Allow .mjs for js module output (#7876)

* Allow .mjs for js module output

Asking for a file of this extension enables EXPORT_ES6 and
MODULARIZE.

Fixes #7853

Author: sbc100

emcc.py

@@ -79,7 +79,7 @@
 
 LIB_PREFIXES = ('', 'lib')
 
-JS_CONTAINING_SUFFIXES = ('js', 'html')
+JS_CONTAINING_SUFFIXES = ('js', 'mjs', 'html')
 EXECUTABLE_SUFFIXES = JS_CONTAINING_SUFFIXES + ('wasm',)
 
 DEFERRED_RESPONSE_FILES = ('EMTERPRETIFY_BLACKLIST', 'EMTERPRETIFY_WHITELIST', 'EMTERPRETIFY_SYNCLIST')
@@ -917,7 +917,12 @@ def detect_fixed_language_mode(args):
     final_ending = ('.' + final_suffix) if len(final_suffix) else ''
 
     # target is now finalized, can finalize other _target s
-    js_target = unsuffixed(target) + '.js'
+    if final_suffix == 'mjs':
+      shared.Settings.EXPORT_ES6 = 1
+      shared.Settings.MODULARIZE = 1
+      js_target = target
+    else:
+      js_target = unsuffixed(target) + '.js'
 
     asm_target = unsuffixed(js_target) + '.asm.js' # might not be used, but if it is, this is the name
     wasm_text_target = asm_target.replace('.asm.js', '.wast') # ditto, might not be used
@@ -1268,7 +1273,11 @@ def check(input_file):
     if shared.Settings.USE_PTHREADS and shared.Settings.WASM and shared.Settings.ALLOW_MEMORY_GROWTH and shared.Settings.WASM_MEM_MAX == -1:
       exit_with_error('If pthreads and memory growth are enabled, WASM_MEM_MAX must be set')
 
-    # When MODULARIZE option is used, currently declare all module exports individually - TODO: this could be optimized
+    if shared.Settings.EXPORT_ES6 and not shared.Settings.MODULARIZE:
+      exit_with_error('EXPORT_ES6 requires MODULARIZE to be set')
+
+    # When MODULARIZE option is used, currently declare all module exports
+    # individually - TODO: this could be optimized
     if shared.Settings.MODULARIZE and not shared.Settings.DECLARE_ASM_MODULE_EXPORTS:
       shared.Settings.DECLARE_ASM_MODULE_EXPORTS = 1
       logger.warning('Enabling -s DECLARE_ASM_MODULE_EXPORTS=1, since MODULARIZE currently requires declaring asm.js/wasm module exports in full')

site/build/text/docs/tools_reference/emcc.txt

@@ -573,15 +573,24 @@ Options that are modified or new in *emcc* are listed below:
    The "target" file name extension defines the output type to be
    generated:
 
-      * <name> **.js** : JavaScript.
+      * <name> **.js** : JavaScript (+ separate **<name>.wasm** file
+        if emitting WebAssembly). (default)
+
+      * <name> **.mjs** : ES6 JavaScript module (+ separate
+        **<name>.wasm** file if emitting WebAssembly).
 
       * <name> **.html** : HTML + separate JavaScript file
-        (**<name>.js**). Having the separate JavaScript file improves
-        page load time.
+        (**<name>.js**; + separate **<name>.wasm** file if emitting
+        WebAssembly).
+
+      * <name> **.bc** : LLVM bitcode.
 
-      * <name> **.bc** : LLVM bitcode (default).
+      * <name> **.o** : LLVM bitcode (same as .bc), unless in
+        *WASM_OBJECT_FILES* mode, in which case it will contain a
+        WebAssembly object.
 
-      * <name> **.o** : LLVM bitcode (same as .bc).
+      * <name> **.wasm** : WebAssembly without JavaScript support
+        code ("standalone wasm").
 
    Note: If "--memory-init-file" is used, a **.mem** file will be
      created in addition to the generated **.js** and/or **.html**

site/source/docs/tools_reference/emcc.rst

@@ -446,6 +446,7 @@ Options that are modified or new in *emcc* are listed below:
   The ``target`` file name extension defines the output type to be generated:
 
     - <name> **.js** : JavaScript (+ separate **<name>.wasm** file if emitting WebAssembly). (default)
+    - <name> **.mjs** : ES6 JavaScript module (+ separate **<name>.wasm** file if emitting WebAssembly).
     - <name> **.html** : HTML + separate JavaScript file (**<name>.js**; + separate **<name>.wasm** file if emitting WebAssembly).
     - <name> **.bc** : LLVM bitcode.
     - <name> **.o** : LLVM bitcode (same as .bc), unless in `WASM_OBJECT_FILES` mode, in which case it will contain a WebAssembly object.

tests/test_other.py

@@ -148,6 +148,15 @@ def test_emcc_generate_config(self):
       self.assertContained('LLVM_ROOT', config_contents)
       os.remove(config_path)
 
+  def test_emcc_output_mjs(self):
+    run_process([PYTHON, EMCC, '-o', 'hello_world.mjs', path_from_root('tests', 'hello_world.c')])
+    with open('hello_world.mjs') as f:
+      output = f.read()
+    self.assertContained('export default Module;', output)
+    # TODO(sbc): Test that this is actually runnable.  We currently don't have
+    # any tests for EXPORT_ES6 but once we do this should be enabled.
+    # self.assertContained('hello, world!', run_js('hello_world.mjs'))
+
   def test_emcc_1(self):
     for compiler, suffix in [(EMCC, '.c'), (EMXX, '.cpp')]:
       # --version