diff --git a/setuptools/_core_metadata.py b/setuptools/_core_metadata.py
new file mode 100644
index 0000000000..c1d41c6680
--- /dev/null
+++ b/setuptools/_core_metadata.py
@@ -0,0 +1,258 @@
+"""
+Handling of Core Metadata for Python packages (including reading and writing).
+
+See: https://packaging.python.org/en/latest/specifications/core-metadata/
+"""
+import os
+import stat
+import textwrap
+from email import message_from_file
+from email.message import Message
+from tempfile import NamedTemporaryFile
+from typing import Optional, List
+
+from distutils.util import rfc822_escape
+
+from . import _normalization
+from .extern.packaging.markers import Marker
+from .extern.packaging.requirements import Requirement
+from .extern.packaging.version import Version
+from .warnings import SetuptoolsDeprecationWarning
+
+
+def get_metadata_version(self):
+    mv = getattr(self, 'metadata_version', None)
+    if mv is None:
+        mv = Version('2.1')
+        self.metadata_version = mv
+    return mv
+
+
+def rfc822_unescape(content: str) -> str:
+    """Reverse RFC-822 escaping by removing leading whitespaces from content."""
+    lines = content.splitlines()
+    if len(lines) == 1:
+        return lines[0].lstrip()
+    return '\n'.join((lines[0].lstrip(), textwrap.dedent('\n'.join(lines[1:]))))
+
+
+def _read_field_from_msg(msg: Message, field: str) -> Optional[str]:
+    """Read Message header field."""
+    value = msg[field]
+    if value == 'UNKNOWN':
+        return None
+    return value
+
+
+def _read_field_unescaped_from_msg(msg: Message, field: str) -> Optional[str]:
+    """Read Message header field and apply rfc822_unescape."""
+    value = _read_field_from_msg(msg, field)
+    if value is None:
+        return value
+    return rfc822_unescape(value)
+
+
+def _read_list_from_msg(msg: Message, field: str) -> Optional[List[str]]:
+    """Read Message header field and return all results as list."""
+    values = msg.get_all(field, None)
+    if values == []:
+        return None
+    return values
+
+
+def _read_payload_from_msg(msg: Message) -> Optional[str]:
+    value = msg.get_payload().strip()
+    if value == 'UNKNOWN' or not value:
+        return None
+    return value
+
+
+def read_pkg_file(self, file):
+    """Reads the metadata values from a file object."""
+    msg = message_from_file(file)
+
+    self.metadata_version = Version(msg['metadata-version'])
+    self.name = _read_field_from_msg(msg, 'name')
+    self.version = _read_field_from_msg(msg, 'version')
+    self.description = _read_field_from_msg(msg, 'summary')
+    # we are filling author only.
+    self.author = _read_field_from_msg(msg, 'author')
+    self.maintainer = None
+    self.author_email = _read_field_from_msg(msg, 'author-email')
+    self.maintainer_email = None
+    self.url = _read_field_from_msg(msg, 'home-page')
+    self.download_url = _read_field_from_msg(msg, 'download-url')
+    self.license = _read_field_unescaped_from_msg(msg, 'license')
+
+    self.long_description = _read_field_unescaped_from_msg(msg, 'description')
+    if self.long_description is None and self.metadata_version >= Version('2.1'):
+        self.long_description = _read_payload_from_msg(msg)
+    self.description = _read_field_from_msg(msg, 'summary')
+
+    if 'keywords' in msg:
+        self.keywords = _read_field_from_msg(msg, 'keywords').split(',')
+
+    self.platforms = _read_list_from_msg(msg, 'platform')
+    self.classifiers = _read_list_from_msg(msg, 'classifier')
+
+    # PEP 314 - these fields only exist in 1.1
+    if self.metadata_version == Version('1.1'):
+        self.requires = _read_list_from_msg(msg, 'requires')
+        self.provides = _read_list_from_msg(msg, 'provides')
+        self.obsoletes = _read_list_from_msg(msg, 'obsoletes')
+    else:
+        self.requires = None
+        self.provides = None
+        self.obsoletes = None
+
+    self.license_files = _read_list_from_msg(msg, 'license-file')
+
+
+def single_line(val):
+    """
+    Quick and dirty validation for Summary pypa/setuptools#1390.
+    """
+    if '\n' in val:
+        # TODO: Replace with `raise ValueError("newlines not allowed")`
+        # after reviewing #2893.
+        msg = "newlines are not allowed in `summary` and will break in the future"
+        SetuptoolsDeprecationWarning.emit("Invalid config.", msg)
+        # due_date is undefined. Controversial change, there was a lot of push back.
+        val = val.strip().split('\n')[0]
+    return val
+
+
+def write_pkg_info(self, base_dir):
+    """Write the PKG-INFO file into the release tree."""
+    temp = ""
+    final = os.path.join(base_dir, 'PKG-INFO')
+    try:
+        # Use a temporary file while writing to avoid race conditions
+        # (e.g. `importlib.metadata` reading `.egg-info/PKG-INFO`):
+        with NamedTemporaryFile("w", encoding="utf-8", dir=base_dir, delete=False) as f:
