config: handle `-p no:plugin` with default plugins

`-p no:capture` should not load its fixtures in the first place.

changelog/4957.bugfix.rst

@@ -0,0 +1,3 @@
+``-p no:plugin`` is handled correctly for default (internal) plugins now, e.g. with ``-p no:capture``.
+
+Previously they were loaded (imported) always, making e.g. the ``capfd`` fixture available.

src/_pytest/config/__init__.py

@@ -147,10 +147,15 @@ builtin_plugins = set(default_plugins)
 builtin_plugins.add("pytester")
 
 
-def get_config():
+def get_config(args=None):
     # subsequent calls to main will create a fresh instance
     pluginmanager = PytestPluginManager()
     config = Config(pluginmanager)
+
+    if args is not None:
+        # Handle any "-p no:plugin" args.
+        pluginmanager.consider_preparse(args)
+
     for spec in default_plugins:
         pluginmanager.import_plugin(spec)
     return config
@@ -178,7 +183,7 @@ def _prepareconfig(args=None, plugins=None):
         msg = "`args` parameter expected to be a list or tuple of strings, got: {!r} (type: {})"
         raise TypeError(msg.format(args, type(args)))
 
-    config = get_config()
+    config = get_config(args)
     pluginmanager = config.pluginmanager
     try:
         if plugins:
@@ -713,7 +718,7 @@ class Config(object):
     @classmethod
     def fromdictargs(cls, option_dict, args):
         """ constructor useable for subprocesses. """
-        config = get_config()
+        config = get_config(args)
         config.option.__dict__.update(option_dict)
         config.parse(args, addopts=False)
         for x in config.option.plugins:

testing/test_config.py

@@ -15,6 +15,7 @@ from _pytest.config.findpaths import determine_setup
 from _pytest.config.findpaths import get_common_ancestor
 from _pytest.config.findpaths import getcfg
 from _pytest.main import EXIT_NOTESTSCOLLECTED
+from _pytest.main import EXIT_TESTSFAILED
 from _pytest.main import EXIT_USAGEERROR
 
 
@@ -1176,3 +1177,15 @@ def test_help_and_version_after_argument_error(testdir):
         ["*pytest*{}*imported from*".format(pytest.__version__)]
     )
     assert result.ret == EXIT_USAGEERROR
+
+
+def test_config_does_not_load_blocked_plugin_from_args(testdir):
+    """This tests that pytest's config setup handles "-p no:X"."""
+    p = testdir.makepyfile("def test(capfd): pass")
+    result = testdir.runpytest(str(p), "-pno:capture", "--tb=native")
+    result.stdout.fnmatch_lines(["E       fixture 'capfd' not found"])
+    assert result.ret == EXIT_TESTSFAILED
+
+    result = testdir.runpytest(str(p), "-pno:capture", "-s")
+    result.stderr.fnmatch_lines(["*: error: unrecognized arguments: -s"])
+    assert result.ret == EXIT_USAGEERROR