nightly 2022-06-29 rejects previously compiling code with missing trait implementations

[hawkw]

Code

The linkerd2-proxy project fails to compile on nighty 2022-06-29 and later due to a "missing" implementation of a trait in an impl Trait return value. The same code compiles successfully on stable and on prior nightlies. The error occurs when using impl Trait as an associated type of a return value that is itself an impl Trait, when the associated type has trait bounds placed on it by the outer impl Trait.

An error like this is generated:

error[E0277]: the trait bound `impl std::marker::Send + Unpin: linkerd_app_core::linkerd_io::AsyncRead` is not satisfied
   --> linkerd/app/inbound/src/lib.rs:158:9
    |
158 | /         impl svc::MakeConnection<
159 | |                 T,
160 | |                 Connection = impl Send + Unpin,
161 | |                 Metadata = impl Send + Unpin,
162 | |                 Error = Error,
163 | |                 Future = impl Send,
164 | |             > + Clone,
    | |_____________________^ the trait `linkerd_app_core::linkerd_io::AsyncRead` is not implemented for `impl std::marker::Send + Unpin`
    |
    = help: the following other types implement trait `linkerd_app_core::linkerd_io::AsyncRead`:
              &[u8]
              &mut T
              Box<T>
              BoxedIo
              BufStream<RW>
              DuplexStream
              EitherIo<L, R>
              Pin<P>
            and 38 others
    = note: required because of the requirements on the impl of `linkerd_app_core::svc::MakeConnection<T>` for `linkerd_app_core::svc::Filter<linkerd_app_core::svc::MapErr<linkerd_app_core::svc::linkerd_stack::Timeout<ConnectTcp>, impl (FnOnce(Box<(dyn std::error::Error + Sync + std::marker::Send + 'static)>) -> Box<(dyn std::error::Error + Sync + std::marker::Send + 'static)>) + Clone>, [closure@linkerd/app/inbound/src/lib.rs:186:38: 193:18]>`

error[E0277]: the trait bound `impl std::marker::Send + Unpin: linkerd_app_core::linkerd_io::AsyncWrite` is not satisfied
   --> linkerd/app/inbound/src/lib.rs:158:9
    |
158 | /         impl svc::MakeConnection<
159 | |                 T,
160 | |                 Connection = impl Send + Unpin,
161 | |                 Metadata = impl Send + Unpin,
162 | |                 Error = Error,
163 | |                 Future = impl Send,
164 | |             > + Clone,
    | |_____________________^ the trait `linkerd_app_core::linkerd_io::AsyncWrite` is not implemented for `impl std::marker::Send + Unpin`
    |
    = help: the following other types implement trait `linkerd_app_core::linkerd_io::AsyncWrite`:
              &mut T
              Box<T>
              BoxedIo
              BufStream<RW>
              DuplexStream
              EitherIo<L, R>
              Pin<P>
              PrefixedIo<I>
            and 38 others
    = note: required because of the requirements on the impl of `linkerd_app_core::svc::MakeConnection<T>` for `linkerd_app_core::svc::Filter<linkerd_app_core::svc::MapErr<linkerd_app_core::svc::linkerd_stack::Timeout<ConnectTcp>, impl (FnOnce(Box<(dyn std::error::Error + Sync + std::marker::Send + 'static)>) -> Box<(dyn std::error::Error + Sync + std::marker::Send + 'static)>) + Clone>, [closure@linkerd/app/inbound/src/lib.rs:186:38: 193:18]>`

