Two conflicting concrete types for the same opaque type

[Nashenas88]

I tried this code:

https://cs.opensource.google/fuchsia/fuchsia/+/main:src/connectivity/network/netcfg/src/lib.rs;l=1762;drc=4a7d2eb6793e61c0021f8dd7dab35804590d36b6
The function beginning with the signature:

async fn create_device_stream(
        &self,
    ) -> Result<
        impl futures::Stream<Item = Result<devices::NetworkDeviceInstance, anyhow::Error>>,
        anyhow::Error,
    > { ... }

I expected to see this happen: Compiles successfully, as it did on previous releases.

Instead, this happened:

error: concrete type differs from previous defining opaque type use
    --> ../../src/connectivity/network/netcfg/src/lib.rs:1765:9
     |
1765 |         impl futures::Stream<Item = Result<devices::NetworkDeviceInstance, anyhow::Error>>,
     |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
     |         |
     |         expected `TryFlattenUnordered<futures::stream::Fuse<TryFilterMap<futures::stream::ErrInto<Watcher, anyhow::Error>, [async block@../../src/connectivity/network/netcfg/src/lib.rs:1782:17: 1843:18], [closure@../../src/connectivity/network/netcfg/src/lib.rs:1780:29: 1780:82]>>>`, got `TryFlattenUnordered<futures::stream::Fuse<TryFilterMap<futures::stream::ErrInto<Watcher, anyhow::Error>, [async block@../../src/connectivity/network/netcfg/src/lib.rs:1782:17: 1843:18], [closure@../../src/connectivity/network/netcfg/src/lib.rs:1780:29: 1780:82]>>>`
     |         this expression supplies two conflicting concrete types for the same opaque type
error: aborting due to previous error

Recompiling with -Zverbose outputs:

