Allow plugins to set tox_root

docs/changelog/2966.bugfix.rst

@@ -0,0 +1 @@
+Fixed an issue where a tox plugin couldn't change the value of ``tox_root``.

src/tox/provision.py

@@ -84,6 +84,11 @@ def add_tox_requires_min_version(reqs: list[Requirement]) -> list[Requirement]:
         desc="Name of the virtual environment used to provision a tox.",
         post_process=add_tox_requires_min_version,
     )
+
+    from tox.plugin.manager import MANAGER
+
+    MANAGER.tox_add_core_config(state.conf.core, state)
+
     requires: list[Requirement] = state.conf.core["requires"]
     missing = _get_missing(requires)
 
@@ -100,10 +105,6 @@ def add_tox_requires_min_version(reqs: list[Requirement]) -> list[Requirement]:
     state.conf.memory_seed_loaders[provision_tox_env].append(loader)
     state.envs._mark_provision(bool(missing), provision_tox_env)
 
-    from tox.plugin.manager import MANAGER
-
-    MANAGER.tox_add_core_config(state.conf.core, state)
-
     if not missing:
         return False
 

tests/plugin/test_plugin.py

@@ -3,6 +3,7 @@
 import logging
 import os
 import sys
+from pathlib import Path
 from typing import Any
 from unittest.mock import patch
 
@@ -102,6 +103,33 @@ def tox_env_teardown(tox_env: ToxEnv) -> None:
     assert result.out.splitlines() == expected, result.out
 
 
+@pytest.mark.parametrize(
+    "dir_name",
+    [
+        "tox_root",
+        "work_dir",
+        "temp_dir",
+    ],
+)
+def test_plugin_can_set_core_conf(
+    tox_project: ToxProjectCreator,
+    mocker: MockerFixture,
+    dir_name: str,
+    tmp_path: Path,
+) -> None:
+    @impl
+    def tox_add_core_config(core_conf: CoreConfigSet, state: State) -> None:  # noqa: U100
+        core_conf.loaders.insert(0, MemoryLoader(**{dir_name: tmp_path}))
+
+    register_inline_plugin(mocker, tox_add_core_config)
+
+    project = tox_project({})
+    result = project.run("c")
+    result.assert_success()
+
+    assert result.state.conf.core[dir_name] == tmp_path
+
+
 def test_plugin_can_read_env_list(tox_project: ToxProjectCreator, mocker: MockerFixture) -> None:
     @impl
     def tox_add_core_config(core_conf: CoreConfigSet, state: State) -> None:  # noqa: U100