diff --git a/libcxx/utils/libcxx/test/dsl.py b/libcxx/utils/libcxx/test/dsl.py
--- a/libcxx/utils/libcxx/test/dsl.py
+++ b/libcxx/utils/libcxx/test/dsl.py
@@ -297,6 +297,9 @@
     """
     assert self.isSupported(config), \
       "Trying to get the name of a feature that is not supported in the given configuration"
+    return self._getNameUnchecked(config)
+
+  def _getNameUnchecked(self, config):
     name = self._name(config) if callable(self._name) else self._name
     if not isinstance(name, str):
       raise ValueError("Feature did not resolve to a name that's a string, got {}".format(name))
@@ -316,7 +319,9 @@
     """
     assert self.isSupported(config), \
       "Trying to enable feature {} that is not supported in the given configuration".format(self._name)
+    self._enableInUnchecked(config)
 
+  def _enableInUnchecked(self, config):
     addTo = lambda subs, sub, flag: [(s, x + ' ' + flag) if s == sub else (s, x) for (s, x) in subs]
     if self._compileFlag:
       compileFlag = self._compileFlag(config) if callable(self._compileFlag) else self._compileFlag
@@ -324,8 +329,19 @@
     if self._linkFlag:
       linkFlag = self._linkFlag(config) if callable(self._linkFlag) else self._linkFlag
       config.substitutions = addTo(config.substitutions, '%{link_flags}', linkFlag)
-    config.available_features.add(self.getName(config))
+    config.available_features.add(self._getNameUnchecked(config))
+
+  def tryEnableIn(self, config):
+    """
+    Try to enable a feature in a TestingConfig (if it is supported).
 
+    This is equivalent to `if isSupported(config): enabledIn(config)`, but
+    is slightly faster since it can omit redundant checks.
+    """
+    if self.isSupported(config):
+      self._enableInUnchecked(config)
+      return True
+    return False
 
 def _str_to_bool(s):
   """
diff --git a/libcxx/utils/libcxx/test/newconfig.py b/libcxx/utils/libcxx/test/newconfig.py
--- a/libcxx/utils/libcxx/test/newconfig.py
+++ b/libcxx/utils/libcxx/test/newconfig.py
@@ -25,9 +25,10 @@
   # Then, apply the automatically-detected features.
   printFeatures = []
   for feature in features:
-    if feature.isSupported(config):
-      feature.enableIn(config)
-      printFeatures.append(feature.getName(config))
+    if feature.tryEnableIn(config):
+      # We can avoid a potentially expensive assertion and use
+      # _getNameUnchecked since we know that the feature is enabled.
+      printFeatures.append(feature._getNameUnchecked(config))
   printFeatures = ["'{}'".format(f) for f in sorted(printFeatures)]
   lit_config.note("Enabling implicitly detected Lit features {}".format(', '.join(printFeatures)))