error: concrete type differs from previous defining opaque type use
    --> ../../src/connectivity/network/netcfg/src/lib.rs:1765:9
     |
1765 |         impl futures::Stream<Item = Result<devices::NetworkDeviceInstance, anyhow::Error>>,
     |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
     |         |
     |         expected `TryFlattenUnordered<futures::stream::Fuse<TryFilterMap<futures::stream::ErrInto<Watcher, anyhow::Error>, [static NetCfg<ReEarlyBound(DefId(0:3068 ~ netcfg[7715]::{impl#13}::create_device_stream::{opaque#0}::'a), 2, 'a)>::create_device_stream::{closure#0}::{closure#2}::{closure#0} upvar_tys=(fuchsia_fs::directory::WatchEvent, PathBuf, InstallerProxy) for<Region(BrAnon(None)), Region(BrAnon(None)), Region(BrAnon(None)), Region(BrAnon(None))> {ResumeTy, fuchsia_fs::directory::WatchEvent, PathBuf, InstallerProxy, &ReLateBound(DebruijnIndex(0), BoundRegion { var: 0, kind: BrAnon(None) }) InstallerProxy, &ReLateBound(DebruijnIndex(0), BoundRegion { var: 1, kind: BrAnon(None) }) PathBuf, Opaque(DefId(0:2630 ~ netcfg[7715]::devices::{impl#4}::get_instance_stream::{opaque#0}), [ReLateBound(DebruijnIndex(0), BoundRegion { var: 2, kind: BrAnon(None) }), ReLateBound(DebruijnIndex(0), BoundRegion { var: 3, kind: BrAnon(None) })]), ()}], [NetCfg<ReEarlyBound(DefId(0:3068 ~ netcfg[7715]::{impl#13}::create_device_stream::{opaque#0}::'a), 2, 'a)>::create_device_stream::{closure#0}::{closure#2} closure_kind_ty=i16 closure_sig_as_fn_ptr_ty=extern "rust-call" fn((WatchMessage,)) -> [static NetCfg<ReEarlyBound(DefId(0:3068 ~ netcfg[7715]::{impl#13}::create_device_stream::{opaque#0}::'a), 2, 'a)>::create_device_stream::{closure#0}::{closure#2}::{closure#0} upvar_tys=(fuchsia_fs::directory::WatchEvent, PathBuf, InstallerProxy) for<Region(BrAnon(None)), Region(BrAnon(None)), Region(BrAnon(None)), Region(BrAnon(None))> {ResumeTy, fuchsia_fs::directory::WatchEvent, PathBuf, InstallerProxy, &ReLateBound(DebruijnIndex(0), BoundRegion { var: 0, kind: BrAnon(None) }) InstallerProxy, &ReLateBound(DebruijnIndex(0), BoundRegion { var: 1, kind: BrAnon(None) }) PathBuf, Opaque(DefId(0:2630 ~ netcfg[7715]::devices::{impl#4}::get_instance_stream::{opaque#0}), [ReLateBound(DebruijnIndex(0), BoundRegion { var: 2, kind: BrAnon(None) }), ReLateBound(DebruijnIndex(0), BoundRegion { var: 3, kind: BrAnon(None) })]), ()}] upvar_tys=(InstallerProxy)]>>>`, got `TryFlattenUnordered<futures::stream::Fuse<TryFilterMap<futures::stream::ErrInto<Watcher, anyhow::Error>, [static NetCfg<ReEarlyBound(DefId(0:626 ~ netcfg[7715]::{impl#13}::'a), 0, 'a)>::create_device_stream::{closure#0}::{closure#2}::{closure#0} upvar_tys=(fuchsia_fs::directory::WatchEvent, PathBuf, InstallerProxy) for<Region(BrAnon(None)), Region(BrAnon(None)), Region(BrAnon(None)), Region(BrAnon(None))> {ResumeTy, fuchsia_fs::directory::WatchEvent, PathBuf, InstallerProxy, &ReLateBound(DebruijnIndex(0), BoundRegion { var: 0, kind: BrAnon(None) }) InstallerProxy, &ReLateBound(DebruijnIndex(0), BoundRegion { var: 1, kind: BrAnon(None) }) PathBuf, Opaque(DefId(0:2630 ~ netcfg[7715]::devices::{impl#4}::get_instance_stream::{opaque#0}), [ReLateBound(DebruijnIndex(0), BoundRegion { var: 2, kind: BrAnon(None) }), ReLateBound(DebruijnIndex(0), BoundRegion { var: 3, kind: BrAnon(None) })]), ()}], [NetCfg<ReEarlyBound(DefId(0:626 ~ netcfg[7715]::{impl#13}::'a), 0, 'a)>::create_device_stream::{closure#0}::{closure#2} closure_kind_ty=i16 closure_sig_as_fn_ptr_ty=extern "rust-call" fn((WatchMessage,)) -> [static NetCfg<ReEarlyBound(DefId(0:626 ~ netcfg[7715]::{impl#13}::'a), 0, 'a)>::create_device_stream::{closure#0}::{closure#2}::{closure#0} upvar_tys=(fuchsia_fs::directory::WatchEvent, PathBuf, InstallerProxy) for<Region(BrAnon(None)), Region(BrAnon(None)), Region(BrAnon(None)), Region(BrAnon(None))> {ResumeTy, fuchsia_fs::directory::WatchEvent, PathBuf, InstallerProxy, &ReLateBound(DebruijnIndex(0), BoundRegion { var: 0, kind: BrAnon(None) }) InstallerProxy, &ReLateBound(DebruijnIndex(0), BoundRegion { var: 1, kind: BrAnon(None) }) PathBuf, Opaque(DefId(0:2630 ~ netcfg[7715]::devices::{impl#4}::get_instance_stream::{opaque#0}), [ReLateBound(DebruijnIndex(0), BoundRegion { var: 2, kind: BrAnon(None) }), ReLateBound(DebruijnIndex(0), BoundRegion { var: 3, kind: BrAnon(None) })]), ()}] upvar_tys=(InstallerProxy)]>>>`
     |         this expression supplies two conflicting concrete types for the same opaque type

error: aborting due to previous error

Which when diffed, shows a change only from instances of
NetCfg<ReEarlyBound(DefId(0:3068 ~ netcfg[7715]::{impl#13}::create_device_stream::{opaque#0}::'a), 2, 'a)> to
NetCfg<ReEarlyBound(DefId(0:626 ~ netcfg[7715]::{impl#13}::'a), 0, 'a)>.