+            temp = f.name
+            self.write_pkg_file(f)
+        permissions = stat.S_IMODE(os.lstat(temp).st_mode)
+        os.chmod(temp, permissions | stat.S_IRGRP | stat.S_IROTH)
+        os.replace(temp, final)  # atomic operation.
+    finally:
+        if temp and os.path.exists(temp):
+            os.remove(temp)
+
+
+# Based on Python 3.5 version
+def write_pkg_file(self, file):  # noqa: C901  # is too complex (14)  # FIXME
+    """Write the PKG-INFO format data to a file object."""
+    version = self.get_metadata_version()
+
+    def write_field(key, value):
+        file.write("%s: %s\n" % (key, value))
+
+    write_field('Metadata-Version', str(version))
+    write_field('Name', self.get_name())
+    write_field('Version', self.get_version())
+
+    summary = self.get_description()
+    if summary:
+        write_field('Summary', single_line(summary))
+
+    optional_fields = (
+        ('Home-page', 'url'),
+        ('Download-URL', 'download_url'),
+        ('Author', 'author'),
+        ('Author-email', 'author_email'),
+        ('Maintainer', 'maintainer'),
+        ('Maintainer-email', 'maintainer_email'),
+    )
+
+    for field, attr in optional_fields:
+        attr_val = getattr(self, attr, None)
+        if attr_val is not None:
+            write_field(field, attr_val)
+
+    license = self.get_license()
+    if license:
+        write_field('License', rfc822_escape(license))
+
+    for project_url in self.project_urls.items():
+        write_field('Project-URL', '%s, %s' % project_url)
+
+    keywords = ','.join(self.get_keywords())
+    if keywords:
+        write_field('Keywords', keywords)
+
+    platforms = self.get_platforms() or []
+    for platform in platforms:
+        write_field('Platform', platform)
+
+    self._write_list(file, 'Classifier', self.get_classifiers())
+
+    # PEP 314
+    self._write_list(file, 'Requires', self.get_requires())
+    self._write_list(file, 'Provides', self.get_provides())
+    self._write_list(file, 'Obsoletes', self.get_obsoletes())
+
+    # Setuptools specific for PEP 345
+    if hasattr(self, 'python_requires'):
+        write_field('Requires-Python', self.python_requires)
+
+    # PEP 566
+    if self.long_description_content_type:
+        write_field('Description-Content-Type', self.long_description_content_type)
+
+    self._write_list(file, 'License-File', self.license_files or [])
+    _write_requirements(self, file)
+
+    long_description = self.get_long_description()
+    if long_description:
+        file.write("\n%s" % long_description)
+        if not long_description.endswith("\n"):
+            file.write("\n")
+
+
+def _write_requirements(self, file):
+    for req in self._normalized_install_requires:
+        file.write(f"Requires-Dist: {req}\n")
+
+    processed_extras = {}
+    for augmented_extra, reqs in self._normalized_extras_require.items():
+        # Historically, setuptools allows "augmented extras": `<extra>:<condition>`
+        unsafe_extra, _, condition = augmented_extra.partition(":")
+        unsafe_extra = unsafe_extra.strip()
+        extra = _normalization.safe_extra(unsafe_extra)
+
+        if extra:
+            _write_provides_extra(file, processed_extras, extra, unsafe_extra)
+        for req in reqs:
+            r = _include_extra(req, extra, condition.strip())
+            file.write(f"Requires-Dist: {r}\n")
+
+    return processed_extras
+
+
+def _include_extra(req: str, extra: str, condition: str) -> Requirement:
+    r = Requirement(req)
+    parts = (
+        f"({r.marker})" if r.marker else None,
+        f"({condition})" if condition else None,
+        f"extra == {extra!r}" if extra else None,
+    )
+    r.marker = Marker(" and ".join(x for x in parts if x))
+    return r
+
+
+def _write_provides_extra(file, processed_extras, safe, unsafe):
+    previous = processed_extras.get(safe)
+    if previous == unsafe:
+        SetuptoolsDeprecationWarning.emit(
+            'Ambiguity during "extra" normalization for dependencies.',
+            f"""
+            {previous!r} and {unsafe!r} normalize to the same value:\n
+                {safe!r}\n
+            In future versions, setuptools might halt the build process.
+            """,
+            see_url="https://peps.python.org/pep-0685/",
+        )
+    else:
+        processed_extras[safe] = unsafe
+        file.write(f"Provides-Extra: {safe}\n")
diff --git a/setuptools/_normalization.py b/setuptools/_normalization.py
index 31899f7ab1..3e94e662ef 100644
--- a/setuptools/_normalization.py
+++ b/setuptools/_normalization.py
@@ -14,6 +14,7 @@
 # https://packaging.python.org/en/latest/specifications/core-metadata/#name
 _VALID_NAME = re.compile(r"^([A-Z0-9]|[A-Z0-9][A-Z0-9._-]*[A-Z0-9])$", re.I)
 _UNSAFE_NAME_CHARS = re.compile(r"[^A-Z0-9.]+", re.I)
+_NON_ALPHANUMERIC = re.compile(r"[^A-Z0-9]+", re.I)
 
