qapi: Improve specificity of type/member descriptions

Error messages describe object members, enumeration values, features,
and variants like ROLE 'NAME', where ROLE is "member", "value",
"feature", or "branch", respectively.  When the member is defined in
another type, e.g. inherited from a base type, we add "of type
'TYPE'".  Example: test case struct-base-clash-deep reports a member
of type 'Sub' clashing with a member of its base type 'Base' as

    struct-base-clash-deep.json: In struct 'Sub':
    struct-base-clash-deep.json:10: member 'name' collides with member 'name' of type 'Base'

Members of implicitly defined types need special treatment.  We don't
want to add "of type 'TYPE'" for them, because their named are made up
and mean nothing to the user.  Instead, we describe members of an
implicitly defined base type as "base member 'NAME'", and command and
event parameters as "parameter 'NAME'".  Example: test case
union-bad-base reports member of a variant's type clashing with a
member of its implicitly defined base type as

    union-bad-base.json: In union 'TestUnion':
    union-bad-base.json:8: member 'string' of type 'TestTypeA' collides with base member 'string'

The next commit will permit unions as variant types.  "base member
'NAME' would then be ambigious: is it the union's base, or is it the
union's variant's base?  One of its test cases would report a clash
between two such bases as "base member 'type' collides with base
member 'type'".  Confusing.

Refine the special treatment: add "of TYPE" even for implicitly
defined types, but massage TYPE and ROLE so they make sense for the
user.

Message-Id: <20230420102619.348173-3-berrange@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>

scripts/qapi/schema.py

@@ -701,6 +701,7 @@ def connect_doc(self, doc):
 
     def describe(self, info):
         role = self.role
+        meta = 'type'
         defined_in = self.defined_in
         assert defined_in
 
@@ -712,13 +713,17 @@ def describe(self, info):
                 # Implicit type created for a command's dict 'data'
                 assert role == 'member'
                 role = 'parameter'
+                meta = 'command'
+                defined_in = defined_in[:-4]
             elif defined_in.endswith('-base'):
                 # Implicit type created for a union's dict 'base'
                 role = 'base ' + role
+                defined_in = defined_in[:-5]
             else:
                 assert False
-        elif defined_in != info.defn_name:
-            return "%s '%s' of type '%s'" % (role, self.name, defined_in)
+
+        if defined_in != info.defn_name:
+            return "%s '%s' of %s '%s'" % (role, self.name, meta, defined_in)
         return "%s '%s'" % (role, self.name)