Unify handling of deprecated settings (emscripten-core#21355)

Unlike `LEGACY_SETTINGS`, deprecated settings can continue to be used
and the values passed in will continue to be honored.

Once folks have stopped using them we can move them to
`LEGACY_SETTINGS`.

test/test_other.py

@@ -8514,7 +8514,7 @@ def test_exported_runtime_methods_metadce(self, use_legacy_name):
       setting_name = 'EXTRA_EXPORTED_RUNTIME_METHODS'
     err = self.run_process([EMCC, test_file('hello_world.c'), '-Os', '-s%s=%s' % (setting_name, ','.join(exports))], stderr=PIPE).stderr
     if use_legacy_name:
-      self.assertContained('warning: EXTRA_EXPORTED_RUNTIME_METHODS is deprecated, please use EXPORTED_RUNTIME_METHODS instead [-Wdeprecated]', err)
+      self.assertContained('warning: EXTRA_EXPORTED_RUNTIME_METHODS is deprecated (please use EXPORTED_RUNTIME_METHODS instead). Please open a bug if you have a continuing need for this setting [-Wdeprecated]', err)
     js = read_file('a.out.js')
     for export in exports:
       self.assertContained(f'Module["{export}"]', js)

tools/link.py

@@ -38,7 +38,7 @@
 from .shared import in_temp, safe_copy, do_replace, OFormat
 from .shared import DEBUG, WINDOWS, DYNAMICLIB_ENDINGS, STATICLIB_ENDINGS
 from .shared import unsuffixed, unsuffixed_basename, get_file_suffix
-from .settings import settings, default_setting, user_settings, JS_ONLY_SETTINGS
+from .settings import settings, default_setting, user_settings, JS_ONLY_SETTINGS, DEPRECATED_SETTINGS
 from .minimal_runtime_shell import generate_minimal_runtime_html
 
 import tools.line_endings
@@ -698,14 +698,11 @@ def phase_linker_setup(options, state, newargs):
 
   final_suffix = get_file_suffix(target)
 
-  if 'SUPPORT_ERRNO' in user_settings:
-    diagnostics.warning('deprecated', 'SUPPORT_ERRNO is deprecated since emscripten no longer uses the setErrNo library function')
-
-  if 'DEMANGLE_SUPPORT' in user_settings:
-    diagnostics.warning('deprecated', 'DEMANGLE_SUPPORT is deprecated since mangled names no longer appear in stack traces')
+  for s, reason in DEPRECATED_SETTINGS.items():
+    if s in user_settings:
+      diagnostics.warning('deprecated', f'{s} is deprecated ({reason}). Please open a bug if you have a continuing need for this setting')
 
   if settings.EXTRA_EXPORTED_RUNTIME_METHODS:
-    diagnostics.warning('deprecated', 'EXTRA_EXPORTED_RUNTIME_METHODS is deprecated, please use EXPORTED_RUNTIME_METHODS instead')
     settings.EXPORTED_RUNTIME_METHODS += settings.EXTRA_EXPORTED_RUNTIME_METHODS
 
   # If no output format was specified we try to deduce the format based on

tools/settings.py

@@ -109,6 +109,16 @@
     'RUNTIME_LINKED_LIBS',
 }.union(PORTS_SETTINGS)
 
+# Unlike `LEGACY_SETTINGS`, deprecated settings can still be used
+# both on the command line and in the emscripten codebase.
+#
+# At some point in the future, once folks have stopped using these
+# settings we can move them to `LEGACY_SETTINGS`.
+DEPRECATED_SETTINGS = {
+    'SUPPORT_ERRNO': 'emscripten no longer uses the setErrNo library function',
+    'EXTRA_EXPORTED_RUNTIME_METHODS': 'please use EXPORTED_RUNTIME_METHODS instead',
+    'DEMANGLE_SUPPORT': 'mangled names no longer appear in stack traces',
+}
 
 # Settings that don't need to be externalized when serializing to json because they
 # are not used by the JS compiler.