 
 def safe_identifier(name: str) -> str:
@@ -92,6 +93,16 @@ def best_effort_version(version: str) -> str:
         return safe_name(v)
 
 
+def safe_extra(extra: str) -> str:
+    """Normalize extra name according to PEP 685
+    >>> safe_extra("_FrIeNdLy-._.-bArD")
+    'friendly-bard'
+    >>> safe_extra("FrIeNdLy-._.-bArD__._-")
+    'friendly-bard'
+    """
+    return _NON_ALPHANUMERIC.sub("-", extra).strip("-").lower()
+
+
 def filename_component(value: str) -> str:
     """Normalize each component of a filename (e.g. distribution/version part of wheel)
     Note: ``value`` needs to be already normalized.
diff --git a/setuptools/command/_requirestxt.py b/setuptools/command/_requirestxt.py
index 435d71dca5..d223737fd4 100644
--- a/setuptools/command/_requirestxt.py
+++ b/setuptools/command/_requirestxt.py
@@ -11,7 +11,6 @@
 from itertools import filterfalse
 from typing import Dict, List, Tuple, Mapping, TypeVar
 
-from .. import _reqs
 from ..extern.jaraco.text import yield_lines
 from ..extern.packaging.requirements import Requirement
 
@@ -20,11 +19,11 @@
 _T = TypeVar("_T")
 _Ordered = Dict[_T, None]
 _ordered = dict
-_StrOrIter = _reqs._StrOrIter
 
 
 def _prepare(
-    install_requires: _StrOrIter, extras_require: Mapping[str, _StrOrIter]
+    install_requires: Dict[str, Requirement],
+    extras_require: Mapping[str, Dict[str, Requirement]],
 ) -> Tuple[List[str], Dict[str, List[str]]]:
     """Given values for ``install_requires`` and ``extras_require``
     create modified versions in a way that can be written in ``requires.txt``
@@ -34,7 +33,7 @@ def _prepare(
 
 
 def _convert_extras_requirements(
-    extras_require: _StrOrIter,
+    extras_require: Dict[str, Dict[str, Requirement]],
 ) -> Mapping[str, _Ordered[Requirement]]:
     """
     Convert requirements in `extras_require` of the form
@@ -45,14 +44,15 @@ def _convert_extras_requirements(
     for section, v in extras_require.items():
         # Do not strip empty sections.
         output[section]
-        for r in _reqs.parse(v):
+        for r in v.values():
             output[section + _suffix_for(r)].setdefault(r)
 
     return output
 
 
 def _move_install_requirements_markers(
-    install_requires: _StrOrIter, extras_require: Mapping[str, _Ordered[Requirement]]
+    install_requires: Dict[str, Requirement],
+    extras_require: Mapping[str, _Ordered[Requirement]],
 ) -> Tuple[List[str], Dict[str, List[str]]]:
     """
     The ``requires.txt`` file has an specific format:
@@ -66,7 +66,7 @@ def _move_install_requirements_markers(
     # divide the install_requires into two sets, simple ones still
     # handled by install_requires and more complex ones handled by extras_require.
 
-    inst_reqs = list(_reqs.parse(install_requires))
+    inst_reqs = install_requires.values()
     simple_reqs = filter(_no_marker, inst_reqs)
     complex_reqs = filterfalse(_no_marker, inst_reqs)
     simple_install_requires = list(map(str, simple_reqs))
@@ -90,8 +90,9 @@ def _suffix_for(req):
 
 def _clean_req(req):
     """Given a Requirement, remove environment markers and return it"""
-    req.marker = None
-    return req
+    r = Requirement(str(req))  # create a copy before modifying.
+    r.marker = None
+    return r
 
 
 def _no_marker(req):
@@ -110,9 +111,10 @@ def append_cr(line):
 
 def write_requirements(cmd, basename, filename):
     dist = cmd.distribution
+    meta = dist.metadata
     data = io.StringIO()
     install_requires, extras_require = _prepare(
-        dist.install_requires or (), dist.extras_require or {}
+        meta._normalized_install_requires, meta._normalized_extras_require
     )
     _write_requirements(data, install_requires)
     for extra in sorted(extras_require):
diff --git a/setuptools/dist.py b/setuptools/dist.py
index 5e05920356..f1d361f1c3 100644
--- a/setuptools/dist.py
+++ b/setuptools/dist.py
@@ -7,12 +7,10 @@
 import os
 import re
 import sys
-import textwrap
 from contextlib import suppress
-from email import message_from_file
 from glob import iglob
 from pathlib import Path
-from typing import TYPE_CHECKING, List, Optional, Set
+from typing import List, Optional, Set
 
 import distutils.cmd
 import distutils.command
@@ -22,7 +20,6 @@
 from distutils.debug import DEBUG
 from distutils.errors import DistutilsOptionError, DistutilsSetupError
 from distutils.fancy_getopt import translate_longopt
-from distutils.util import rfc822_escape
 from distutils.util import strtobool
 
 from .extern.more_itertools import partition, unique_everseen
@@ -42,184 +39,6 @@
 from .warnings import InformationOnly, SetuptoolsDeprecationWarning
 
