[libc++] Improve the verbosity of configuration errors when a compiler flag is not supported (#66379)

If an assertion fails during the configuration of the test suite because
the compiler doesn't support a flag (or the compiler configuration is
borked entirely), we will now print additional context to help debug the
issue instead of just saying "The compiler doesn't support the flag" and
bailing.

Author: ldionne

libcxx/utils/libcxx/test/dsl.py

@@ -203,23 +203,28 @@ def programSucceeds(config, program, args=None):
 
 
 @_memoizeExpensiveOperation(lambda c, f: (c.substitutions, c.environment, f))
+def tryCompileFlag(config, flag):
+    """
+    Try using the given compiler flag and return the exit code along with stdout and stderr.
+    """
+    # fmt: off
+    with _makeConfigTest(config) as test:
+        out, err, exitCode, timeoutInfo, _ = _executeWithFakeConfig(test, [
+            "%{{cxx}} -xc++ {} -Werror -fsyntax-only %{{flags}} %{{compile_flags}} {}".format(os.devnull, flag)
+        ])
+        return exitCode, out, err
+    # fmt: on
+
+
 def hasCompileFlag(config, flag):
     """
     Return whether the compiler in the configuration supports a given compiler flag.
 
     This is done by executing the %{cxx} substitution with the given flag and
     checking whether that succeeds.
     """
-    with _makeConfigTest(config) as test:
-        out, err, exitCode, timeoutInfo, _ = _executeWithFakeConfig(
-            test,
-            [
-                "%{{cxx}} -xc++ {} -Werror -fsyntax-only %{{flags}} %{{compile_flags}} {}".format(
-                    os.devnull, flag
-                )
-            ],
-        )
-        return exitCode == 0
+    (exitCode, _, _) = tryCompileFlag(config, flag)
+    return exitCode == 0
 
 
 @_memoizeExpensiveOperation(lambda c, s: (c.substitutions, c.environment, s))
@@ -348,10 +353,15 @@ def _getSubstitution(substitution, config):
 def _appendToSubstitution(substitutions, key, value):
     return [(k, v + " " + value) if k == key else (k, v) for (k, v) in substitutions]
 
-
 def _prependToSubstitution(substitutions, key, value):
     return [(k, value + " " + v) if k == key else (k, v) for (k, v) in substitutions]
 
+def _ensureFlagIsSupported(config, flag):
+    (exitCode, out, err) = tryCompileFlag(config, flag)
+    assert (
+        exitCode == 0
+    ), f"Trying to enable compiler flag {flag}, which is not supported. stdout was:\n{out}\n\nstderr was:\n{err}"
+
 
 class ConfigAction(object):
     """
@@ -427,9 +437,7 @@ def __init__(self, flag):
 
     def applyTo(self, config):
         flag = self._getFlag(config)
-        assert hasCompileFlag(
-            config, flag
-        ), "Trying to enable flag {}, which is not supported".format(flag)
+        _ensureFlagIsSupported(config, flag)
         config.substitutions = _appendToSubstitution(
             config.substitutions, "%{flags}", flag
         )
@@ -473,9 +481,7 @@ def __init__(self, flag):
 
     def applyTo(self, config):
         flag = self._getFlag(config)
-        assert hasCompileFlag(
-            config, flag
-        ), "Trying to enable compile flag {}, which is not supported".format(flag)
+        _ensureFlagIsSupported(config, flag)
         config.substitutions = _appendToSubstitution(
             config.substitutions, "%{compile_flags}", flag
         )
@@ -497,9 +503,7 @@ def __init__(self, flag):
 
     def applyTo(self, config):
         flag = self._getFlag(config)
-        assert hasCompileFlag(
-            config, flag
-        ), "Trying to enable link flag {}, which is not supported".format(flag)
+        _ensureFlagIsSupported(config, flag)
         config.substitutions = _appendToSubstitution(
             config.substitutions, "%{link_flags}", flag
         )
@@ -521,9 +525,7 @@ def __init__(self, flag):
 
     def applyTo(self, config):
         flag = self._getFlag(config)
-        assert hasCompileFlag(
-            config, flag
-        ), "Trying to enable link flag {}, which is not supported".format(flag)
+        _ensureFlagIsSupported(config, flag)
         config.substitutions = _prependToSubstitution(
             config.substitutions, "%{link_flags}", flag
         )