Meta

Error output is from rustc commit 139b49b
Likely related to #113661, CC @oli-obk @lcnr

[Nashenas88]

I'm working to find a minimized test-case.

[compiler-errors]

Probably fixed by #114574.

[Nashenas88]

I was able to minimize (see on Playground):

use futures; // 0.3.28
use futures::{Stream, StreamExt as _, TryStreamExt as _};
use std::pin::Pin;
use std::task::{Context, Poll};

struct A<'a> {
    dat: &'a (),
}

struct Streamer;

impl Streamer {
    async fn new() -> Self {
        Self
    }
}

impl Stream for Streamer {
    type Item = Result<(), ()>;

    fn poll_next(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
        todo!()
    }
}

struct B;

impl B {
    async fn get_stream() -> impl Stream<Item = Result<Self, ()>> {
        futures::stream::try_unfold((), |()| async move { Ok(Some((B, ()))) })
    }
}

impl<'a> A<'a> {
    async fn a(&self) -> Result<impl Stream<Item=Result<B, ()>>, ()> {
        let stream_of_streams = Streamer::new().await.err_into::<()>().try_filter_map(move |()| {
            async move {
                let stream = B::get_stream().await;
                Ok(Some(stream.filter_map(move |r| {
                    futures::future::ready(match r {
                        Ok(b) => Some(Ok(b)),
                        _ => None,
                    })
                }).boxed()
                ))
            }
        }).fuse().try_flatten_unordered(1);
        Ok(stream_of_streams)
    }
}

Gives:

error: concrete type differs from previous defining opaque type use
  --> src/lib.rs:35:33
   |
35 |     async fn a(&self) -> Result<impl Stream<Item=Result<B, ()>>, ()> {
   |                                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |                                 |
   |                                 expected `TryFlattenUnordered<futures::stream::Fuse<TryFilterMap<futures::stream::ErrInto<Streamer, ()>, [async block@src/lib.rs:37:13: 46:14], [closure@src/lib.rs:36:87: 36:96]>>>`, got `TryFlattenUnordered<futures::stream::Fuse<TryFilterMap<futures::stream::ErrInto<Streamer, ()>, [async block@src/lib.rs:37:13: 46:14], [closure@src/lib.rs:36:87: 36:96]>>>`
   |                                 this expression supplies two conflicting concrete types for the same opaque type

error: could not compile `playground` (lib) due to previous error

With -Zverbose:

