Display pattern and sys.platform with platform mismatch

docs/changelog/1176.feature.rst

@@ -0,0 +1 @@
+Display pattern and ``sys.platform`` with platform mismatch - by :user:`blueyed`.

docs/changelog/examples.rst

@@ -8,11 +8,10 @@ file ``544.feature.rst``::
 
     ``tox --version`` now shows information about all registered plugins - by :user:`obestwalter`
 
-
 file ``571.bugfix.rst``::
 
     ``skip_install`` overrides ``usedevelop`` (``usedevelop`` is an option to choose the
     installation type if the package is installed and ``skip_install`` determines if it should be
     installed at all) - by :user:`ferdonline`
 
-.. see tox/pyproject.toml for all available categories
+.. see pyproject.toml for all available categories  (tool.towncrier.type)

src/tox/session/__init__.py

@@ -222,7 +222,9 @@ def _summary(self):
                     exit_code = 1
                     report = reporter.error
             elif status == "platform mismatch":
-                msg = " {}: {}".format(venv.envconfig.envname, str(status))
+                msg = " {}: {} ({!r} does not match {!r})".format(
+                    venv.envconfig.envname, str(status), sys.platform, venv.envconfig.platform
+                )
                 report = reporter.skip
             elif status and status == "ignored failed command":
                 msg = "  {}: {}".format(venv.envconfig.envname, str(status))

tests/unit/test_z_cmdline.py

@@ -190,7 +190,9 @@ def test_skip_platform_mismatch(cmd, initproj):
     result = cmd()
     assert not result.ret
     assert any(
-        "SKIPPED:  python: platform mismatch" == l for l in result.outlines
+        "SKIPPED:  python: platform mismatch ({!r} does not match 'x123')".format(sys.platform)
+        == l
+        for l in result.outlines
     ), result.outlines