Add requirement parsing in setuptools.build_meta

This fixes GH #1682 by porting the pkg_resources requirement parsing
logic into setuptools.build_meta, so that all valid requirement
specifiers passed to setup_requires will be added to the
get_requires_for_build_* function outputs.

Fixes GH #1682

setuptools/build_meta.py

@@ -36,6 +36,8 @@ import contextlib
 import setuptools
 import distutils
 
+from setuptools._vendor import six
+
 __all__ = ['get_requires_for_build_sdist',
            'get_requires_for_build_wheel',
            'prepare_metadata_for_build_wheel',
@@ -51,7 +53,9 @@ class SetupRequirementsError(BaseException):
 
 class Distribution(setuptools.dist.Distribution):
     def fetch_build_eggs(self, specifiers):
-        raise SetupRequirementsError(specifiers)
+        specifier_list = self._parse_requirements(specifiers)
+
+        raise SetupRequirementsError(specifier_list)
 
     @classmethod
     @contextlib.contextmanager
@@ -68,6 +72,45 @@ class Distribution(setuptools.dist.Distribution):
         finally:
             distutils.core.Distribution = orig
 
+    def _yield_lines(self, strs):
+        """Yield non-empty/non-comment lines of a string or sequence"""
+        if isinstance(strs, six.string_types):
+            for s in strs.splitlines():
+                s = s.strip()
+                # skip blank lines/comments
+                if s and not s.startswith('#'):
+                    yield s
+        else:
+            for ss in strs:
+                for s in self._yield_lines(ss):
+                    yield s
+
+    def _parse_requirements(self, strs):
+        """Parse requirement specifiers into a list of requirement strings
+
+        This is forked from pkg_resources.parse_requirements.
+
+        `strs` must be a string, or a (possibly-nested) iterable thereof.
+        """
+        # create a steppable iterator, so we can handle \-continuations
+        lines = iter(self._yield_lines(strs))
+
+        requirements = []
+
+        for line in lines:
+            # Drop comments -- a hash without a space may be in a URL.
+            if ' #' in line:
+                line = line[:line.find(' #')]
+            # If there is a line continuation, drop it, and append the next line.
+            if line.endswith('\\'):
+                line = line[:-2].strip()
+                try:
+                    line += next(lines)
+                except StopIteration:
+                    return
+            requirements.append(line)
+
+        return requirements
 
 def _to_str(s):
     """

setuptools/tests/test_build_meta.py

@@ -288,21 +288,17 @@ class TestBuildMetaBackend:
             build_backend.build_sdist("temp")
 
     @pytest.mark.parametrize('setup_literal, requirements', [
-        pytest.param("'foo'", ['foo'], marks=pytest.mark.xfail),
+        ("'foo'", ['foo']),
         ("['foo']", ['foo']),
-        pytest.param(r"'foo\n'", ['foo'], marks=pytest.mark.xfail),
-        pytest.param(r"'foo\n\n'", ['foo'], marks=pytest.mark.xfail),
+        (r"'foo\n'", ['foo']),
+        (r"'foo\n\n'", ['foo']),
         ("['foo', 'bar']", ['foo', 'bar']),
-        pytest.param(r"'# Has a comment line\nfoo'",
-                     ['foo'], marks=pytest.mark.xfail),
-        pytest.param(r"'foo # Has an inline comment'",
-                     ['foo'], marks=pytest.mark.xfail),
-        pytest.param(r"'foo \\\n >=3.0'",
-                     ['foo>=3.0'], marks=pytest.mark.xfail),
-        pytest.param(r"'foo\nbar'", ['foo', 'bar'], marks=pytest.mark.xfail),
-        pytest.param(r"'foo\nbar\n'", ['foo', 'bar'], marks=pytest.mark.xfail),
-        pytest.param(r"['foo\n', 'bar\n']",
-                     ['foo', 'bar'], marks=pytest.mark.xfail),
+        (r"'# Has a comment line\nfoo'", ['foo']),
+        (r"'foo # Has an inline comment'", ['foo']),
+        (r"'foo \\\n >=3.0'", ['foo>=3.0']),
+        (r"'foo\nbar'", ['foo', 'bar']),
+        (r"'foo\nbar\n'", ['foo', 'bar']),
+        (r"['foo\n', 'bar\n']", ['foo', 'bar']),
     ])
     def test_setup_requires(self, setup_literal, requirements, tmpdir_cwd):