 
-if TYPE_CHECKING:
-    from email.message import Message
-
-
-def get_metadata_version(self):
-    mv = getattr(self, 'metadata_version', None)
-    if mv is None:
-        mv = Version('2.1')
-        self.metadata_version = mv
-    return mv
-
-
-def rfc822_unescape(content: str) -> str:
-    """Reverse RFC-822 escaping by removing leading whitespaces from content."""
-    lines = content.splitlines()
-    if len(lines) == 1:
-        return lines[0].lstrip()
-    return '\n'.join((lines[0].lstrip(), textwrap.dedent('\n'.join(lines[1:]))))
-
-
-def _read_field_from_msg(msg: "Message", field: str) -> Optional[str]:
-    """Read Message header field."""
-    value = msg[field]
-    if value == 'UNKNOWN':
-        return None
-    return value
-
-
-def _read_field_unescaped_from_msg(msg: "Message", field: str) -> Optional[str]:
-    """Read Message header field and apply rfc822_unescape."""
-    value = _read_field_from_msg(msg, field)
-    if value is None:
-        return value
-    return rfc822_unescape(value)
-
-
-def _read_list_from_msg(msg: "Message", field: str) -> Optional[List[str]]:
-    """Read Message header field and return all results as list."""
-    values = msg.get_all(field, None)
-    if values == []:
-        return None
-    return values
-
-
-def _read_payload_from_msg(msg: "Message") -> Optional[str]:
-    value = msg.get_payload().strip()
-    if value == 'UNKNOWN' or not value:
-        return None
-    return value
-
-
-def read_pkg_file(self, file):
-    """Reads the metadata values from a file object."""
-    msg = message_from_file(file)
-
-    self.metadata_version = Version(msg['metadata-version'])
-    self.name = _read_field_from_msg(msg, 'name')
-    self.version = _read_field_from_msg(msg, 'version')
-    self.description = _read_field_from_msg(msg, 'summary')
-    # we are filling author only.
-    self.author = _read_field_from_msg(msg, 'author')
-    self.maintainer = None
-    self.author_email = _read_field_from_msg(msg, 'author-email')
-    self.maintainer_email = None
-    self.url = _read_field_from_msg(msg, 'home-page')
-    self.download_url = _read_field_from_msg(msg, 'download-url')
-    self.license = _read_field_unescaped_from_msg(msg, 'license')
-
-    self.long_description = _read_field_unescaped_from_msg(msg, 'description')
-    if self.long_description is None and self.metadata_version >= Version('2.1'):
-        self.long_description = _read_payload_from_msg(msg)
-    self.description = _read_field_from_msg(msg, 'summary')
-
-    if 'keywords' in msg:
-        self.keywords = _read_field_from_msg(msg, 'keywords').split(',')
-
-    self.platforms = _read_list_from_msg(msg, 'platform')
-    self.classifiers = _read_list_from_msg(msg, 'classifier')
-
-    # PEP 314 - these fields only exist in 1.1
-    if self.metadata_version == Version('1.1'):
-        self.requires = _read_list_from_msg(msg, 'requires')
-        self.provides = _read_list_from_msg(msg, 'provides')
-        self.obsoletes = _read_list_from_msg(msg, 'obsoletes')
-    else:
-        self.requires = None
-        self.provides = None
-        self.obsoletes = None
-
-    self.license_files = _read_list_from_msg(msg, 'license-file')
-
-
-def single_line(val):
-    """
-    Quick and dirty validation for Summary pypa/setuptools#1390.
-    """
-    if '\n' in val:
-        # TODO: Replace with `raise ValueError("newlines not allowed")`
-        # after reviewing #2893.
-        msg = "newlines are not allowed in `summary` and will break in the future"
-        SetuptoolsDeprecationWarning.emit("Invalid config.", msg)
-        # due_date is undefined. Controversial change, there was a lot of push back.
-        val = val.strip().split('\n')[0]
-    return val
-
-
-# Based on Python 3.5 version
-def write_pkg_file(self, file):  # noqa: C901  # is too complex (14)  # FIXME
-    """Write the PKG-INFO format data to a file object."""
-    version = self.get_metadata_version()
-
-    def write_field(key, value):
-        file.write("%s: %s\n" % (key, value))
-
-    write_field('Metadata-Version', str(version))
-    write_field('Name', self.get_name())
-    write_field('Version', self.get_version())
-
-    summary = self.get_description()
-    if summary:
-        write_field('Summary', single_line(summary))
-
-    optional_fields = (
-        ('Home-page', 'url'),
-        ('Download-URL', 'download_url'),
-        ('Author', 'author'),
-        ('Author-email', 'author_email'),
-        ('Maintainer', 'maintainer'),
-        ('Maintainer-email', 'maintainer_email'),
-    )
-
-    for field, attr in optional_fields:
-        attr_val = getattr(self, attr, None)
-        if attr_val is not None:
-            write_field(field, attr_val)
-
-    license = self.get_license()
-    if license:
-        write_field('License', rfc822_escape(license))
-
-    for project_url in self.project_urls.items():
-        write_field('Project-URL', '%s, %s' % project_url)
-
-    keywords = ','.join(self.get_keywords())
-    if keywords:
-        write_field('Keywords', keywords)
-
-    platforms = self.get_platforms() or []
-    for platform in platforms:
-        write_field('Platform', platform)
-
-    self._write_list(file, 'Classifier', self.get_classifiers())
-
-    # PEP 314
-    self._write_list(file, 'Requires', self.get_requires())
-    self._write_list(file, 'Provides', self.get_provides())
-    self._write_list(file, 'Obsoletes', self.get_obsoletes())
-
-    # Setuptools specific for PEP 345
-    if hasattr(self, 'python_requires'):
-        write_field('Requires-Python', self.python_requires)
-
-    # PEP 566
-    if self.long_description_content_type:
-        write_field('Description-Content-Type', self.long_description_content_type)
-    if self.provides_extras:
-        for extra in self.provides_extras:
-            write_field('Provides-Extra', extra)
-
-    self._write_list(file, 'License-File', self.license_files or [])
-
-    long_description = self.get_long_description()
-    if long_description:
-        file.write("\n%s" % long_description)
-        if not long_description.endswith("\n"):
-            file.write("\n")
-
-
 sequence = tuple, list
 
 
@@ -444,6 +263,11 @@ class Distribution(_Distribution):
         'provides_extras': OrderedSet,
         'license_file': lambda: None,
         'license_files': lambda: None,
+        # Both install_requires and extras_require are needed to write PKG-INFO,
+        # So we take this opportunity to cache parsed requirement objects.
+        # These attributes are not part of the public API and intended for internal use.
+        '_normalized_install_requires': dict,  # Dict[str, Requirement]
+        '_normalized_extras_require': dict,  # Dict[str, Dict[str, Requirement]]
     }
 
     _patched_dist = None
@@ -582,9 +406,17 @@ def _normalize_requires(self):
         """Make sure requirement-related attributes exist and are normalized"""
         install_requires = getattr(self, "install_requires", None) or []
         extras_require = getattr(self, "extras_require", None) or {}
-        self.install_requires = list(map(str, _reqs.parse(install_requires)))
+        meta = self.metadata
+        meta._normalized_install_requires = {
+            str(r): r for r in _reqs.parse(install_requires)
+        }
+        meta._normalized_extras_require = {
+            k: {str(r): r for r in _reqs.parse(v or [])}
+            for k, v in extras_require.items()
+        }
+        self.install_requires = list(meta._normalized_install_requires)
         self.extras_require = {
-            k: list(map(str, _reqs.parse(v or []))) for k, v in extras_require.items()
+            k: list(v) for k, v in meta._normalized_extras_require.items()
         }
 
     def _finalize_license_files(self):
diff --git a/setuptools/monkey.py b/setuptools/monkey.py
index 9464de46ab..2ab98c178a 100644
--- a/setuptools/monkey.py
+++ b/setuptools/monkey.py
@@ -99,11 +99,16 @@ def patch_all():
 
 
 def _patch_distribution_metadata():
-    from . import dist
+    from . import _core_metadata
 
