Further fixes for the Serializable type

- Fixed Serializable to properly resolve structured composite types.
- Fixed missing generic types in Serializable usages.
- Fixed issue where interfaces were not being stubified.

types/defines/rpc.d.ts

@@ -32,6 +32,9 @@ declare namespace Rpc {
   export type Stubable = RpcTargetBranded | ((...args: any[]) => any);
 
   // Types that can be passed over RPC
+  // The reason for using a generic type here is to build a serializable subset of structured
+  //   cloneable composite types. This allows types defined with the "interface" keyword to pass the
+  //   serializable check as well. Otherwise, only types defined with the "type" keyword would pass.
   type Serializable<T> =
     // Structured cloneables
     | void
@@ -48,9 +51,12 @@ declare namespace Rpc {
     | Error
     | RegExp
     // Structured cloneable composites
-    | Map<Serializable<T>, Serializable<T>>
-    | Set<Serializable<T>>
-    | ReadonlyArray<Serializable<T>>
+    | Map<
+        T extends Map<infer U, unknown> ? Serializable<U> : never,
+        T extends Map<unknown, infer U> ? Serializable<U> : never
+      >
+    | Set<T extends Set<infer U> ? Serializable<U> : never>
+    | ReadonlyArray<T extends ReadonlyArray<infer U> ? Serializable<U> : never>
     | {
         [K in keyof T]: K extends number | string ? Serializable<T[K]> : never;
       }
@@ -80,7 +86,8 @@ declare namespace Rpc {
     : T extends Set<infer V> ? Set<Stubify<V>>
     : T extends Array<infer V> ? Array<Stubify<V>>
     : T extends ReadonlyArray<infer V> ? ReadonlyArray<Stubify<V>>
-    : T extends { [key: string | number]: unknown } ? { [K in keyof T]: Stubify<T[K]> }
+    // When using "unknown" instead of "any", interfaces are not stubified.
+    : T extends { [key: string | number]: any } ? { [K in keyof T]: Stubify<T[K]> }
     : T;
 
   // Recursively rewrite all `Stub<T>`s with the corresponding `T`s.
@@ -227,7 +234,7 @@ declare module "cloudflare:workers" {
   };
 
   export type WorkflowStep = {
-    do: <T extends Rpc.Serializable>(
+    do: <T extends Rpc.Serializable<T>>(
       name: string,
       callback: () => Promise<T>,
       config?: WorkflowStepConfig
@@ -240,7 +247,7 @@ declare module "cloudflare:workers" {
 
   export abstract class Workflow<
     Env = unknown,
-    T extends Rpc.Serializable | unknown = unknown,
+    T extends Rpc.Serializable<T> | unknown = unknown,
   > implements Rpc.WorkflowBranded
   {
     [Rpc.__WORKFLOW_BRAND]: never;