Add a settings to disable all polyfills

ChangeLog.md

@@ -20,6 +20,10 @@ See docs/process.md for more on how version tagging works.
 
 3.1.2
 -----
+- A new setting, `POLYFILL`, was added which is on by default but can be disabled
+  (via `-sNO_POLYFILL`) to prevent emscripten from outputing needed polyfills.
+  For default browser targets, no polyfills are needed so this option only has
+  meaning when targeting older browsers.
 - `EVAL_CTORS` has been rewritten and improved. The main differences from before
   are that it is much more capable (it can now eval parts of functions and not
   just all or nothing, and it can eval more wasm constructs like globals). It is

src/polyfill/promise.js

@@ -23,6 +23,10 @@
 // THE SOFTWARE.
 //==============================================================================
 
+#if !POLYFILL
+assert(false, "this file should never be included unless POLYFILL is set");
+#endif
+
 /** @suppress{duplicate} This is already defined in from Closure's built-in
     externs.zip//es6.js, Closure should not yell when seeing this again. */
 var Promise = (function() {

src/settings.js

@@ -1971,6 +1971,14 @@ var ALLOW_UNIMPLEMENTED_SYSCALLS = 1;
 // [link]
 var TRUSTED_TYPES = 0;
 
+// When targeting older browsers emscripten will sometimes require that
+// polyfills be included in the output.  If you would prefer to take care of
+// polyfilling yourself via some other mechanism you can prevent emscripten
+// from generating these by passing `-sNO_POLYFILL` or `-sPOLYFILL=0`
+// With default browser targets emscripten does not need any polyfills so this
+// settings is *only* needed when also explicitly targeting older browsers.
+var POLYFILL = 1;
+
 //===========================================
 // Internal, used for testing only, from here
 //===========================================

src/shell.js

@@ -36,12 +36,14 @@ if (!Module) /** @suppress{checkTypes}*/Module = {"__EMSCRIPTEN_PRIVATE_MODULE_E
 var Module = typeof {{{ EXPORT_NAME }}} !== 'undefined' ? {{{ EXPORT_NAME }}} : {};
 #endif // USE_CLOSURE_COMPILER
 
+#if POLYFILL
 #if ((MAYBE_WASM2JS && WASM != 2) || MODULARIZE) && (MIN_CHROME_VERSION < 33 || MIN_EDGE_VERSION < 12 || MIN_FIREFOX_VERSION < 29 || MIN_IE_VERSION != TARGET_NOT_SUPPORTED || MIN_SAFARI_VERSION < 80000) // https://caniuse.com/#feat=promises
 // Include a Promise polyfill for legacy browsers. This is needed either for
 // wasm2js, where we polyfill the wasm API which needs Promises, or when using
 // modularize which creates a Promise for when the module is ready.
 #include "polyfill/promise.js"
 #endif
+#endif
 
 // See https://caniuse.com/mdn-javascript_builtins_object_assign
 #if MIN_CHROME_VERSION < 45 || MIN_EDGE_VERSION < 12 || MIN_FIREFOX_VERSION < 34 || MIN_IE_VERSION != TARGET_NOT_SUPPORTED || MIN_SAFARI_VERSION < 90000

tests/test_other.py

@@ -10022,9 +10022,14 @@ def test(args, expect_fail):
       return self.run_js('a.out.js', assert_returncode=NON_ZERO if expect_fail else 0)
 
     # we fail without legacy support
-    self.assertNotContained('hello, world!', test([], expect_fail=True))
+    test([], expect_fail=True)
+
     # but work with it
-    self.assertContained('hello, world!', test(['-sLEGACY_VM_SUPPORT'], expect_fail=False))
+    output = test(['-sLEGACY_VM_SUPPORT'], expect_fail=False)
+    self.assertContained('hello, world!', output)
+
+    # unless we explictly disable polyfills
+    test(['-sLEGACY_VM_SUPPORT', '-sNO_POLYFILL'], expect_fail=True)
 
   def test_webgpu_compiletest(self):
     for args in [[], ['-sASSERTIONS'], ['-sASSERTIONS', '--closure=1'], ['-sMAIN_MODULE=1']]: