🧪 Add a test injecting ephemeral envs w/ plugin

Author: webknjaz

tests/plugin/test_inline.py

@@ -4,7 +4,9 @@
 
 if TYPE_CHECKING:
     from tox.config.cli.parser import ToxParser
+    from tox.config.sets import ConfigSet
     from tox.pytest import ToxProjectCreator
+    from tox.session.state import State
 
 
 def test_inline_tox_py(tox_project: ToxProjectCreator) -> None:
@@ -22,3 +24,37 @@ def tox_add_option(parser: ToxParser) -> None:
     result = project.run("-h")
     result.assert_success()
     assert "--magic" in result.out
+
+
+def test_toxfile_py_w_ephemeral_envs(tox_project: ToxProjectCreator) -> None:
+    """Ensure additional ephemeral tox envs can be plugin-injected."""
+    def plugin() -> None:  # pragma: no cover # the code is copied to a python file
+        from tox.config.loader.memory import MemoryLoader  # noqa: PLC0415
+        from tox.plugin import impl  # noqa: PLC0415
+
+        env_name = "sentinel-env-name"
+
+        @impl
+        def tox_extend_envs() -> tuple[str]:
+            return (env_name,)
+
+        @impl
+        def tox_add_core_config(core_conf: ConfigSet, state: State) -> None:  # noqa: ARG001
+            in_memory_config_loader = MemoryLoader(
+                base=["sentinel-base"],
+                description="sentinel-description",
+            )
+            state.conf.memory_seed_loaders[env_name].append(
+                in_memory_config_loader,  # src/tox/provision.py:provision()
+            )
+
+    project = tox_project({"toxfile.py": plugin})
+
+    tox_list_result = project.run("list", "-qq")
+    tox_list_result.assert_success()
+    expected_additional_env_txt = "\n\nadditional environments:\nsentinel-env-name -> sentinel-description"
+    assert expected_additional_env_txt in tox_list_result.out
+
+    tox_config_result = project.run("config", "-e", "sentinel-env-name", "-qq")
+    tox_config_result.assert_success()
+    assert "base = sentinel-base" in tox_config_result.out