_arg
+                ("Invalid module system: " ^ s
+               ^ ". Use: commonjs, esmodule, or es6-global")),
+      "*internal* Set module system: commonjs, esmodule, es6-global" );
+    ( "-bs-suffix",
+      string_call (fun s -> Js_config.default_suffix := s),
+      "*internal* Set import file suffix: .js, .mjs, .cjs" );
     ( "-bs-package-output",
       string_call Js_packages_state.update_npm_package_path,
-      "*internal* Set npm-output-path: [opt_module]:path, for example: \
-       'lib/cjs', 'amdjs:lib/amdjs', 'es6:lib/es6' " );
+      "*internal* Set output path (when combined with -bs-module-system and \
+       -bs-suffix)" );
     ( "-bs-ast",
       unit_call (fun _ ->
           Js_config.binary_ast := true;

compiler/common/js_config.ml

@@ -52,6 +52,8 @@ let jsx_version = ref None
 let jsx_module = ref React
 let jsx_preserve = ref false
 let js_stdout = ref true
+let default_module_system = ref Ext_module_system.Commonjs
+let default_suffix = ref Literals.suffix_js
 let all_module_aliases = ref false
 let no_stdlib = ref false
 let no_export = ref false

compiler/common/js_config.mli

@@ -84,6 +84,10 @@ val jsx_preserve : bool ref
 
 val js_stdout : bool ref
 
+val default_module_system : Ext_module_system.t ref
+
+val default_suffix : string ref
+
 val all_module_aliases : bool ref
 
 val no_stdlib : bool ref

compiler/core/js_packages_info.ml

@@ -192,13 +192,21 @@ let add_npm_package_path (packages_info : t) (s : string) : t =
     in
     let m =
       match Ext_string.split ~keep_empty:true s ':' with
-      | [path] -> {module_system = Esmodule; path; suffix = Literals.suffix_js}
+      (* NEW: Just path - use configured module system and suffix *)
+      | [path] ->
+        {
+          module_system = !Js_config.default_module_system;
+          path;
+          suffix = !Js_config.default_suffix;
+        }
+      (* OLD: module_system:path - use configured suffix *)
       | [module_system; path] ->
         {
           module_system = handle_module_system module_system;
           path;
-          suffix = Literals.suffix_js;
+          suffix = !Js_config.default_suffix;
         }
+      (* OLD: Full format - all explicit *)
       | [module_system; path; suffix] ->
         {module_system = handle_module_system module_system; path; suffix}
       | _ -> Bsc_args.bad_arg ("invalid npm package path: " ^ s)

compiler/core/js_packages_info.mli

@@ -50,6 +50,9 @@ val is_empty : t -> bool
 
 val dump_packages_info : Format.formatter -> t -> unit
 
+val module_system_of_string : string -> module_system option
+(** Parse module system from string (commonjs, esmodule, es6, es6-global) *)
+
 val add_npm_package_path : t -> string -> t
 (** used by command line option 
     e.g [-bs-package-output commonjs:xx/path]

compiler/core/lam_compile_main.ml

@@ -292,7 +292,8 @@ let lambda_as_module
   : unit = 
   let package_info = Js_packages_state.get_packages_info () in 
   if Js_packages_info.is_empty package_info && !Js_config.js_stdout then begin    
-    Js_dump_program.dump_deps_program ~output_prefix Commonjs (lambda_output) stdout
+    (* Use configured module system instead of hardcoded Commonjs *)
+    Js_dump_program.dump_deps_program ~output_prefix !Js_config.default_module_system (lambda_output) stdout
   end else
     Js_packages_info.iter package_info (fun {module_system; path; suffix} -> 
         let output_chan chan  = 
@@ -303,12 +304,40 @@ let lambda_as_module
         let basename =  
           Ext_namespace.change_ext_ns_suffix (Filename.basename output_prefix) suffix
         in
+        (* Construct target path:
+           - For bsb (path="."): extract source subdir from output_prefix
+           - For rewatch: path already contains full directory from file_path.parent()
+           - basename is the final filename *)
         let target_file = 
-          (Lazy.force Ext_path.package_dir //
-           path //
-           basename
-           (* #913 only generate little-case js file *)
-          ) in     
+          (* Check if path is a base output directory (bsb mode) vs full path (rewatch mode)
+             Use starts_with to handle both Unix (lib/es6) and Windows (lib\es6) paths *)
+          let is_base_path = 
+            path = "." || 
+            Ext_string.starts_with path "lib" && 
+            (Ext_string.contain_substring path "bs" || 
+             Ext_string.contain_substring path "js" || 
+             Ext_string.contain_substring path "es6")
+          in
+          if is_base_path then
+            (* Legacy bsb mode: path is base dir, extract source subdir from output_prefix *)
+            let source_subdir = Filename.dirname output_prefix in
+            (* When source_subdir is ".", don't include it in the path to avoid "././" *)
+            if source_subdir = "." then
+              (Lazy.force Ext_path.package_dir //
+               path //
+               basename)
+            else
+              (Lazy.force Ext_path.package_dir //
+               path //
+               source_subdir //
+               basename)
+          else
+            (* Rewatch mode: path already contains full directory *)
+            (Lazy.force Ext_path.package_dir //
+             path //
+             basename
+             (* #913 only generate little-case js file *)
+            ) in     
         (if not !Clflags.dont_write_files then 
            Ext_pervasives.with_file_as_chan
              target_file output_chan );

compiler/core/lam_compile_primitive.ml

@@ -46,7 +46,8 @@ let get_module_system () =
   let package_info = Js_packages_state.get_packages_info () in
   let module_system =
     if Js_packages_info.is_empty package_info && !Js_config.js_stdout then
-      [Ext_module_system.Commonjs]
+      (* Use configured module system instead of hardcoded Commonjs *)
+      [!Js_config.default_module_system]
     else
       Js_packages_info.map package_info (fun {module_system} -> module_system)
   in