fix PEP 518 support when pip is installed in the user site

Do not use the build environment isolation during the build dependencies installation.

news/5524.bugfix

@@ -0,0 +1 @@
+Fix PEP 518 support when pip is installed in the user site.

src/pip/_internal/operations/prepare.py

@@ -144,8 +144,10 @@ class IsSDist(DistAbstraction):
             )
 
         if should_isolate:
-            with self.req.build_env as prefix:
-                _install_build_reqs(finder, prefix, build_requirements)
+            with self.req.build_env:
+                pass
+            _install_build_reqs(finder, self.req.build_env.path,
+                                build_requirements)
         else:
             self.req.build_env = NoOpBuildEnvironment(no_clean=False)
 

tests/conftest.py

@@ -137,20 +137,24 @@ def isolate(tmpdir):
         )
 
 
-@pytest.yield_fixture(scope='session')
-def virtualenv_template(tmpdir_factory):
-    tmpdir = Path(str(tmpdir_factory.mktemp('virtualenv')))
-    # Copy over our source tree so that each virtual environment is self
-    # contained
-    pip_src = tmpdir.join("pip_src").abspath
+@pytest.fixture(scope='session')
+def pip_src(tmpdir_factory):
+    pip_src = Path(str(tmpdir_factory.mktemp('pip_src'))).join('pip_src')
+    # Copy over our source tree so that each use is self contained
     shutil.copytree(
         SRC_DIR,
-        pip_src,
+        pip_src.abspath,
         ignore=shutil.ignore_patterns(
             "*.pyc", "__pycache__", "contrib", "docs", "tasks", "*.txt",
             "tests", "pip.egg-info", "build", "dist", ".tox", ".git",
         ),
     )
+    return pip_src
+
+
+@pytest.yield_fixture(scope='session')
+def virtualenv_template(tmpdir_factory, pip_src):
+    tmpdir = Path(str(tmpdir_factory.mktemp('virtualenv')))
     # Create the virtual environment
     venv = VirtualEnvironment.create(
         tmpdir.join("venv_orig"),

tests/functional/test_install.py

@@ -45,6 +45,29 @@ def test_pep518_uses_build_env(script, data, original_setuptools):
         )
 
 
+def test_pep518_with_user_pip(script, virtualenv, pip_src, data):
+    virtualenv.system_site_packages = True
+    script.pip("install", "--ignore-installed", "--user", pip_src)
+    system_pip_dir = script.site_packages_path / 'pip'
+    system_pip_dir.rmtree()
+    system_pip_dir.mkdir()
+    with open(system_pip_dir / '__init__.py', 'w') as fp:
+        fp.write('raise ImportError\n')
+    to_install = data.src.join("pep518-3.0")
+    for command in ('install', 'wheel'):
+        kwargs = {}
+        if sys.version_info[:2] == (3, 3):
+            # Ignore Python 3.3 deprecation warning...
+            kwargs['expect_stderr'] = True
+        script.run(
+            "python", "-c",
+            "import pip._internal; pip._internal.main(["
+            "%r, " "'-f', %r, " "%r, "
+            "])" % (command, str(data.packages), str(to_install)),
+            **kwargs
+        )
+
+
 @pytest.mark.network
 def test_pip_second_command_line_interface_works(script, data):
     """