Define standard 'persistent' annotation.

c++/src/capnp/persistent.capnp

@@ -48,9 +48,10 @@ interface Persistent@0xc8cb212fcd9f5691(SturdyRef, Owner) {
   # the capability's interface should NOT inherit `Persistent`; instead, just perform a cast at
   # runtime. It's not type-safe, but trying to be type-safe in these cases will likely lead to
   # tears. In cases where a particular interface only makes sense on persistent capabilities, it
-  # *might* make sense to explicitly inherit it. But, even in these cases, you probably don't want
-  # to specify the `SturdyRef` parameter, since this type may differ from app to app or even host
-  # to host.
+  # still should not explicitly inherit Persistent because the `SturdyRef` and `Owner` types will
+  # vary between realms (they may even be different at the call site than they are on the
+  # implementation). Instead, mark persistent interfaces with the $persistent annotation (defined
+  # below).
   #
   # Sealing
   # -------
@@ -123,3 +124,16 @@ interface RealmGateway(InternalRef, ExternalRef, InternalOwner, ExternalOwner) {
   # Given an internal capability, save it and return an external reference. Used when someone
   # outside the realm tries to save a capability from inside the realm.
 }
+
+annotation persistent(interface, field) :Void;
+# Apply this annotation to interfaces for objects that will always be persistent, instead of
+# extending the Persistent capability, since the correct type parameters to Persistent depend on
+# the realm, which is orthogonal to the interface type and therefore should not be defined
+# along-side it.
+#
+# You may also apply this annotation to a capability-typed field which will always contain a
+# persistent capability, but where the capability's interface itself is not already marked
+# persistent.
+#
+# Note that absence of the $persistent annotation doesn't mean a capability of that type isn't
+# persistent; it just means not *all* such capabilities are persistent.

c++/src/capnp/persistent.capnp.c++

@@ -482,5 +482,35 @@ const ::capnp::_::RawSchema s_ecafa18b482da3aa = {
   2, 2, i_ecafa18b482da3aa, nullptr, nullptr, { &s_ecafa18b482da3aa, nullptr, bd_ecafa18b482da3aa, 0, sizeof(bd_ecafa18b482da3aa) / sizeof(bd_ecafa18b482da3aa[0]), nullptr }
 };
 #endif  // !CAPNP_LITE
+static const ::capnp::_::AlignedData<21> b_f622595091cafb67 = {
+  {   0,   0,   0,   0,   5,   0,   6,   0,
+    103, 251, 202, 145,  80,  89,  34, 246,
+     23,   0,   0,   0,   5,   0,  32,   1,
+    215, 238,  63, 152,  54,   8,  99, 184,
+      0,   0,   0,   0,   0,   0,   0,   0,
+      0,   0,   0,   0,   0,   0,   0,   0,
+     21,   0,   0,   0,  18,   1,   0,   0,
+     37,   0,   0,   0,   7,   0,   0,   0,
+      0,   0,   0,   0,   0,   0,   0,   0,
+     32,   0,   0,   0,   2,   0,   1,   0,
+      0,   0,   0,   0,   0,   0,   0,   0,
+      0,   0,   0,   0,   0,   0,   0,   0,
+     99,  97, 112, 110, 112,  47, 112, 101,
+    114, 115, 105, 115, 116, 101, 110, 116,
+     46,  99,  97, 112, 110, 112,  58, 112,
+    101, 114, 115, 105, 115, 116, 101, 110,
+    116,   0,   0,   0,   0,   0,   0,   0,
+      0,   0,   0,   0,   1,   0,   1,   0,
+      0,   0,   0,   0,   0,   0,   0,   0,
+      0,   0,   0,   0,   0,   0,   0,   0,
+      0,   0,   0,   0,   0,   0,   0,   0, }
+};
+::capnp::word const* const bp_f622595091cafb67 = b_f622595091cafb67.words;
+#if !CAPNP_LITE
+const ::capnp::_::RawSchema s_f622595091cafb67 = {
+  0xf622595091cafb67, b_f622595091cafb67.words, 21, nullptr, nullptr,
+  0, 0, nullptr, nullptr, nullptr, { &s_f622595091cafb67, nullptr, nullptr, 0, 0, nullptr }
+};
+#endif  // !CAPNP_LITE
 }  // namespace schemas
 }  // namespace capnp

c++/src/capnp/persistent.capnp.h

@@ -23,6 +23,7 @@ CAPNP_DECLARE_SCHEMA(b76848c18c40efbf);
 CAPNP_DECLARE_SCHEMA(84ff286cd00a3ed4);
 CAPNP_DECLARE_SCHEMA(f0c2cc1d3909574d);
 CAPNP_DECLARE_SCHEMA(ecafa18b482da3aa);
+CAPNP_DECLARE_SCHEMA(f622595091cafb67);
 
 }  // namespace schemas
 }  // namespace capnp