     """Patch write_pkg_file and read_pkg_file for higher metadata standards"""
-    for attr in ('write_pkg_file', 'read_pkg_file', 'get_metadata_version'):
-        new_val = getattr(dist, attr)
+    for attr in (
+        'write_pkg_info',
+        'write_pkg_file',
+        'read_pkg_file',
+        'get_metadata_version',
+    ):
+        new_val = getattr(_core_metadata, attr)
         setattr(distutils.dist.DistributionMetadata, attr, new_val)
 
 
diff --git a/setuptools/tests/config/test_apply_pyprojecttoml.py b/setuptools/tests/config/test_apply_pyprojecttoml.py
index 8a654ff94a..50099df491 100644
--- a/setuptools/tests/config/test_apply_pyprojecttoml.py
+++ b/setuptools/tests/config/test_apply_pyprojecttoml.py
@@ -45,6 +45,7 @@ def test_apply_pyproject_equivalent_to_setupcfg(url, monkeypatch, tmp_path):
 
     dist_toml = pyprojecttoml.apply_configuration(makedist(tmp_path), pyproject_example)
     dist_cfg = setupcfg.apply_configuration(makedist(tmp_path), setupcfg_example)
+    _port_tests_require(dist_cfg)
 
     pkg_info_toml = core_metadata(dist_toml)
     pkg_info_cfg = core_metadata(dist_cfg)
@@ -448,3 +449,15 @@ def core_metadata(dist) -> str:
         result.append(line + "\n")
 
