[Part 8/N] Interledger Stream: Futures 0.3 Transition

[gakonst]

Moving the discussion about the pending_requests removal here: #585 (comment)

[bstrie]

What changed that made these 'static?

[gakonst]

This has to do with us pushing the next.handle_request future to the PendingRequests vector. Removing the statics results in:

   --> crates/interledger-stream/src/client.rs:243:29
    |
176 | impl<S, A> SendMoneyFuture<S, A>
    |      - help: consider adding an explicit lifetime bound `S: 'static`...
...
243 |                     future: request,
    |                             ^^^^^^^
    |
note: ...so that the type `impl core::future::future::Future` will meet its required lifetime bounds
   --> crates/interledger-stream/src/client.rs:243:29
    |
243 |                     future: request,
    |                             ^^^^^^^

error[E0310]: the parameter type `A` may not live long enough
   --> crates/interledger-stream/src/client.rs:243:29
    |
176 | impl<S, A> SendMoneyFuture<S, A>
    |         - help: consider adding an explicit lifetime bound `A: 'static`...
...
243 |                     future: request,
    |                             ^^^^^^^
    |
note: ...so that the type `impl core::future::future::Future` will meet its required lifetime bounds
   --> crates/interledger-stream/src/client.rs:243:29
    |
243 |                     future: request,
    |                             ^^^^^^^

(Note that I've tried rewriting this without pin_project (ie replace self.project() with unsafe { self.get_unchecked_mut() }, and there's no lifetime requirement introduced by it.)

My intuition about this is:

1. SendMoneyFuture has a vector of pending requests.
2. Each pending request has a future associated with it, which has static lifetime
3. As a result the SendMoneyFuture struct must also live for static.
4. This means that the associated accounts and service also must live for static.

It is not clear to me why this was not required on master before though.

[bstrie]

Does this or any of the other 'static bounds cause any problems anywhere else? If not then let's not worry about it, but if so then we could try a few things. I just know that 'static has a tendency to become "infectious" and I have a lot of code yet to review, so I'm not sure if this is going to cause problems downstream.

[bstrie]

More statics whose necessity I'm unsure of.

[gakonst]

Alternatively to what I wrote above, you try doing the following:

diff --git i/crates/interledger-stream/src/client.rs w/crates/interledger-stream/src/client.rs
index 641e067e..89eafb65 100644
--- i/crates/interledger-stream/src/client.rs
+++ w/crates/interledger-stream/src/client.rs
@@ -73,8 +73,8 @@ pub async fn send_money<S, A>(
     source_amount: u64,
 ) -> Result<(StreamDelivery, S), Error>
 where
-    S: IncomingService<A> + Clone + 'static,
-    A: Account + 'static,
+    S: IncomingService<A> + Clone,
+    A: Account,
 
@@ -233,10 +231,10 @@ where
 
             // Send it!
             self.congestion_controller.prepare(amount);
-            if let Some(ref next) = self.next {
-                let mut next = next.clone();
+            if let Some(ref mut next) = self.next {
                 let from = self.from_account.clone();
                 let request = Box::pin(async move {
                     next.handle_request(IncomingRequest { from, prepare }).await

But this results in the following error, which makes sense! The Pin<Box<T>> must live for static, and we cannot guarantee that. Or can we? (note: we must make it a pinned future to poll the pending requests)

error[E0495]: cannot infer an appropriate lifetime due to conflicting requirements
   --> crates/interledger-stream/src/client.rs:234:25
    |
234 |             if let Some(ref mut next) = self.next {
    |                         ^^^^^^^^^^^^
    |
note: first, the lifetime cannot outlive the anonymous lifetime #1 defined on the method body at 183:5...
   --> crates/interledger-stream/src/client.rs:183:5
    |
183 | /     fn try_send_money(&mut self) -> Result<bool, Error> {
184 | |         let mut sent_packets = false;
185 | |         loop {
186 | |             let amount = min(
...   |
251 | |         Ok(sent_packets)
252 | |     }
    | |_____^
note: ...so that reference does not outlive borrowed content
   --> crates/interledger-stream/src/client.rs:234:25
    |
234 |             if let Some(ref mut next) = self.next {
    |                         ^^^^^^^^^^^^
    = note: but, the lifetime must be valid for the static lifetime...
    = note: ...so that the expression is assignable:
            expected std::pin::Pin<std::boxed::Box<(dyn core::future::future::Future<Output = std::result::Result<interledger_packet::packet::Fulfill, interledger_packet::packet::Reject>> + 'static)>>
               found std::pin::Pin<std::boxed::Box<dyn core::future::future::Future<Output = std::result::Result<interledger_packet::packet::Fulfill, interledger_packet::packet::Reject>>>>