cache_dir: use $TOX_ENV_DIR/ prefix if set

Fixes https://github.com/pytest-dev/pytest/issues/4270

changelog/4270.feature.rst

@@ -0,0 +1,3 @@
+The ``cache_dir`` option uses ``$TOX_ENV_DIR`` as prefix (if set in the environment).
+
+This uses a different cache per tox environment by default.

src/_pytest/cacheprovider.py

@@ -9,6 +9,7 @@ from __future__ import division
 from __future__ import print_function
 
 import json
+import os
 from collections import OrderedDict
 
 import attr
@@ -275,7 +276,10 @@ def pytest_addoption(parser):
         dest="cacheclear",
         help="remove all cache contents at start of test run.",
     )
-    parser.addini("cache_dir", default=".pytest_cache", help="cache directory path.")
+    cache_dir_default = ".pytest_cache"
+    if "TOX_ENV_DIR" in os.environ:
+        cache_dir_default = os.path.join(os.environ["TOX_ENV_DIR"], cache_dir_default)
+    parser.addini("cache_dir", default=cache_dir_default, help="cache directory path.")
     group.addoption(
         "--lfnf",
         "--last-failed-no-failures",

testing/test_cacheprovider.py

@@ -14,6 +14,17 @@ import pytest
 pytest_plugins = ("pytester",)
 
 
+@pytest.fixture(scope="module", autouse=True)
+def handle_env():
+    """Ensure env is like most of the tests expect it, i.e. not using tox."""
+    orig_env = os.environ.pop("TOX_ENV_DIR", None)
+
+    yield
+
+    if orig_env is not None:
+        os.environ["TOX_ENV_DIR"] = orig_env
+
+
 class TestNewAPI(object):
     def test_config_cache_makedir(self, testdir):
         testdir.makeini("[pytest]")
@@ -148,15 +159,17 @@ class TestNewAPI(object):
         assert testdir.tmpdir.join("custom_cache_dir").isdir()
 
 
-def test_cache_reportheader(testdir):
-    testdir.makepyfile(
-        """
-        def test_hello():
-            pass
-    """
-    )
+@pytest.mark.parametrize("env", ((), ("TOX_ENV_DIR", "/tox_env_dir")))
+def test_cache_reportheader(env, testdir, monkeypatch):
+    testdir.makepyfile("""def test_foo(): pass""")
+    if env:
+        monkeypatch.setenv(*env)
+        expected = os.path.join(env[1], ".pytest_cache")
+    else:
+        monkeypatch.delenv("TOX_ENV_DIR", raising=False)
+        expected = ".pytest_cache"
     result = testdir.runpytest("-v")
-    result.stdout.fnmatch_lines(["cachedir: .pytest_cache"])
+    result.stdout.fnmatch_lines(["cachedir: %s" % expected])
 
 
 def test_cache_reportheader_external_abspath(testdir, tmpdir_factory):