     return "".join(result)
+
+
+def _port_tests_require(dist):
+    """
+    ``ini2toml`` "forward fix" deprecated tests_require definitions by moving
+    them into an extra called ``testing``.
+    """
+    tests_require = getattr(dist, "tests_require", None) or []
+    if tests_require:
+        dist.tests_require = []
+        dist.extras_require.setdefault("testing", []).extend(tests_require)
+        dist._finalize_requires()
diff --git a/setuptools/tests/test_core_metadata.py b/setuptools/tests/test_core_metadata.py
new file mode 100644
index 0000000000..5c7982a71b
--- /dev/null
+++ b/setuptools/tests/test_core_metadata.py
@@ -0,0 +1,380 @@
+import functools
+import io
+import importlib
+from email import message_from_string
+
+import pytest
+
+from setuptools import sic, _reqs
+from setuptools.dist import Distribution
+from setuptools._core_metadata import rfc822_escape, rfc822_unescape
+from setuptools.command.egg_info import egg_info, write_requirements
+
+
+EXAMPLE_BASE_INFO = dict(
+    name="package",
+    version="0.0.1",
+    author="Foo Bar",
+    author_email="foo@bar.net",
+    long_description="Long\ndescription",
+    description="Short description",
+    keywords=["one", "two"],
+)
+
+
+@pytest.mark.parametrize(
+    'content, result',
+    (
+        pytest.param(
+            "Just a single line",
+            None,
+            id="single_line",
+        ),
+        pytest.param(
+            "Multiline\nText\nwithout\nextra indents\n",
+            None,
+            id="multiline",
+        ),
+        pytest.param(
+            "Multiline\n    With\n\nadditional\n  indentation",
+            None,
+            id="multiline_with_indentation",
+        ),
+        pytest.param(
+            "  Leading whitespace",
+            "Leading whitespace",
+            id="remove_leading_whitespace",
+        ),
+        pytest.param(
+            "  Leading whitespace\nIn\n    Multiline comment",
+            "Leading whitespace\nIn\n    Multiline comment",
+            id="remove_leading_whitespace_multiline",
+        ),
+    ),
+)
+def test_rfc822_unescape(content, result):
+    assert (result or content) == rfc822_unescape(rfc822_escape(content))
+
+
+def __read_test_cases():
+    base = EXAMPLE_BASE_INFO
+
+    params = functools.partial(dict, base)
+
+    test_cases = [
+        ('Metadata version 1.0', params()),
+        (
+            'Metadata Version 1.0: Short long description',
+            params(
+                long_description='Short long description',
+            ),
+        ),
+        (
+            'Metadata version 1.1: Classifiers',
+            params(
+                classifiers=[
+                    'Programming Language :: Python :: 3',
+                    'Programming Language :: Python :: 3.7',
+                    'License :: OSI Approved :: MIT License',
+                ],
+            ),
+        ),
+        (
+            'Metadata version 1.1: Download URL',
+            params(
+                download_url='https://example.com',
+            ),
+        ),
+        (
+            'Metadata Version 1.2: Requires-Python',
+            params(
+                python_requires='>=3.7',
+            ),
+        ),
+        pytest.param(
+            'Metadata Version 1.2: Project-Url',
+            params(project_urls=dict(Foo='https://example.bar')),
+            marks=pytest.mark.xfail(
+                reason="Issue #1578: project_urls not read",
+            ),
+        ),
+        (
+            'Metadata Version 2.1: Long Description Content Type',
+            params(
+                long_description_content_type='text/x-rst; charset=UTF-8',
+            ),
+        ),
+        (
+            'License',
+            params(
+                license='MIT',
+            ),
+        ),
+        (
+            'License multiline',
+            params(
+                license='This is a long license \nover multiple lines',
+            ),
+        ),
+        pytest.param(
+            'Metadata Version 2.1: Provides Extra',
+            params(provides_extras=['foo', 'bar']),
+            marks=pytest.mark.xfail(reason="provides_extras not read"),
+        ),
+        (
+            'Missing author',
+            dict(
+                name='foo',
+                version='1.0.0',
+                author_email='snorri@sturluson.name',
+            ),
+        ),
+        (
+            'Missing author e-mail',
+            dict(
+                name='foo',
+                version='1.0.0',
+                author='Snorri Sturluson',
+            ),
+        ),
+        (
+            'Missing author and e-mail',
+            dict(
+                name='foo',
+                version='1.0.0',
+            ),
+        ),
+        (
+            'Bypass normalized version',
+            dict(
+                name='foo',
+                version=sic('1.0.0a'),
+            ),
+        ),
+    ]
+
+    return test_cases
+
+
+@pytest.mark.parametrize('name,attrs', __read_test_cases())
+def test_read_metadata(name, attrs):
+    dist = Distribution(attrs)
+    metadata_out = dist.metadata
+    dist_class = metadata_out.__class__
+
+    # Write to PKG_INFO and then load into a new metadata object
+    PKG_INFO = io.StringIO()
+
+    metadata_out.write_pkg_file(PKG_INFO)
+
+    PKG_INFO.seek(0)
+    metadata_in = dist_class()
+    metadata_in.read_pkg_file(PKG_INFO)
+
+    tested_attrs = [
+        ('name', dist_class.get_name),
+        ('version', dist_class.get_version),
+        ('author', dist_class.get_contact),
+        ('author_email', dist_class.get_contact_email),
+        ('metadata_version', dist_class.get_metadata_version),
+        ('provides', dist_class.get_provides),
+        ('description', dist_class.get_description),
+        ('long_description', dist_class.get_long_description),
+        ('download_url', dist_class.get_download_url),
+        ('keywords', dist_class.get_keywords),
+        ('platforms', dist_class.get_platforms),
+        ('obsoletes', dist_class.get_obsoletes),
+        ('requires', dist_class.get_requires),
+        ('classifiers', dist_class.get_classifiers),
+        ('project_urls', lambda s: getattr(s, 'project_urls', {})),
+        ('provides_extras', lambda s: getattr(s, 'provides_extras', set())),
+    ]
+
+    for attr, getter in tested_attrs:
+        assert getter(metadata_in) == getter(metadata_out)
+
+
+def __maintainer_test_cases():
+    attrs = {"name": "package", "version": "1.0", "description": "xxx"}
+
+    def merge_dicts(d1, d2):
+        d1 = d1.copy()
+        d1.update(d2)
+
+        return d1
+
+    test_cases = [
+        ('No author, no maintainer', attrs.copy()),
+        (
