mark: deprecate a couple undocumented -k syntaxes

The `-k '-expr'` syntax is an old alias to `-k 'not expr'`. It's also
not a very convenient to have syntax that start with `-` on the CLI.
Deprecate it and suggest replacing with `not`.

---

The `-k 'expr:'` syntax discards all items until the first match and
keeps all subsequent, e.g. `-k foo` with

    test_bar
    test_foo
    test_baz

results in `test_foo`, `test_baz`. That's a bit weird, so deprecate it
without a replacement. If someone complains we can reconsider or devise
a better alternative.

changelog/TBD.deprecation.rst

@@ -0,0 +1,5 @@
+The special ``-k '-expr'`` syntax to ``-k`` is deprecated. Use ``-k 'not expr'``
+instead.
+
+The special ``-k 'expr:'`` syntax to ``-k`` is deprecated. Please open an issue
+if you use this and want a replacement.

src/_pytest/deprecated.py

@@ -75,3 +75,13 @@
     "The TerminalReporter.writer attribute is deprecated, use TerminalReporter._tw instead at your own risk.\n"
     "See https://docs.pytest.org/en/latest/deprecations.html#terminalreporter-writer for more information."
 )
+
+
+MINUS_K_DASH = PytestDeprecationWarning(
+    "The `-k '-expr'` syntax to -k is deprecated.\nUse `-k 'not expr'` instead."
+)
+
+MINUS_K_COLON = PytestDeprecationWarning(
+    "The `-k 'expr:'` syntax to -k is deprecated.\n"
+    "Please open an issue if you use this and want a replacement."
+)

src/_pytest/mark/__init__.py

@@ -1,4 +1,5 @@
 """ generic mechanism for marking and selecting python functions. """
+import warnings
 from typing import Optional
 
 from .legacy import matchkeyword
@@ -13,6 +14,8 @@
 from _pytest.config import Config
 from _pytest.config import hookimpl
 from _pytest.config import UsageError
+from _pytest.deprecated import MINUS_K_COLON
+from _pytest.deprecated import MINUS_K_DASH
 from _pytest.store import StoreKey
 
 __all__ = ["Mark", "MarkDecorator", "MarkGenerator", "get_empty_parameterset_mark"]
@@ -107,9 +110,13 @@ def deselect_by_keyword(items, config):
         return
 
     if keywordexpr.startswith("-"):
+        # To be removed in pytest 7.0.0.
+        warnings.warn(MINUS_K_DASH, stacklevel=2)
         keywordexpr = "not " + keywordexpr[1:]
     selectuntil = False
     if keywordexpr[-1:] == ":":
+        # To be removed in pytest 7.0.0.
+        warnings.warn(MINUS_K_COLON, stacklevel=2)
         selectuntil = True
         keywordexpr = keywordexpr[:-1]
 

testing/deprecated_test.py

@@ -164,3 +164,27 @@ def test__fillfuncargs_is_deprecated() -> None:
         match="The `_fillfuncargs` function is deprecated",
     ):
         pytest._fillfuncargs(mock.Mock())
+
+
+def test_minus_k_dash_is_deprecated(testdir) -> None:
+    threepass = testdir.makepyfile(
+        test_threepass="""
+        def test_one(): assert 1
+        def test_two(): assert 1
+        def test_three(): assert 1
+    """
+    )
+    result = testdir.runpytest("-k=-test_two", threepass)
+    result.stdout.fnmatch_lines(["*The `-k '-expr'` syntax*deprecated*"])
+
+
+def test_minus_k_colon_is_deprecated(testdir) -> None:
+    threepass = testdir.makepyfile(
+        test_threepass="""
+        def test_one(): assert 1
+        def test_two(): assert 1
+        def test_three(): assert 1
+    """
+    )
+    result = testdir.runpytest("-k", "test_two:", threepass)
+    result.stdout.fnmatch_lines(["*The `-k 'expr:'` syntax*deprecated*"])