Fix Module.name from full path without drive letter

Fix #7628

Author: nicoddemus

changelog/7628.bugfix.rst

@@ -0,0 +1 @@
+Fix test collection when a full path without a drive letter was passed to pytest on Windows (for example ``\projects\tests\test.py`` instead of ``c:\projects\tests\pytest.py``).

src/_pytest/nodes.py

@@ -515,6 +515,7 @@ def __init__(
         session: Optional["Session"] = None,
         nodeid: Optional[str] = None,
     ) -> None:
+        fspath = py.path.local(os.path.abspath(str(fspath)))
         name = fspath.basename
         if parent is not None:
             rel = fspath.relto(parent.fspath)

testing/test_collection.py

@@ -1426,3 +1426,59 @@ def test_modules_not_importable_as_side_effect(self, testdir):
                 "* 1 failed in *",
             ]
         )
+
+
+class TestCollectionFullPathWithoutDriveLetter:
+    def setup_test_file(self, testdir):
+        testdir.makepyfile(
+            **{
+                "project/tests/dummy_test.py": """
+                    def test(fix):
+                        assert fix == 1
+                """
+            }
+        )
+        fn = testdir.tmpdir.join("project/tests/dummy_test.py")
+        assert fn.isfile()
+
+        drive, path = os.path.splitdrive(str(fn))
+        return path
+
+    def setup_conftest(self, testdir):
+        testdir.makepyfile(
+            **{
+                "project/conftest.py": """
+                    import pytest
+                    @pytest.fixture
+                    def fix(): return 1
+                """,
+            }
+        )
+
+    def test_module_name(self, testdir):
+        """Module name resolves correctly from full path without drive letter (#7628)
+
+        Full path without drive letter makes py.path.local.relto, during Module construction,
+        to incorrectly report that the module path is not relative to the Session root.
+        """
+        path = self.setup_test_file(testdir)
+
+        [item], _ = testdir.inline_genitems(path)
+        assert item.name == "test"
+        assert item.parent.name == "project/tests/dummy_test.py"
+
+    def test_module_full_path_without_drive_integration(self, testdir):
+        """Collect and run test using full path except for the drive letter (#7628)
+
+        Integration test as reported to make sure we don't regress in other areas covered by
+        the test above.
+        """
+        self.setup_conftest(testdir)
+        path = self.setup_test_file(testdir)
+        result = testdir.runpytest(path, "-v")
+        result.stdout.fnmatch_lines(
+            [
+                os.path.join("project", "tests", "dummy_test.py") + "::test PASSED *",
+                "* 1 passed in *",
+            ]
+        )