option ref: support optional part with '[=value]'

This supports a commonly used format when it comes to optional argument:
-fauto-profile[=path], where `path` is optional part.

Extends: sphinx-doc#10840

doc/usage/restructuredtext/domains.rst

@@ -1777,7 +1777,8 @@ There is a set of directives allowing documenting command-line programs:
 
    .. versionchanged:: 5.3
 
-      One can cross-reference including an option value: ``:option:`--module=foobar```.
+      One can cross-reference including an option value: ``:option:`--module=foobar```,
+      or ``:option:`--module[=foobar]```.
 
    Use :confval:`option_emphasise_placeholders` for parsing of
    "variable part" of a literal text (similarly to the :rst:role:`samp` role).

sphinx/domains/std.py

@@ -942,11 +942,20 @@ def _resolve_option_xref(self, env: "BuildEnvironment", fromdocname: str,
                              node: pending_xref, contnode: Element) -> Optional[Element]:
         progname = node.get('std:program')
         target = target.strip()
-        docname, labelid = self.progoptions.get((progname, target), ('', ''))
+        needles = [target]
         # for :option:`-foo=bar` search for -foo option directive
-        if not docname and '=' in target:
-            target2 = target[:target.find('=')]
-            docname, labelid = self.progoptions.get((progname, target2), ('', ''))
+        idx = target.find('=')
+        if idx != -1:
+            needles.append(target[:idx])
+        # for :option:`-foo[=bar]` search for -foo option directive
+        idx = target.find('[=')
+        if idx != -1:
+            needles.append(target[:idx])
+        for needle in needles:
+            docname, labelid = self.progoptions.get((progname, needle), ('', ''))
+            if docname:
+                break
+
         if not docname:
             commands = []
             while ws_re.search(target):

tests/roots/test-root/objects.txt

@@ -214,7 +214,7 @@ Test repeated option directive.
 
   My secret API.
 
-Reference the first option :option:`-mapi=secret`.
+Reference the first option :option:`-mapi=secret` or :option:`-mapi[=xxx]`.
 
 
 User markup

tests/test_build_html.py

@@ -1771,6 +1771,8 @@ def test_option_reference_with_value(app, status, warning):
     assert ('<span class="pre">-mapi</span></span><span class="sig-prename descclassname">'
             '</span><a class="headerlink" href="#cmdoption-git-commit-mapi"') in content
     assert 'first option <a class="reference internal" href="#cmdoption-git-commit-mapi">' in content
+    assert ('<a class="reference internal" href="#cmdoption-git-commit-mapi">'
+            '<code class="xref std std-option docutils literal notranslate"><span class="pre">-mapi[=xxx]</span></code></a>') in content
 
 
 @pytest.mark.sphinx('html', testroot='theming')