Add @thrift annotations: AllowLegacyTypedefUri AllowLegacyOptionalCustomDefaultValue

Reviewed By: hchokshi, thedavekwon

Differential Revision: D78716098

fbshipit-source-id: 74dee5e95cf96c720b1026902ad83e6c7f99e039

Author: aristidisp

thrift/annotation/thrift.thrift

@@ -257,3 +257,54 @@ struct AllowReservedFilename {}
  */
 @scope.Struct
 struct RuntimeAnnotation {}
+
+/**
+ * Allows the Thrift compiler to add a URI to the target typedef. 
+ *
+ * Use of this annotation is strongly DISCOURAGED, and is provided for
+ * backwards-compatibility purposes only.
+ *
+ * Indeed, Thrift IDL [typedefs](https://github.com/facebook/fbthrift/blob/main/thrift/doc/idl/index.md#typedefs)
+ * do not correspond to the set of user-defined types that can have unique URIs
+ * per the [Thrift Object Model](https://github.com/facebook/fbthrift/blob/main/thrift/doc/object-model/index.md#thrift-uri)
+ * While it may seem like typedefs correspond to
+ * [Opaque Alias Types](https://github.com/facebook/fbthrift/blob/main/thrift/doc/object-model/index.md#opaque-alias-types),
+ * that is actually incorrect, as the "aliased" type that a typedef introduces
+ * is considered identical - at the Object Model level - to the original type.
+ *
+ * This annotation is introduced to allow "grandfathering in" existing typedef
+ * URIs in preparation for the thrift compiler to reject such cases in the
+ * future (unless this annotation is specified).
+ *
+ * This annoation MUST NOT be applied to a typedef for which no URI is
+ * specified (either explicitly via @thrift.Uri, or implicitly through a
+ * non-empty
+ * [package declaration](https://github.com/facebook/fbthrift/blob/main/thrift/doc/idl/index.md#package-declaration)).
+ */
+@scope.Typedef
+struct AllowLegacyTypedefUri {}
+
+/**
+ * Allows the target field of a structured user-defined type (i.e., struct,
+ * union or exception), whose qualifier is `optional`, to have a custom default
+ * value specified in IDL.
+ *
+ * Use of this annotation is strongly DISCOURAGED, as custom default values for
+ * optional fields are both non-sensical and dangerous:
+ *   - non-sensical because, by definition, the "default" state of an optional
+ *     field is to have no value (i.e., be "absent") - as explicitly specified
+ *     in the [Thrift Object Model](https://github.com/facebook/fbthrift/blob/main/thrift/doc/object-model/index.md#structured-types).
+ *   - dangerous because in practice, the runtime behavior of the generated code
+ *     for optional fields with custom default values is inconsistent (sometimes
+ *     even for the same programming language!).
+ *
+ * This annotation is merely introduced to allow existing use cases to be
+ * grandfathered into the new compiler validation logic, which will reject
+ * optional fields with custom default values unless this annotation is
+ * specified.
+ *
+ * This annotation MUST NOT be applied to a field whose qualifier is not
+ * optional, or that doesn't have a custom default value.
+ */
+@scope.Field
+struct AllowUnsafeOptionalCustomDefaultValue {}