[PATCH v2 06/12] qapi/source: Add builtin null-object sentinel

[John Snow <jsnow@redhat.com>]

We use None to represent an object that has no source information
because it's a builtin. This complicates interface typing, since many
interfaces expect that there is an info object available to print errors
with.

Introduce a special QAPISourceInfo that represents these built-ins so
that if an error should so happen to occur relating to one of these
builtins that we will be able to print its information, and interface
typing becomes simpler: you will always have a source info object.

This object will evaluate as False, so "if info" remains a valid
idiomatic construct.

NB: It was intentional to not allow empty constructors or similar to
create "empty" source info objects; callers must explicitly invoke
'builtin()' to pro-actively opt into using the sentinel. This should
prevent use-by-accident.

Signed-off-by: John Snow <jsnow@redhat.com>
---
 scripts/qapi/source.py | 20 +++++++++++++++++++-
 1 file changed, 19 insertions(+), 1 deletion(-)

diff --git a/scripts/qapi/source.py b/scripts/qapi/source.py
index d7a79a9b8ae..a049b73b57b 100644
--- a/scripts/qapi/source.py
+++ b/scripts/qapi/source.py
@@ -11,7 +11,12 @@
 
 import copy
 import sys
-from typing import List, Optional, TypeVar
+from typing import (
+    List,
+    Optional,
+    Type,
+    TypeVar,
+)
 
 
 class QAPISchemaPragma:
@@ -41,6 +46,17 @@ def __init__(self, fname: str, line: int,
         self.defn_meta: Optional[str] = None
         self.defn_name: Optional[str] = None
 
+    @classmethod
+    def builtin(cls: Type[T]) -> T:
+        """
+        Create an instance corresponding to a built-in definition.
+        """
+        return cls('', -1, None)
+
+    def __bool__(self) -> bool:
+        # "if info" is false if @info corresponds to a built-in definition.
+        return bool(self.fname)
+
     def set_defn(self, meta: str, name: str) -> None:
         self.defn_meta = meta
         self.defn_name = name
@@ -73,4 +89,6 @@ def include_path(self) -> str:
         return ret
 
     def __str__(self) -> str:
+        if not bool(self):
+            return '[builtin]'
         return self.include_path() + self.in_defn() + self.loc()
-- 
2.26.2