pip: 22.1: Editable installs for packages with extras & flit causes `UndefinedEnvironmentName`

Description

First of all, thanks for pip and for taking the time to read this issue!

We use editable installs on sigstore-python to provide a convenient development environment. For example, either with or without the dev extra:

pip install --editable .
pip install --editable .[dev]

Our dependencies are entirely managed within a pyproject.toml, with a small stub setup.py to help pip do an editable install.

Up until pip==22.1, these commands worked just fine. Upon upgrading to 22.1 (released today), we get this error:

  File "/Users/william/devel/sigstore-python/env/lib/python3.10/site-packages/pip/_vendor/packaging/markers.py", line 215, in _get_env
    raise UndefinedEnvironmentName(
pip._vendor.packaging.markers.UndefinedEnvironmentName: 'extra' does not exist in evaluation environment.

(I’ve truncated the traceback. I’ll paste the full-length trace below.)

When I run with --debug, I see that the dev extra is being passed in as an environment marker, which then fails since extra is not included in the marker environment.

Expected behavior

I expect pip install --editable . to behave the same as it did in the last stable release (22.0.4), i.e. install correctly with no errors.

pip version

22.1

Python version

3.10.0

OS

macOS 12.3.1

How to Reproduce

These steps should reproduce the failure:

git clone https://github.com/sigstore/sigstore-python && cd sigstore-python
make dev

If that fails to pull down the latest pip for whatever reason, you can do it explicitly:

git clone https://github.com/sigstore/sigstore-python && cd sigstore-python
python -m venv env && source env/bin/activate
pip install --upgrade pip
pip install --editable .

Output

Obtaining file:///Users/william/devel/sigstore-python
  Installing build dependencies: started
  Installing build dependencies: finished with status 'done'
  Checking if build backend supports build_editable: started
  Checking if build backend supports build_editable: finished with status 'done'
  Getting requirements to build editable: started
  Getting requirements to build editable: finished with status 'done'
ERROR: Exception:
Traceback (most recent call last):
  File "/Users/william/devel/sigstore-python/env/lib/python3.10/site-packages/pip/_internal/cli/base_command.py", line 167, in exc_logging_wrapper
    status = run_func(*args)
  File "/Users/william/devel/sigstore-python/env/lib/python3.10/site-packages/pip/_internal/cli/req_command.py", line 205, in wrapper
    return func(self, options, args)
  File "/Users/william/devel/sigstore-python/env/lib/python3.10/site-packages/pip/_internal/commands/install.py", line 340, in run
    requirement_set = resolver.resolve(
  File "/Users/william/devel/sigstore-python/env/lib/python3.10/site-packages/pip/_internal/resolution/resolvelib/resolver.py", line 75, in resolve
    collected = self.factory.collect_root_requirements(root_reqs)
  File "/Users/william/devel/sigstore-python/env/lib/python3.10/site-packages/pip/_internal/resolution/resolvelib/factory.py", line 506, in collect_root_requirements
    req = self._make_requirement_from_install_req(
  File "/Users/william/devel/sigstore-python/env/lib/python3.10/site-packages/pip/_internal/resolution/resolvelib/factory.py", line 468, in _make_requirement_from_install_req
    cand = self._make_candidate_from_link(
  File "/Users/william/devel/sigstore-python/env/lib/python3.10/site-packages/pip/_internal/resolution/resolvelib/factory.py", line 188, in _make_candidate_from_link
    self._editable_candidate_cache[link] = EditableCandidate(
  File "/Users/william/devel/sigstore-python/env/lib/python3.10/site-packages/pip/_internal/resolution/resolvelib/candidates.py", line 316, in __init__
    super().__init__(
  File "/Users/william/devel/sigstore-python/env/lib/python3.10/site-packages/pip/_internal/resolution/resolvelib/candidates.py", line 161, in __init__
    self.dist = self._prepare()
  File "/Users/william/devel/sigstore-python/env/lib/python3.10/site-packages/pip/_internal/resolution/resolvelib/candidates.py", line 230, in _prepare
    dist = self._prepare_distribution()
  File "/Users/william/devel/sigstore-python/env/lib/python3.10/site-packages/pip/_internal/resolution/resolvelib/candidates.py", line 326, in _prepare_distribution
    return self._factory.preparer.prepare_editable_requirement(self._ireq)
  File "/Users/william/devel/sigstore-python/env/lib/python3.10/site-packages/pip/_internal/operations/prepare.py", line 543, in prepare_editable_requirement
    dist = _get_prepared_distribution(
  File "/Users/william/devel/sigstore-python/env/lib/python3.10/site-packages/pip/_internal/operations/prepare.py", line 57, in _get_prepared_distribution
    abstract_dist.prepare_distribution_metadata(finder, build_isolation)
  File "/Users/william/devel/sigstore-python/env/lib/python3.10/site-packages/pip/_internal/distributions/sdist.py", line 45, in prepare_distribution_metadata
    self._install_build_reqs(finder)
  File "/Users/william/devel/sigstore-python/env/lib/python3.10/site-packages/pip/_internal/distributions/sdist.py", line 114, in _install_build_reqs
    conflicting, missing = self.req.build_env.check_requirements(build_reqs)
  File "/Users/william/devel/sigstore-python/env/lib/python3.10/site-packages/pip/_internal/build_env.py", line 178, in check_requirements
    if req.marker is not None and not req.marker.evaluate():
  File "/Users/william/devel/sigstore-python/env/lib/python3.10/site-packages/pip/_vendor/packaging/markers.py", line 304, in evaluate
    return _evaluate_markers(self._markers, current_environment)
  File "/Users/william/devel/sigstore-python/env/lib/python3.10/site-packages/pip/_vendor/packaging/markers.py", line 234, in _evaluate_markers
    lhs_value = _get_env(environment, lhs.value)
  File "/Users/william/devel/sigstore-python/env/lib/python3.10/site-packages/pip/_vendor/packaging/markers.py", line 215, in _get_env
    raise UndefinedEnvironmentName(
pip._vendor.packaging.markers.UndefinedEnvironmentName: 'extra' does not exist in evaluation environment.

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