rustc after 1.91 hangs during type inference case

If there's any more information I can give, I'm happy to investigate! Sorry for the length of the reproduction, I couldn't find anything shorter.

Code

I tried this code:

use std::marker::PhantomData;

pub trait Marker {}
impl Marker for () {}

pub trait Relation {
    type Domain: Marker;
    type Codomain: Marker;
}

pub trait Follow<R: Relation, E: Marker, const DIR: bool> {}

impl<R: Relation> Follow<R, R::Codomain, true> for R::Domain {}
impl<R: Relation> Follow<R, R::Domain, false> for R::Codomain {}

// this bound on T is somehow important?
pub trait Container<T: Marker>: IntoIterator<Item = T> {
    type WithReplacedMarker<N: Marker>: Container<N>;
    type IDContainer;
}

impl<T: Marker> Container<T> for Option<T> {
    type WithReplacedMarker<N: Marker> = Option<N>;
    type IDContainer = Option<()>;
}

struct StubQueryable<E: Marker, C: Container<E>>(PhantomData<(E, C)>);

impl<E: Marker, C: Container<E>> Queryable for StubQueryable<E, C> {
    type Output = E;
    type Container = C;
}

pub trait Queryable {
    type Output: Marker;
    type Container: Container<Self::Output>;

    fn finish(&self) -> <Self::Container as Container<Self::Output>>::IDContainer {
        todo!()
    }

    fn follow<R: Relation, E: Marker, const DIR: bool>(
        &self,
        _r: &R,
    ) -> impl Queryable<Output = E, Container = Option<E>> {
        StubQueryable::<E, Option<E>>(PhantomData)
    }
}


struct R;
impl Relation for R {
    type Domain = ();
    type Codomain = ();
}

fn run_on(q: &impl Queryable<Output = (), Container = Option<()>>) {
    // this line causes rustc to hang:
    q.follow(&R).finish();
}

I expected to see this happen: Compile error due to inability to determine the value of the DIR generic parameter.

Instead, this happened: rustc hangs in a presumably-infinite loop (at least 60 seconds instead of 40ms)

Version it worked on

It most recently worked on: 1.91.0

Version with regression

rustc --version --verbose:

rustc 1.92.0 (ded5c06cf 2025-12-08)
binary: rustc
commit-hash: ded5c06cf21d2b93bffd5d884aa6e96934ee4234
commit-date: 2025-12-08
host: x86_64-unknown-linux-gnu
release: 1.92.0
LLVM version: 21.1.3

(The same happens for 1.94, 1.96-nightly, 1.97, and 1.99-nightly.)

@rustbot

-Znext-solver=globally fixes this

error[E0599]: the method `finish` exists for associated type `impl Queryable<Container = Option<_>>`, but its trait bounds were not satisfied
  --> hang-clean.rs:32:16
   |
32 |     q.follow().finish();
   |                ^^^^^^ method cannot be called on `impl Queryable<Container = Option<_>>` due to unsatisfied trait bounds
   |
   = note: the following trait bounds were not satisfied:
           `Option<_>: Container<<impl Queryable<Container = Option<_>> as Queryable>::Output>`

reduced

trait Container<T> {
    type IDContainer;
}

impl<T> Container<T> for Option<T> {
    type IDContainer = ();
}

trait Queryable {
    type Output;
    type Container;

    fn finish(&self) -> <Self::Container as Container<Self::Output>>::IDContainer;
}

fn follow<E>() -> impl Queryable<Container = Option<E>> {
    loop {}
}

fn main() {
    follow().finish();
}

@rustbot claim

seems like duplicate for #157516

Found that the regression was introduced by #142712, kind of subtle...

Rollup merge of rust-lang#159774 - InvalidPathException:fix-old-solve…

Unrolled build for #159774

@InvalidPathException

@InvalidPathException Was #159774 intended to close this one too?

Closed by #159774

Rollup merge of #159774 - InvalidPathException:fix-old-solver-hang, r…

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rustc after 1.91 hangs during type inference case #159750

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rustc after 1.91 hangs during type inference case #159750

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InvalidPathException commented on Jul 23, 2026

InvalidPathException commented on Jul 23, 2026

InvalidPathException commented on Jul 23, 2026

ojeda commented on Jul 27, 2026

apiraino commented on Jul 27, 2026

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