+            'Author (no e-mail), no maintainer',
+            merge_dicts(attrs, {'author': 'Author Name'}),
+        ),
+        (
+            'Author (e-mail), no maintainer',
+            merge_dicts(
+                attrs, {'author': 'Author Name', 'author_email': 'author@name.com'}
+            ),
+        ),
+        (
+            'No author, maintainer (no e-mail)',
+            merge_dicts(attrs, {'maintainer': 'Maintainer Name'}),
+        ),
+        (
+            'No author, maintainer (e-mail)',
+            merge_dicts(
+                attrs,
+                {
+                    'maintainer': 'Maintainer Name',
+                    'maintainer_email': 'maintainer@name.com',
+                },
+            ),
+        ),
+        (
+            'Author (no e-mail), Maintainer (no-email)',
+            merge_dicts(
+                attrs, {'author': 'Author Name', 'maintainer': 'Maintainer Name'}
+            ),
+        ),
+        (
+            'Author (e-mail), Maintainer (e-mail)',
+            merge_dicts(
+                attrs,
+                {
+                    'author': 'Author Name',
+                    'author_email': 'author@name.com',
+                    'maintainer': 'Maintainer Name',
+                    'maintainer_email': 'maintainer@name.com',
+                },
+            ),
+        ),
+        (
+            'No author (e-mail), no maintainer (e-mail)',
+            merge_dicts(
+                attrs,
+                {
+                    'author_email': 'author@name.com',
+                    'maintainer_email': 'maintainer@name.com',
+                },
+            ),
+        ),
+        ('Author unicode', merge_dicts(attrs, {'author': '鉄沢寛'})),
+        ('Maintainer unicode', merge_dicts(attrs, {'maintainer': 'Jan Łukasiewicz'})),
+    ]
+
+    return test_cases
+
+
+@pytest.mark.parametrize('name,attrs', __maintainer_test_cases())
+def test_maintainer_author(name, attrs, tmpdir):
+    tested_keys = {
+        'author': 'Author',
+        'author_email': 'Author-email',
+        'maintainer': 'Maintainer',
+        'maintainer_email': 'Maintainer-email',
+    }
+
+    # Generate a PKG-INFO file
+    dist = Distribution(attrs)
+    fn = tmpdir.mkdir('pkg_info')
+    fn_s = str(fn)
+
+    dist.metadata.write_pkg_info(fn_s)
+
+    with io.open(str(fn.join('PKG-INFO')), 'r', encoding='utf-8') as f:
+        raw_pkg_lines = f.readlines()
+
+    # Drop blank lines and strip lines from default description
+    pkg_lines = list(filter(None, raw_pkg_lines[:-2]))
+
+    pkg_lines_set = set(pkg_lines)
+
+    # Duplicate lines should not be generated
+    assert len(pkg_lines) == len(pkg_lines_set)
+
+    for fkey, dkey in tested_keys.items():
+        val = attrs.get(dkey, None)
+        if val is None:
+            for line in pkg_lines:
+                assert not line.startswith(fkey + ':')
+        else:
+            line = '%s: %s' % (fkey, val)
+            assert line in pkg_lines_set
+
+
+def test_parity_with_metadata_from_pypa_wheel(tmp_path):
+    attrs = dict(
+        **EXAMPLE_BASE_INFO,
+        # Example with complex requirement definition
+        python_requires=">=3.8",
+        install_requires="""
+        packaging==23.0
+        ordered-set==3.1.1
+        more-itertools==8.8.0; extra == "other"
+        jaraco.text==3.7.0
+        importlib-resources==5.10.2; python_version<"3.8"
+        importlib-metadata==6.0.0 ; python_version<"3.8"
+        colorama>=0.4.4; sys_platform == "win32"
+        """,
+        extras_require={
+            "testing": """
+                pytest >= 6
+                pytest-checkdocs >= 2.4
+                pytest-flake8 ; \\
+                        # workaround for tholo/pytest-flake8#87
+                        python_version < "3.12"
+                ini2toml[lite]>=0.9
+                """,
+            "other": [],
+        }
+    )
+    # Generate a PKG-INFO file using setuptools
+    dist = Distribution(attrs)
+    with io.StringIO() as fp:
+        dist.metadata.write_pkg_file(fp)
+        pkg_info = fp.getvalue()
+
+    # Ensure Requires-Dist is present
+    expected = [
+        'Metadata-Version:',
+        'Requires-Python: >=3.8',
+        'Provides-Extra: other',
+        'Provides-Extra: testing',
+        'Requires-Dist: pytest-flake8; python_version < "3.12" and extra == "testing"',
+        'Requires-Dist: more-itertools==8.8.0; extra == "other"',
+        'Requires-Dist: ini2toml[lite]>=0.9; extra == "testing"',
+    ]
+    for line in expected:
+        assert line in pkg_info
+
+    # Generate a METADATA file using pypa/wheel for comparisson
+    wheel_metadata = importlib.import_module("wheel.metadata")
+    pkginfo_to_metadata = getattr(wheel_metadata, "pkginfo_to_metadata", None)
+
+    if pkginfo_to_metadata is None:
+        pytest.xfail(
+            "wheel.metadata.pkginfo_to_metadata is undefined, "
+            "(this is likely to be caused by API changes in pypa/wheel"
+        )
+
+    # Generate an simplified "egg-info" dir for pypa/wheel to convert
+    egg_info_dir = tmp_path / "pkg.egg-info"
+    egg_info_dir.mkdir(parents=True)
+    (egg_info_dir / "PKG-INFO").write_text(pkg_info, encoding="utf-8")
+    write_requirements(egg_info(dist), egg_info_dir, egg_info_dir / "requires.txt")
+
+    # Get pypa/wheel generated METADATA but normalize requirements formatting
+    metadata_msg = pkginfo_to_metadata(egg_info_dir, egg_info_dir / "PKG-INFO")
+    metadata_deps = set(_reqs.parse(metadata_msg.get_all("Requires-Dist")))
+    metadata_extras = set(metadata_msg.get_all("Provides-Extra"))
+    del metadata_msg["Requires-Dist"]
+    del metadata_msg["Provides-Extra"]
+    pkg_info_msg = message_from_string(pkg_info)
+    pkg_info_deps = set(_reqs.parse(pkg_info_msg.get_all("Requires-Dist")))
+    pkg_info_extras = set(pkg_info_msg.get_all("Provides-Extra"))
+    del pkg_info_msg["Requires-Dist"]
+    del pkg_info_msg["Provides-Extra"]
+
+    # Compare setuptools PKG-INFO x pypa/wheel METADATA
+    assert metadata_msg.as_string() == pkg_info_msg.as_string()
+    assert metadata_deps == pkg_info_deps
+    assert metadata_extras == pkg_info_extras
diff --git a/setuptools/tests/test_dist.py b/setuptools/tests/test_dist.py
index f8f7996486..0caaef6578 100644
--- a/setuptools/tests/test_dist.py
+++ b/setuptools/tests/test_dist.py
@@ -1,7 +1,5 @@
-import io
 import collections
 import re
-import functools
 import os
 import urllib.request
 import urllib.parse
@@ -9,10 +7,7 @@
 from setuptools.dist import (
     check_package_data,
     check_specifier,
-    rfc822_escape,
-    rfc822_unescape,
 )
-from setuptools import sic
 from setuptools import Distribution
 
