Split out list of essential plugins

Fixes https://github.com/pytest-dev/pytest/issues/4976.

src/_pytest/config/__init__.py

@@ -112,13 +112,18 @@ def directory_arg(path, optname):
     return path
 
 
-default_plugins = (
+# Plugins that cannot be disabled via "-p no:X" currently.
+essential_plugins = (
     "mark",
     "main",
-    "terminal",
     "runner",
     "python",
     "fixtures",
+    "helpconfig",  # Provides -p.
+)
+
+default_plugins = essential_plugins + (
+    "terminal",  # Has essential options, but xdist uses -pno:terminal.
     "debugging",
     "unittest",
     "capture",
@@ -127,7 +132,6 @@ default_plugins = (
     "monkeypatch",
     "recwarn",
     "pastebin",
-    "helpconfig",
     "nose",
     "assertion",
     "junitxml",
@@ -143,7 +147,6 @@ default_plugins = (
     "reports",
 )
 
-
 builtin_plugins = set(default_plugins)
 builtin_plugins.add("pytester")
 
@@ -496,6 +499,9 @@ class PytestPluginManager(PluginManager):
     def consider_pluginarg(self, arg):
         if arg.startswith("no:"):
             name = arg[3:]
+            if name in essential_plugins:
+                raise UsageError("plugin %s cannot be disabled" % name)
+
             # PR #4304 : remove stepwise if cacheprovider is blocked
             if name == "cacheprovider":
                 self.set_blocked("stepwise")

src/_pytest/terminal.py

@@ -173,6 +173,7 @@ def getreportopt(config):
     return reportopts
 
 
+@pytest.hookimpl(trylast=True)  # after _pytest.runner
 def pytest_report_teststatus(report):
     if report.passed:
         letter = "."

testing/test_config.py

@@ -1205,20 +1205,12 @@ def test_config_does_not_load_blocked_plugin_from_args(testdir):
     [
         x
         for x in _pytest.config.default_plugins
-        if x
+        if x not in _pytest.config.essential_plugins
-        not in [
-            "fixtures",
-            "helpconfig",  # Provides -p.
-            "main",
-            "mark",
-            "python",
-            "runner",
-            "terminal",  # works in OK case (no output), but not with failures.
-        ]
     ],
 )
 def test_config_blocked_default_plugins(testdir, plugin):
     if plugin == "debugging":
+        # Fixed in xdist master (after 1.27.0).
         # https://github.com/pytest-dev/pytest-xdist/pull/422
         try:
             import xdist  # noqa: F401
@@ -1230,8 +1222,10 @@ def test_config_blocked_default_plugins(testdir, plugin):
     p = testdir.makepyfile("def test(): pass")
     result = testdir.runpytest(str(p), "-pno:%s" % plugin)
     assert result.ret == EXIT_OK
+    if plugin != "terminal":
         result.stdout.fnmatch_lines(["* 1 passed in *"])
 
+    if plugin != "terminal":  # fails to report due to its options being used elsewhere.
         p = testdir.makepyfile("def test(): assert 0")
         result = testdir.runpytest(str(p), "-pno:%s" % plugin)
         assert result.ret == EXIT_TESTSFAILED

testing/test_pluginmanager.py

@@ -9,6 +9,7 @@ import types
 
 import pytest
 from _pytest.config import PytestPluginManager
+from _pytest.config.exceptions import UsageError
 from _pytest.main import EXIT_NOTESTSCOLLECTED
 from _pytest.main import Session
 
@@ -314,6 +315,9 @@ class TestPytestPluginManagerBootstrapming(object):
         # Handles -p without following arg (when used without argparse).
         pytestpm.consider_preparse(["-p"])
 
+        with pytest.raises(UsageError, match="^plugin main cannot be disabled$"):
+            pytestpm.consider_preparse(["-p", "no:main"])
+
     def test_plugin_prevent_register(self, pytestpm):
         pytestpm.consider_preparse(["xyz", "-p", "no:abc"])
         l1 = pytestpm.get_plugins()