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

@@ -40,6 +40,8 @@
 from . import contexts
 from .textwrap import DALS
 
+import py
+
 
 @pytest.fixture(autouse=True)
 def pip_disable_index(monkeypatch):
@@ -1109,3 +1111,43 @@ 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)
+
+    # Patching $HOME for 2 reasons:
+    # 1. setuptools/command/easy_install.py:create_home_path
+    #    tries creating directories in $HOME
+    #    given `self.config_vars['DESTDIRS'] = "/home/user/.pyenv/versions/3.9.10 /home/user/.pyenv/versions/3.9.10/lib /home/user/.pyenv/versions/3.9.10/lib/python3.9 /home/user/.pyenv/versions/3.9.10/lib/python3.9/lib-dynload"``
+    #    it will `makedirs("/home/user/.pyenv/versions/3.9.10 /home/user/.pyenv/versions/3.9.10/lib /home/user/.pyenv/versions/3.9.10/lib/python3.9 /home/user/.pyenv/versions/3.9.10/lib/python3.9/lib-dynload")``
+    # 2. We are going to force `site` to update site.USER_BASE and site.USER_SITE
+    #    To point inside our new home
+    monkeypatch.setenv('HOME', str(tmpdir / 'home'))
+    monkeypatch.setattr('site.USER_BASE', None)
+    monkeypatch.setattr('site.USER_SITE', None)
+    user_site = py.path.local(site.getusersitepackages())
+    user_site.ensure_dir()
+
+    sys_prefix = (tmpdir / 'sys_prefix').ensure_dir()
+    monkeypatch.setattr('sys.prefix', str(sys_prefix))
+
+    ## Sanity check:
+    assert sys_prefix.listdir() == []
+    assert user_site.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 user site
+    with pytest.raises(AssertionError):
+        assert {f.basename for f in user_site.listdir()} == {'UNKNOWN.egg-link'}