 from .textwrap import DALS
@@ -77,251 +72,6 @@ def sdist_with_index(distname, version):
 )
 
 
-def __read_test_cases():
-    base = EXAMPLE_BASE_INFO
-
-    params = functools.partial(dict, base)
-
-    test_cases = [
-        ('Metadata version 1.0', params()),
-        (
-            'Metadata Version 1.0: Short long description',
-            params(
-                long_description='Short long description',
-            ),
-        ),
-        (
-            'Metadata version 1.1: Classifiers',
-            params(
-                classifiers=[
-                    'Programming Language :: Python :: 3',
-                    'Programming Language :: Python :: 3.7',
-                    'License :: OSI Approved :: MIT License',
-                ],
-            ),
-        ),
-        (
-            'Metadata version 1.1: Download URL',
-            params(
-                download_url='https://example.com',
-            ),
-        ),
-        (
-            'Metadata Version 1.2: Requires-Python',
-            params(
-                python_requires='>=3.7',
-            ),
-        ),
-        pytest.param(
-            'Metadata Version 1.2: Project-Url',
-            params(project_urls=dict(Foo='https://example.bar')),
-            marks=pytest.mark.xfail(
-                reason="Issue #1578: project_urls not read",
-            ),
-        ),
-        (
-            'Metadata Version 2.1: Long Description Content Type',
-            params(
-                long_description_content_type='text/x-rst; charset=UTF-8',
-            ),
-        ),
-        (
-            'License',
-            params(
-                license='MIT',
-            ),
-        ),
-        (
-            'License multiline',
-            params(
-                license='This is a long license \nover multiple lines',
-            ),
-        ),
-        pytest.param(
-            'Metadata Version 2.1: Provides Extra',
-            params(provides_extras=['foo', 'bar']),
-            marks=pytest.mark.xfail(reason="provides_extras not read"),
-        ),
-        (
-            'Missing author',
-            dict(
-                name='foo',
-                version='1.0.0',
-                author_email='snorri@sturluson.name',
-            ),
-        ),
-        (
-            'Missing author e-mail',
-            dict(
-                name='foo',
-                version='1.0.0',
-                author='Snorri Sturluson',
-            ),
-        ),
-        (
-            'Missing author and e-mail',
-            dict(
-                name='foo',
-                version='1.0.0',
-            ),
-        ),
-        (
-            'Bypass normalized version',
-            dict(
-                name='foo',
-                version=sic('1.0.0a'),
-            ),
-        ),
-    ]
-
-    return test_cases
-
-
-@pytest.mark.parametrize('name,attrs', __read_test_cases())
-def test_read_metadata(name, attrs):
-    dist = Distribution(attrs)
-    metadata_out = dist.metadata
-    dist_class = metadata_out.__class__
-
-    # Write to PKG_INFO and then load into a new metadata object
-    PKG_INFO = io.StringIO()
-
-    metadata_out.write_pkg_file(PKG_INFO)
-
-    PKG_INFO.seek(0)
-    metadata_in = dist_class()
-    metadata_in.read_pkg_file(PKG_INFO)
-
-    tested_attrs = [
-        ('name', dist_class.get_name),
-        ('version', dist_class.get_version),
-        ('author', dist_class.get_contact),
-        ('author_email', dist_class.get_contact_email),
-        ('metadata_version', dist_class.get_metadata_version),
-        ('provides', dist_class.get_provides),
-        ('description', dist_class.get_description),
-        ('long_description', dist_class.get_long_description),
-        ('download_url', dist_class.get_download_url),
-        ('keywords', dist_class.get_keywords),
-        ('platforms', dist_class.get_platforms),
-        ('obsoletes', dist_class.get_obsoletes),
-        ('requires', dist_class.get_requires),
-        ('classifiers', dist_class.get_classifiers),
-        ('project_urls', lambda s: getattr(s, 'project_urls', {})),
-        ('provides_extras', lambda s: getattr(s, 'provides_extras', set())),
-    ]
-
-    for attr, getter in tested_attrs:
-        assert getter(metadata_in) == getter(metadata_out)
-
-
-def __maintainer_test_cases():
-    attrs = {"name": "package", "version": "1.0", "description": "xxx"}
-
-    def merge_dicts(d1, d2):
-        d1 = d1.copy()
-        d1.update(d2)
-
-        return d1
-
-    test_cases = [
-        ('No author, no maintainer', attrs.copy()),
-        (
-            'Author (no e-mail), no maintainer',
-            merge_dicts(attrs, {'author': 'Author Name'}),
-        ),
-        (
-            'Author (e-mail), no maintainer',
-            merge_dicts(
-                attrs, {'author': 'Author Name', 'author_email': 'author@name.com'}
-            ),
-        ),
-        (
-            'No author, maintainer (no e-mail)',
-            merge_dicts(attrs, {'maintainer': 'Maintainer Name'}),
-        ),
-        (
-            'No author, maintainer (e-mail)',
-            merge_dicts(
-                attrs,
-                {
-                    'maintainer': 'Maintainer Name',
-                    'maintainer_email': 'maintainer@name.com',
-                },
-            ),
-        ),
-        (
-            'Author (no e-mail), Maintainer (no-email)',
-            merge_dicts(
-                attrs, {'author': 'Author Name', 'maintainer': 'Maintainer Name'}
-            ),
-        ),
-        (
-            'Author (e-mail), Maintainer (e-mail)',
-            merge_dicts(
-                attrs,
-                {
-                    'author': 'Author Name',
-                    'author_email': 'author@name.com',
-                    'maintainer': 'Maintainer Name',
-                    'maintainer_email': 'maintainer@name.com',
-                },
-            ),
-        ),
-        (
-            'No author (e-mail), no maintainer (e-mail)',
-            merge_dicts(
-                attrs,
-                {
-                    'author_email': 'author@name.com',
-                    'maintainer_email': 'maintainer@name.com',
-                },
-            ),
-        ),
-        ('Author unicode', merge_dicts(attrs, {'author': '鉄沢寛'})),
-        ('Maintainer unicode', merge_dicts(attrs, {'maintainer': 'Jan Łukasiewicz'})),
-    ]
-
-    return test_cases
-
-
-@pytest.mark.parametrize('name,attrs', __maintainer_test_cases())
-def test_maintainer_author(name, attrs, tmpdir):
-    tested_keys = {
-        'author': 'Author',
-        'author_email': 'Author-email',
-        'maintainer': 'Maintainer',
-        'maintainer_email': 'Maintainer-email',
-    }
-
-    # Generate a PKG-INFO file
-    dist = Distribution(attrs)
-    fn = tmpdir.mkdir('pkg_info')
-    fn_s = str(fn)
-
-    dist.metadata.write_pkg_info(fn_s)
-
-    with io.open(str(fn.join('PKG-INFO')), 'r', encoding='utf-8') as f:
-        raw_pkg_lines = f.readlines()
-
-    # Drop blank lines and strip lines from default description
-    pkg_lines = list(filter(None, raw_pkg_lines[:-2]))
-
-    pkg_lines_set = set(pkg_lines)
-
-    # Duplicate lines should not be generated
-    assert len(pkg_lines) == len(pkg_lines_set)
-
-    for fkey, dkey in tested_keys.items():
-        val = attrs.get(dkey, None)
-        if val is None:
-            for line in pkg_lines:
-                assert not line.startswith(fkey + ':')
-        else:
-            line = '%s: %s' % (fkey, val)
-            assert line in pkg_lines_set
-
-
 def test_provides_extras_deterministic_order():
     extras = collections.OrderedDict()
     extras['a'] = ['foo']
@@ -407,40 +157,6 @@ def test_check_specifier():
         dist = Distribution(attrs)
 
 
-@pytest.mark.parametrize(
-    'content, result',
-    (
-        pytest.param(
-            "Just a single line",
-            None,
-            id="single_line",
-        ),
-        pytest.param(
-            "Multiline\nText\nwithout\nextra indents\n",
-            None,
-            id="multiline",
-        ),
-        pytest.param(
-            "Multiline\n    With\n\nadditional\n  indentation",
-            None,
-            id="multiline_with_indentation",
-        ),
-        pytest.param(
-            "  Leading whitespace",
-            "Leading whitespace",
-            id="remove_leading_whitespace",
-        ),
-        pytest.param(
-            "  Leading whitespace\nIn\n    Multiline comment",
-            "Leading whitespace\nIn\n    Multiline comment",
-            id="remove_leading_whitespace_multiline",
-        ),
-    ),
-)
-def test_rfc822_unescape(content, result):
-    assert (result or content) == rfc822_unescape(rfc822_escape(content))
-
-
 def test_metadata_name():
     with pytest.raises(DistutilsSetupError, match='missing.*name'):
         Distribution()._validate_metadata()