This patch adds a warning (which is a build failure under our current
build settings) whenever a QAPI cross-reference fails to resolve.

This applies to any cross-references of the form :qapi:{role}:`foo`,
which covers all of the automatically generated references by the qapi
domain, and any such references that are manually written into the
documentation rst files.

Cross-references of the form `foo` do not use this system, but are
already configured to issue a warning (Again, a build failure) if the
cross-reference isn't found anywhere.

Adds warnings that look like the following:

docs/qapi/index.rst:48: WARNING: qapi:type reference target not found: 'footype' [ref.qapi]
docs/qapi/index.rst:50: WARNING: qapi:mod reference target not found: 'foomod' [ref.qapi]

Signed-off-by: John Snow <jsnow@redhat.com>
---
 docs/sphinx/qapi_domain.py | 23 +++++++++++++++++++++++
 1 file changed, 23 insertions(+)

diff --git a/docs/sphinx/qapi_domain.py b/docs/sphinx/qapi_domain.py
index d3487c5dfeb..9a11a2dcbe0 100644
--- a/docs/sphinx/qapi_domain.py
+++ b/docs/sphinx/qapi_domain.py
@@ -882,6 +882,29 @@ def resolve_xref(
         matches = self.find_obj(modname, target, typ)
 
         if not matches:
+            # Normally, we could pass warn_dangling=True to QAPIXRefRole(),
+            # but that will trigger on references to these built-in types,
+            # which we'd like to ignore instead.
+
+            # Take care of that warning here instead, so long as the
+            # reference isn't to one of our built-in core types.
+            if target not in (
+                "string",
+                "number",
+                "int",
+                "boolean",
+                "null",
+                "value",
+                "q_empty",
+            ):
+                logger.warning(
+                    __("qapi:%s reference target not found: %r"),
+                    typ,
+                    target,
+                    type="ref",
+                    subtype="qapi",
+                    location=node,
+                )
             return None
 
         if len(matches) > 1:
-- 
2.48.1