Fix Module.name from full path without drive letter

Fix #7628

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/main.py

@@ -31,6 +31,7 @@ from _pytest.config import UsageError
 from _pytest.config.argparsing import Parser
 from _pytest.fixtures import FixtureManager
 from _pytest.outcomes import exit
+from _pytest.pathlib import absolutepath
 from _pytest.pathlib import Path
 from _pytest.pathlib import visit
 from _pytest.reports import CollectReport
@@ -693,15 +694,15 @@ class Session(nodes.FSCollector):
         strpath, *parts = str(arg).split("::")
         if self.config.option.pyargs:
             strpath = self._tryconvertpyarg(strpath)
-        relpath = strpath.replace("/", os.sep)
-        fspath = self.config.invocation_dir.join(relpath, abs=True)
-        if not fspath.check():
+        fspath = Path(str(self.config.invocation_dir), strpath)
+        fspath = absolutepath(fspath)
+        if not fspath.exists():
             if self.config.option.pyargs:
                 raise UsageError(
                     "file or package not found: " + arg + " (missing __init__.py?)"
                 )
             raise UsageError("file not found: " + arg)
-        return (fspath, parts)
+        return py.path.local(str(fspath)), parts
 
     def matchnodes(
         self, matching: Sequence[Union[nodes.Item, nodes.Collector]], names: List[str],

testing/test_collection.py

@@ -1426,3 +1426,42 @@ class TestImportModeImportlib:
                 "* 1 failed in *",
             ]
         )
+
+
+def test_module_full_path_without_drive(testdir):
+    """Collect and run test using full path except for the drive letter (#7628)
+
+    Passing a full path without a drive letter would trigger a bug in py.path.local
+    where it would keep the full path without the drive letter around, instead of resolving
+    to the full path, resulting in fixtures node ids not matching against test node ids correctly.
+    """
+    testdir.makepyfile(
+        **{
+            "project/conftest.py": """
+                import pytest
+                @pytest.fixture
+                def fix(): return 1
+            """,
+        }
+    )
+
+    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))
+
+    result = testdir.runpytest(path, "-v")
+    result.stdout.fnmatch_lines(
+        [
+            os.path.join("project", "tests", "dummy_test.py") + "::test PASSED *",
+            "* 1 passed in *",
+        ]
+    )