error: concrete type differs from previous defining opaque type use
  --> ../../src/connectivity/network/netcfg/src/lib2.rs:35:33
   |
35 |     async fn a(&self) -> Result<impl Stream<Item = Result<B, ()>>, ()> {
   |                                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |                                 |
   |                                 expected `TryFlattenUnordered<futures::stream::Fuse<TryFilterMap<futures::stream::ErrInto<Streamer, ()>, [static A<ReEarlyBound(DefId(0:47 ~ lib2[3a14]::{impl#3}::a::{opaque#0}::'a), 2, 'a)>::a::{closure#0}::{closure#0}::{closure#0} upvar_tys=() {ResumeTy, Opaque(DefId(0:42 ~ lib2[3a14]::{impl#2}::get_stream::{opaque#0}), []), ()}], [A<ReEarlyBound(DefId(0:47 ~ lib2[3a14]::{impl#3}::a::{opaque#0}::'a), 2, 'a)>::a::{closure#0}::{closure#0} closure_kind_ty=i16 closure_sig_as_fn_ptr_ty=extern "rust-call" fn(((),)) -> [static A<ReEarlyBound(DefId(0:47 ~ lib2[3a14]::{impl#3}::a::{opaque#0}::'a), 2, 'a)>::a::{closure#0}::{closure#0}::{closure#0} upvar_tys=() {ResumeTy, Opaque(DefId(0:42 ~ lib2[3a14]::{impl#2}::get_stream::{opaque#0}), []), ()}] upvar_tys=()]>>>`, got `TryFlattenUnordered<futures::stream::Fuse<TryFilterMap<futures::stream::ErrInto<Streamer, ()>, [static A<ReEarlyBound(DefId(0:32 ~ lib2[3a14]::{impl#3}::'a), 0, 'a)>::a::{closure#0}::{closure#0}::{closure#0} upvar_tys=() {ResumeTy, Opaque(DefId(0:42 ~ lib2[3a14]::{impl#2}::get_stream::{opaque#0}), []), ()}], [A<ReEarlyBound(DefId(0:32 ~ lib2[3a14]::{impl#3}::'a), 0, 'a)>::a::{closure#0}::{closure#0} closure_kind_ty=i16 closure_sig_as_fn_ptr_ty=extern "rust-call" fn(((),)) -> [static A<ReEarlyBound(DefId(0:32 ~ lib2[3a14]::{impl#3}::'a), 0, 'a)>::a::{closure#0}::{closure#0}::{closure#0} upvar_tys=() {ResumeTy, Opaque(DefId(0:42 ~ lib2[3a14]::{impl#2}::get_stream::{opaque#0}), []), ()}] upvar_tys=()]>>>`
   |                                 this expression supplies two conflicting concrete types for the same opaque type

error: aborting due to previous error; 1 warning emitted

Main diff of:
A<ReEarlyBound(DefId(0:47 ~ lib2[3a14]::{impl#3}::a::{opaque#0}::'a), 2, 'a)> vs
A<ReEarlyBound(DefId(0:32 ~ lib2[3a14]::{impl#3}::'a), 0, 'a)>

[compiler-errors]

Ideally this would be minimized to not rely on any external crates, but I can at least test that locally.

[Nashenas88]

I was able to minimize this further (See on Playground):

use futures::{Stream, StreamExt as _};

struct A<'a> {
    dat: &'a (),
}

impl<'a> A<'a> {
    async fn a(&self) -> impl Stream<Item = ()> {
        futures::stream::repeat(())
            .map(|()| futures::stream::repeat(()))
            .flatten_unordered(None)
    }
}

Verbose:

error: concrete type differs from previous defining opaque type use
 --> src/lib.rs:9:26
  |
9 |     async fn a(&self) -> impl Stream<Item = ()> {
  |                          ^^^^^^^^^^^^^^^^^^^^^^
  |                          |
  |                          expected `stream::stream::flatten_unordered::FlattenUnorderedWithFlowController<futures::stream::Map<futures::stream::Repeat<()>, [A<ReEarlyBound(DefId(0:18 ~ minim[be95]::{impl#0}::a::{opaque#0}::'a), 2, 'a)>::a::{closure#0}::{closure#0} closure_kind_ty=i16 closure_sig_as_fn_ptr_ty=extern "rust-call" fn(((),)) -> futures::stream::Repeat<()> upvar_tys=()]>, ()>`, got `stream::stream::flatten_unordered::FlattenUnorderedWithFlowController<futures::stream::Map<futures::stream::Repeat<()>, [A<ReEarlyBound(DefId(0:11 ~ minim[be95]::{impl#0}::'a), 0, 'a)>::a::{closure#0}::{closure#0} closure_kind_ty=i16 closure_sig_as_fn_ptr_ty=extern "rust-call" fn(((),)) -> futures::stream::Repeat<()> upvar_tys=()]>, ()>`
  |                          this expression supplies two conflicting concrete types for the same opaque type