Add substitution {/} to os.sep

Closes tox-dev#1700

docs/config.rst

@@ -669,6 +669,12 @@ Globally available substitutions
     OS-specific path separator (``:`` os \*nix family, ``;`` on Windows). May be used in ``setenv``,
     when target variable is path variable (e.g. PATH or PYTHONPATH).
 
+``{/}``
+    OS-specific directory separator (``/`` os \*nix family, ``\\`` on Windows).
+    Useful for deriving filenames from preset paths, as arguments for commands
+    that requires ``\\`` on Windows. e.g. ``{distdir}{/}file.txt``.
+    It is not usually needed when using commands written in Python.
+
 Substitutions for virtualenv-related sections
 +++++++++++++++++++++++++++++++++++++++++++++
 

src/tox/config/__init__.py

@@ -1811,6 +1811,8 @@ def _replace_match(self, match):
                 "Malformed substitution; no substitution type provided",
             )
 
+        if not sub_type and not g["default_value"] and sub_value == "/":
+            return os.sep
         if sub_type == "env":
             return self._replace_env(match)
         if sub_type == "tty":

tests/unit/config/test_config.py

@@ -2792,7 +2792,8 @@ def test_multiple_homedir_relative_local_indexservers(self, newconfig):
 
 class TestConfigConstSubstitutions:
     @pytest.mark.parametrize("pathsep", [":", ";"])
-    def test_replace_pathsep_unix(self, monkeypatch, newconfig, pathsep):
+    def test_replace_pathsep(self, monkeypatch, newconfig, pathsep):
+        """Replace {:} with OS path separator."""
         monkeypatch.setattr("os.pathsep", pathsep)
         config = newconfig(
             """
@@ -2813,6 +2814,29 @@ def test_pathsep_regex(self):
         assert mdict["substitution_value"] == ""
         assert mdict["default_value"] == ""
 
+    @pytest.mark.parametrize("dirsep", ["\\", "\\\\"])
+    def test_dirsep_replace(self, monkeypatch, newconfig, dirsep):
+        """Replace {/} with OS directory separator."""
+        monkeypatch.setattr("os.sep", dirsep)
+        config = newconfig(
+            """
+        [testenv]
+        setenv =
+            VAR = dira{/}subdirb{/}subdirc
+        """,
+        )
+        envconfig = config.envconfigs["python"]
+        assert envconfig.setenv["VAR"] == dirsep.join(["dira", "subdirb", "subdirc"])
+
+    def test_dirsep_regex(self):
+        """Sanity check for regex behavior for directory separator."""
+        regex = tox.config.Replacer.RE_ITEM_REF
+        match = next(regex.finditer("{/}"))
+        mdict = match.groupdict()
+        assert mdict["sub_type"] is None
+        assert mdict["substitution_value"] == "/"
+        assert mdict["default_value"] is None
+
 
 class TestParseEnv:
     def test_parse_recreate(self, newconfig):