error[E0277]: the trait bound `impl std::marker::Send + Unpin: linkerd_app_core::linkerd_io::AsyncRead` is not satisfied
  --> linkerd/app/outbound/src/tcp/connect.rs:39:9
   |
39 | /         impl svc::MakeConnection<
40 | |                 T,
41 | |                 Connection = impl Send + Unpin,
42 | |                 Metadata = ConnectMeta,
43 | |                 Error = Error,
44 | |                 Future = impl Send,
45 | |             > + Clone,
   | |_____________________^ the trait `linkerd_app_core::linkerd_io::AsyncRead` is not implemented for `impl std::marker::Send + Unpin`
   |
   = help: the following other types implement trait `linkerd_app_core::linkerd_io::AsyncRead`:
             &[u8]
             &mut T
             Box<T>
             BoxedIo
             DuplexStream
             EitherIo<L, R>
             Pin<P>
             PrefixedIo<I>
           and 38 others
   = note: required because of the requirements on the impl of `MakeConnection<T>` for `linkerd_app_core::transport::linkerd_transport_metrics::Client<linkerd_app_core::transport::Metrics, BoxFuture<linkerd_app_core::svc::MapErr<linkerd_app_core::svc::linkerd_stack::Timeout<OpaqueTransport<linkerd_app_core::linkerd_tls::Client<linkerd_app_core::identity::NewClient, C>>>, impl (FnOnce(Box<(dyn std::error::Error + Sync + std::marker::Send + 'static)>) -> Box<(dyn std::error::Error + Sync + std::marker::Send + 'static)>) + Clone>>>`

error[E0277]: the trait bound `impl std::marker::Send + Unpin: linkerd_app_core::linkerd_io::AsyncWrite` is not satisfied
  --> linkerd/app/outbound/src/tcp/connect.rs:39:9
   |
39 | /         impl svc::MakeConnection<
40 | |                 T,
41 | |                 Connection = impl Send + Unpin,
42 | |                 Metadata = ConnectMeta,
43 | |                 Error = Error,
44 | |                 Future = impl Send,
45 | |             > + Clone,
   | |_____________________^ the trait `linkerd_app_core::linkerd_io::AsyncWrite` is not implemented for `impl std::marker::Send + Unpin`
   |
   = help: the following other types implement trait `linkerd_app_core::linkerd_io::AsyncWrite`:
             &mut T
             Box<T>
             BoxedIo
             DuplexStream
             EitherIo<L, R>
             Pin<P>
             PrefixedIo<I>
             ScopedIo<I>
           and 38 others
   = note: required because of the requirements on the impl of `MakeConnection<T>` for `linkerd_app_core::transport::linkerd_transport_metrics::Client<linkerd_app_core::transport::Metrics, BoxFuture<linkerd_app_core::svc::MapErr<linkerd_app_core::svc::linkerd_stack::Timeout<OpaqueTransport<linkerd_app_core::linkerd_tls::Client<linkerd_app_core::identity::NewClient, C>>>, impl (FnOnce(Box<(dyn std::error::Error + Sync + std::marker::Send + 'static)>) -> Box<(dyn std::error::Error + Sync + std::marker::Send + 'static)>) + Clone>>>`

For more information about this error, try `rustc --explain E0277`.

Unfortunately, I don't have a minimal reproducer yet; I'd like to find one, but for now, checking out linkerd/linkerd2-proxy@0288b9c and running cargo +nigihtly-2022-06-29 build will fail with this error.

Version it worked on

The most recent nightly that can compile the linkerd2-proxy codebase successfully is 2022-06-28. Successful CI build: https://github.com/linkerd/linkerd2-proxy/runs/7088482069?check_suite_focus=true

Version with regression

Nightly 2022-06-29 and later fails to compile the same codebase. Failing CI build: https://github.com/linkerd/linkerd2-proxy/runs/7107925369?check_suite_focus=true

We ran cargo bisect-rustc and it appears that commit 00ebeb8 introduced the regression.

searched nightlies: from nightly-2022-06-28 to nightly-2022-06-30
regressed nightly: nightly-2022-06-29
searched commit range: 2f3ddd9...8308806
regressed commit: 00ebeb8

bisected with cargo-bisect-rustc v0.6.3

Host triple: x86_64-unknown-linux-gnu
Reproduce with:

cargo bisect-rustc --start 2022-06-28 --end 2022-06-30 --test-dir ../linkerd2-proxy -- build 

[hawkw]

Started attempting to find a minimal reproduction; this playground does not reproduce the issue, so clearly there is something more complex involved than just returning an impl Trait as an associated type of another impl Trait...

