Include more details in a pip freeze warning message

news/6513.bugfix

@@ -0,0 +1,2 @@
+Include more details in the log message if ``pip freeze`` can't generate a
+requirement string for a particular distribution.

src/pip/_internal/operations/freeze.py

@@ -60,10 +60,14 @@ def freeze(
                                             user_only=user_only):
         try:
             req = FrozenRequirement.from_dist(dist)
-        except RequirementParseError:
+        except RequirementParseError as exc:
+            # We include dist rather than dist.project_name because the
+            # dist string includes more information, like the version and
+            # location. We also include the exception message to aid
+            # troubleshooting.
             logger.warning(
-                "Could not parse requirement: %s",
-                dist.project_name
+                'Could not generate requirement for distribution %r: %s',
+                dist, exc
             )
             continue
         if exclude_editable and req.editable:

tests/functional/test_freeze.py

@@ -108,12 +108,19 @@ def fake_install(pkgname, dest):
             '...{}==1.0...'.format(pkgname.replace('_', '-'))
         )
     for pkgname in invalid_pkgnames:
-        _check_output(
-            result.stderr,
-            '...Could not parse requirement: {}\n...'.format(
-                pkgname.replace('_', '-')
-            )
+        # Check that the full distribution repr is present.
+        dist_repr = '{} 1.0 ('.format(pkgname.replace('_', '-'))
+        expected = (
+            '...Could not generate requirement for '
+            'distribution {}...'.format(dist_repr)
         )
+        _check_output(result.stderr, expected)
+
+    # Also check that the parse error details occur at least once.
+    # We only need to find one occurrence to know that exception details
+    # are logged.
+    expected = '...site-packages): Parse error at "...'
+    _check_output(result.stderr, expected)
 
 
 @pytest.mark.git