Add the .. qapi:alternate:: directive, object, and qapi:alt:`name`
cross-reference role.

Add the "Choices:" field list for describing alternate choices. Like
other field lists that reference QAPI types, a forthcoming commit will
add cross-referencing support to this field.

RFC: In the future, it would be nice to directly inline Alternates as
part of the type information in the containing object (i.e. directly in
arguments/members) - but that's a task for another series. For now, the
branch "names" are documented just like qapidoc.py does, even though
this information is superfluous for user documentation. Room for future
improvement, but not now.

Signed-off-by: John Snow <jsnow@redhat.com>
---
 docs/sphinx/qapi-domain.py | 20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/docs/sphinx/qapi-domain.py b/docs/sphinx/qapi-domain.py
index e37e300e4e6..e0c31b2ee4d 100644
--- a/docs/sphinx/qapi-domain.py
+++ b/docs/sphinx/qapi-domain.py
@@ -304,6 +304,23 @@ class QAPIEnum(QAPIObject):
     )
 
 
+class QAPIAlternate(QAPIObject):
+    """Description of a QAPI Alternate."""
+
+    doc_field_types = QAPIObject.doc_field_types.copy()
+    doc_field_types.extend(
+        [
+            # :choice type name: descr
+            TypedField(
+                "choice",
+                label=_("Choices"),
+                names=("choice",),
+                can_collapse=False,
+            ),
+        ]
+    )
+
+
 class QAPIModule(SphinxDirective):
     """
     Directive to mark description of a new module.
@@ -465,6 +482,7 @@ class QAPIDomain(Domain):
         "module": ObjType(_("module"), "mod", "obj"),
         "command": ObjType(_("command"), "cmd", "obj"),
         "enum": ObjType(_("enum"), "enum", "obj", "type"),
+        "alternate": ObjType(_("alternate"), "alt", "obj", "type"),
     }
 
     # Each of these provides a rST directive,
@@ -473,6 +491,7 @@ class QAPIDomain(Domain):
         "module": QAPIModule,
         "command": QAPICommand,
         "enum": QAPIEnum,
+        "alternate": QAPIAlternate,
     }
 
     # These are all cross-reference roles; e.g.
@@ -482,6 +501,7 @@ class QAPIDomain(Domain):
         "mod": QAPIXRefRole(),
         "cmd": QAPIXRefRole(),
         "enum": QAPIXRefRole(),
+        "alt": QAPIXRefRole(),
         # reference any data type (excludes modules, commands, events)
         "type": QAPIXRefRole(),
         "obj": QAPIXRefRole(),  # reference *any* type of QAPI object.
-- 
2.47.0