[compiler-errors]

Guessing it's #97346

[compiler-errors]

@hawkw, are there rustdocs for linkerd2-proxy (or, specifically, one of the failing crates like linkerd2-proxy/linkerd/app/inbound)? There's quite a few types here, and those would help, but I couldn't seem to find a rustdoc-style documentation from the website. I also may just be looking in the wrong place.

edit: nvm i just built them myself.

[compiler-errors]

Soooooo, I think I got it to compile. This is not a correct soltuion, because it causes a bunch of UI tests to fail. Draft PR: #99547

This has to do with the fact that when we Equate::tys on an inference variable and an opaque type, we end up just inferring that inference variable to opaque type, instead of (what we usually do, which is) causing us to constrain the opaque type to the value of the other ty (the infer, in this case).

This means that when we later shallow_resolve a predicate that looks like <_#1t as AsyncRead> (i.e. some inferred type must implement AsyncRead), we get <impl Send + Unpin as AsyncRead> -- without the usual opaque type registering machinery taking place.

side-note: does it usually take 3 minutes to compile linkerd/app/outbound?

[oli-obk]

we end up just inferring that inference variable to opaque type, instead of (what we usually do, which is) causing us to constrain the opaque type to the value of the other ty (the infer, in this case).

That's on purpose. The only time we don't do that is for return types so that we can merge the types from all the return sites.

how did we miss this in the crater run :/

[compiler-errors]

@oli-obk linkerd isn't on crates.io i think? 🤔

[oli-obk]

we run github crates, too, and I was certain I have seen linkerd in crater reports before

[oli-obk]

{
  "name": "gridgentoo.linkerd2-proxy.f7a8ee98c63b6729e02f54a5182a9b7f56695922",
  "url": "https://github.com/gridgentoo/linkerd2-proxy/tree/f7a8ee98c63b6729e02f54a5182a9b7f56695922",
  "krate": {
    "GitHub": {
      "org": "gridgentoo",
      "name": "linkerd2-proxy",
      "sha": "f7a8ee98c63b6729e02f54a5182a9b7f56695922"
    }
  },
  "res": "build-fail",
  "runs": [
    {
      "res": "build-fail:compiler-error(unused_must_use)",
      "log": "master%2343d9f3859e0204e764161ee085a360274b5f3e9a/gh/gridgentoo.linkerd2-proxy"
    },
    {
      "res": "build-fail:compiler-error(unused_must_use)",
      "log": "try%237306d3110618f84a6a4662b9e2cc4fd0d9b02da8/gh/gridgentoo.linkerd2-proxy"
    }
  ]
}

uhm... did crater not recognize the regression because it hit a deny for a lint?

also it seems like it didn't actually run on the sub-crates like inbound (at least I can't find it in the results).

nominating for t-types discussion, as there was a regression we didn't know about.

I think what all users that hit this desire is not actually impl Send, but "an opaque type that satisfies the bounds of the associated type, PLUS Send".

[compiler-errors]

f7a8ee98c63b6729e02f54a5182a9b7f56695922 seems really old

[apiraino]

WG-prioritization assigning priority (Zulip discussion).

@rustbot label -I-prioritize +P-high

[oli-obk]

Let's revert for now, this seems more useful in practice than we thought

[olix0r]

side-note: does it usually take 3 minutes to compile linkerd/app/outbound?

yes 🥲

[LucioFranco]

Btw starting to see this in beta now as well. https://github.com/tower-rs/tower/runs/7789919663?check_suite_focus=true

[compiler-errors]

Thanks, yeah, there is a revert PR in flight but the author is on vacation.

In the mean time, I've beta-backport nominated the revert PR and that'll likely get approved, fixing this in a few more weeks.

Sorry for the delay.

[LucioFranco]

No problem! Thanks for the quick response 😄