Default to EXECUTABLE=1 when generating a JS file with no extension (#26088)

Followup to #26085 which added initial support for `-sEXECUTABLE`.

The assumption here is that if we are generating a file with no
extension (or something called `a.out`) then we are trying to build
something that is directly runnable.

Author: sbc100

ChangeLog.md

@@ -22,7 +22,8 @@ See docs/process.md for more on how version tagging works.
 -----------------------
 - compiler-rt was updated to LLVM 21.1.8. (#26405)
 - A new `-sEXECUTABLE` setting was added which adds a #! line to the resulting
-  JavaScript and makes it executable. (#26085)
+  JavaScript and makes it executable.  This setting defaults to true when the
+  output filename has no extension, or ends in `.out` (e.g. `a.out`) (#26085)
 
 4.0.23 - 01/10/26
 -----------------

docs/emcc.txt

@@ -585,6 +585,11 @@ Options that are modified or new in *emcc* are listed below:
    These rules only apply when linking.  When compiling to object code
    (See *-c* below) the name of the output file is irrelevant.
 
+   Note: Linking to a file with no extension (or a file ending in
+   ".out", like "a.out") will cause the generated JavaScript file to
+   be exectuable, and include a "#!" line to make it runnable
+   directly.
+
 "-c"
    [compile] Tells *emcc* to emit an object file which can then be
    linked with other object files to produce an executable.

site/source/docs/tools_reference/emcc.rst

@@ -574,6 +574,10 @@ Options that are modified or new in *emcc* are listed below:
   These rules only apply when linking.  When compiling to object code (See `-c`
   below) the name of the output file is irrelevant.
 
+  Note: Linking to a file with no extension (or a file ending in ``.out``, like
+  ``a.out``) will cause the generated JavaScript file to be exectuable, and
+  include a ``#!`` line to make it runnable directly.
+
 .. _emcc-c:
 
 ``-c``

test/test_other.py

@@ -15283,14 +15283,28 @@ def test_binary_encode(self, extra):
     write_file('test.js', read_file(path_from_root('src/binaryDecode.js')) + '\nvar src = ' + binary_encoded + ';\n' + test_js)
     self.assertContained('OK', self.run_js('test.js'))
 
-  @no_windows('depends on UNIX shbang feature')
-  def test_executable(self):
-    # First test without -sEXECUTABLE
+  @crossplatform
+  def test_executable_output_default(self):
+    # By default there should not be a #! line included
     self.run_process([EMCC, test_file('hello_world.c')])
+    self.assertFalse(read_file('a.out.js').startswith('#!'))
     self.assertNotContained('#!/usr/bin/env node', read_file('a.out.js'))
 
-    # Now, test with -sEXECUTABLE
-    self.run_process([EMCC, test_file('hello_world.c'), '-sEXECUTABLE'])
-    self.assertContained('#!/usr/bin/env node', read_file('a.out.js'))
-    output = self.run_process([os.path.abspath('a.out.js')], stdout=PIPE).stdout
+  @crossplatform
+  @no_windows('depends on UNIX shbang feature')
+  @parameterized({
+    '': (['-sEXECUTABLE', '-o', 'out.js'],),
+    # -sEXECUTABLE is implied if output filename has no extension
+    'no_extension': (['-o', 'foo'],),
+    # -sEXECUTABLE is implied for `.out` extension, e.g. a.out.
+    'a_out': (['-o', 'a.out'],),
+  })
+  def test_executable_output(self, args):
+    js_filename = args[-1]
+    self.run_process([EMCC, test_file('hello_world.c')] + args)
+    self.assertContained('#!/usr/bin/env node', read_file(js_filename))
+    output = self.run_process([os.path.abspath(js_filename)], stdout=PIPE).stdout
     self.assertContained('hello, world!', output)
+
+  def test_executable_requires_node(self):
+    self.assert_fail([EMCC, test_file('hello_world.c'), '-sEXECUTABLE', '-sENVIRONMENT=web'], 'emcc: error: EXECUTABLE requires `node` in ENVRIONMENT')

tools/link.py

@@ -931,6 +931,14 @@ def phase_linker_setup(options, linker_args):  # noqa: C901, PLR0912, PLR0915
       if s in user_settings:
         diagnostics.warning('unused-command-line-argument', f'{s} is only valid when generating JavaScript output')
 
+  # When there is no final suffix or the suffix is `.out` (as in `a.out`) then default to
+  # making the resulting file exectuable.
+  if settings.ENVIRONMENT_MAY_BE_NODE and options.oformat == OFormat.JS and final_suffix in ('', '.out'):
+    default_setting('EXECUTABLE', 1)
+
+  if settings.EXECUTABLE and not settings.ENVIRONMENT_MAY_BE_NODE:
+    exit_with_error('EXECUTABLE requires `node` in ENVRIONMENT')
+
   if options.oformat == OFormat.MJS:
     default_setting('EXPORT_ES6', 1)