Deprecate use of session.install() without a virtual env

docs/config.rst

@@ -149,6 +149,14 @@ If you want to disable virtualenv creation altogether, you can set ``python`` to
     def tests(session):
         pass
 
+Use of :func:`session.install()` is deprecated without a virtualenv since it modifies the global Python environment. If this is what you really want, use :func:`session.run()` and pip instead.
+
+.. code-block:: python
+
+    @nox.session(python=False)
+    def tests(session):
+        session.run("pip", "install", "nox")
+
 You can also specify that the virtualenv should *always* be reused instead of recreated every time:
 
 .. code-block:: python

nox/sessions.py

@@ -20,6 +20,7 @@
 import re
 import sys
 import unicodedata
+import warnings
 from typing import (
     Any,
     Callable,
@@ -488,6 +489,15 @@ def install(self, *args: str, **kwargs: Any) -> None:
             raise ValueError(
                 "A session without a virtualenv can not install dependencies."
             )
+        if isinstance(venv, PassthroughEnv):
+            warnings.warn(
+                f"Session {self.name} does not have a virtual environment, "
+                "so use of session.install() is deprecated since it would modify "
+                "the global Python environment. If you're really sure that is "
+                'what you want to do, use session.run("pip", "install", ...) instead.',
+                category=FutureWarning,
+                stacklevel=2,
+            )
         if not args:
             raise ValueError("At least one argument required to install().")
 

tests/test_sessions.py

@@ -578,6 +578,39 @@ class SessionNoSlots(nox.sessions.Session):
                 external="error",
             )
 
+    def test_install_no_venv_deprecated(self):
+        runner = nox.sessions.SessionRunner(
+            name="test",
+            signatures=["test"],
+            func=mock.sentinel.func,
+            global_config=_options.options.namespace(posargs=[]),
+            manifest=mock.create_autospec(nox.manifest.Manifest),
+        )
+        runner.venv = mock.create_autospec(nox.virtualenv.PassthroughEnv)
+        runner.venv.env = {}
+
+        class SessionNoSlots(nox.sessions.Session):
+            pass
+
+        session = SessionNoSlots(runner=runner)
+
+        with mock.patch.object(session, "_run", autospec=True) as run:
+            with pytest.warns(
+                FutureWarning,
+                match=r"use of session\.install\(\) is deprecated since it would modify the global Python environment",
+            ):
+                session.install("requests", "urllib3")
+            run.assert_called_once_with(
+                "python",
+                "-m",
+                "pip",
+                "install",
+                "requests",
+                "urllib3",
+                silent=True,
+                external="error",
+            )
+
     def test_notify(self):
         session, runner = self.make_session_and_runner()