Add warning for potential extras_require misconfiguration

Fixes: #3467

setuptools/config/setupcfg.py

@@ -14,6 +14,7 @@
                     Optional, Tuple, TypeVar, Union)
 
 from distutils.errors import DistutilsOptionError, DistutilsFileError
+from setuptools.extern.packaging.requirements import Requirement, InvalidRequirement
 from setuptools.extern.packaging.version import Version, InvalidVersion
 from setuptools.extern.packaging.specifiers import SpecifierSet
 from setuptools._deprecation_warning import SetuptoolsDeprecationWarning
@@ -174,6 +175,43 @@ def parse_configuration(
     return meta, options
 
 
+def warn_accidental_env_marker_misconfig(section_name, section_options, parsed):
+    """Because users sometimes misinterpret this configuration:
+
+    [options.extras_require]
+    foo = bar;python_version<"4"
+
+    It looks like one requirement with an environment marker
+    but because there is no newline, it's parsed as two requirements
+    with a semicolon as separator.
+
+    Therefore, if:
+        * input string does not contain a newline AND
+        * parsed result contains two requirements AND
+        * parsing of the two parts from the result ("<first>;<second>")
+        leads in a valid Requirement with a valid marker
+    a UserWarning is shown to inform the user about the possible problem.
+    """
+
+    for name, (file, requirements) in section_options.items():
+        if "\n" not in requirements and len(parsed[name]) == 2:
+            original_requirements_str = ";".join(parsed[name])
+            try:
+                req = Requirement(original_requirements_str)
+            except InvalidRequirement:
+                pass
+            else:
+                if req.marker is None:
+                    continue
+                msg = (
+                    f"One of the parsed requirements in {section_name} section "
+                    f"looks like a valid environment marker: '{parsed[name][1]}'\n"
+                    "Make sure that the config is correct and check "
+                    "https://setuptools.pypa.io/en/latest/userguide/declarative_config.html#opt-2"  # noqa: E501
+                )
+                warnings.warn(msg, UserWarning)
+
+
 class ConfigHandler(Generic[Target]):
     """Handles metadata supplied in configuration files."""
 
@@ -421,6 +459,13 @@ def parse_section(self, section_options):
             try:
                 self[name] = value
 
+                if name == "install_requires":
+                    warn_accidental_env_marker_misconfig(
+                        "install_requires",
+                        {name: (_, value)},
+                        {name: self.target_obj.install_requires},
+                    )
+
             except KeyError:
                 pass  # Keep silent for a new option may appear anytime.
 
@@ -702,6 +747,9 @@ def parse_section_extras_require(self, section_options):
             section_options,
             self._parse_requirements_list,
         )
+
+        warn_accidental_env_marker_misconfig("extras_require", section_options, parsed)
+
         self['extras_require'] = parsed
 
     def parse_section_data_files(self, section_options):

setuptools/tests/config/test_setupcfg.py

@@ -716,6 +716,51 @@ def test_extras_require(self, tmpdir):
             }
             assert dist.metadata.provides_extras == set(['pdf', 'rest'])
 
+    @pytest.mark.parametrize(
+        "config",
+        [
+            "[options.extras_require]\nfoo = bar;python_version<'3'",
+            "[options.extras_require]\nfoo = bar;os_name=='linux'",
+            "[options.extras_require]\nfoo = bar;python_version<'3'\n",
+            "[options.extras_require]\nfoo = bar;os_name=='linux'\n",
+            "[options]\ninstall_requires = bar;python_version<'3'",
+            "[options]\ninstall_requires = bar;os_name=='linux'",
+            "[options]\ninstall_requires = bar;python_version<'3'\n",
+            "[options]\ninstall_requires = bar;os_name=='linux'\n",
+        ],
+    )
+    def test_warn_accidental_env_marker_misconfig(self, config, tmpdir):
+        fake_env(tmpdir, config)
+        match = (
+            r"One of the parsed requirements in (install_requires|extras_require) "
+            "section looks like a valid environment marker.*"
+        )
+        with pytest.warns(UserWarning, match=match):
+            with get_dist(tmpdir) as _:
+                pass
+
+    @pytest.mark.parametrize(
+        "config",
+        [
+            "[options.extras_require]\nfoo =\n    bar;python_version<'3'",
+            "[options.extras_require]\nfoo = bar;baz\nboo = xxx;yyy",
+            "[options.extras_require]\nfoo =\n    bar;python_version<'3'\n",
+            "[options.extras_require]\nfoo = bar;baz\nboo = xxx;yyy\n",
+            "[options.extras_require]\nfoo =\n    bar\n    python_version<'3'\n",
+            "[options]\ninstall_requires =\n    bar;python_version<'3'",
+            "[options]\ninstall_requires = bar;baz\nboo = xxx;yyy",
+            "[options]\ninstall_requires =\n    bar;python_version<'3'\n",
+            "[options]\ninstall_requires = bar;baz\nboo = xxx;yyy\n",
+            "[options]\ninstall_requires =\n    bar\n    python_version<'3'\n",
+        ],
+    )
+    def test_nowarn_accidental_env_marker_misconfig(self, config, tmpdir, recwarn):
+        fake_env(tmpdir, config)
+        with get_dist(tmpdir) as _:
+            pass
+        # The examples are valid, no warnings shown
+        assert not any(w.category == UserWarning for w in recwarn)
+
     def test_dash_preserved_extras_require(self, tmpdir):
         fake_env(tmpdir, '[options.extras_require]\n' 'foo-a = foo\n' 'foo_b = test\n')