Test: editable install \w --user&build isolation

Add a new test and confirm that it
fails in the expected manner

setuptools/tests/test_easy_install.py

@@ -1109,3 +1109,32 @@ def test_use_correct_python_version_string(tmpdir, tmpdir_cwd, monkeypatch):
     assert cmd.config_vars['py_version'] == '3.10.1'
     assert cmd.config_vars['py_version_short'] == '3.10'
     assert cmd.config_vars['py_version_nodot'] == '310'
+
+
+def test_editable_install_honors_user_under_build_isolation(setup_context, monkeypatch, tmpdir):
+    ''' `setup.py develop` should honor `--user` even under build isolation'''
+
+    ## Arrange:
+    # Pretend that build isolation was enabled
+    # e.g pip sets the environment varible PYTHONNOUSERSITE=1
+    monkeypatch.setattr('site.ENABLE_USER_SITE', False)
+    # The tests should not write to the real ~/.local/lib/ or /usr/local/lib
+    USER_BASE = (tmpdir / 'user-base').ensure_dir()
+    monkeypatch.setattr('site.USER_BASE', str(USER_BASE))
+    sys_prefix = (tmpdir / 'sys_prefix').ensure_dir()
+    monkeypatch.setattr('sys.prefix', str(sys_prefix))
+
+    ## Sanity check:
+    assert sys_prefix.listdir() == []
+    assert USER_BASE.listdir() == []
+
+    ## Act:
+    run_setup('setup.py', ['develop', '--user'])
+
+    ## Assert:
+    # Should not install to sys.prefix
+    with pytest.raises(AssertionError):
+        assert sys_prefix.listdir() == []
+    # Should install to site.USER_BASE
+    with pytest.raises(AssertionError):
+        assert (USER_BASE / 'lib').isdir()