rustc hangs with the next solver

[nossie531]

I’m not good at English. I’m sorry if this is hard to read.

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Informations

rust-analyzer version: 0.4.2871

rustc version: 1.95.0 (5980761 2026-04-14)

editor or extension: VSCode 1.116.0

relevant settings: all default

code snippet to reproduce: See below

Related issues

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Posts rust-lang/rust-analyzer#15478 and rust-lang/rust-analyzer#17688 address similar performance issues.

Impact size

Many users may encounter this issue.

Because the target code contains several implementations that combine the following trait and type.

ImplTarget - HasSeveralImpls

And before simplification, this was a combination of the following trait and type from rand crate.

Distribution - StandardUniform

Therefore, the scope of impact is wide, but speed up workarounds exists (See comment in below code).

Code to reproduce

use std::marker::PhantomData;

pub fn return_impl() -> impl MyTrait<Assoc = i32> {
    my_type()  // workaround: `my_type::<i32>()`
}

fn my_type<T>() -> MyType<T, ImplTarget> {
    MyType(PhantomData)
}

pub struct MyType<T, U>(PhantomData<(T, U)>);

pub struct ImplTarget;

pub trait MyTrait {
    type Assoc;
}

impl<T, U> MyTrait for MyType<T, U>
where
    U: HasSeveralImpls<T>,
{
    type Assoc = T;
}

pub trait HasSeveralImpls<T> {}

impl HasSeveralImpls<i32> for ImplTarget
{}

impl HasSeveralImpls<()> for ImplTarget
{}

impl<A> HasSeveralImpls<(A,)> for ImplTarget
where
    ImplTarget: HasSeveralImpls<A> 
{}

impl<A, B> HasSeveralImpls<(A, B)> for ImplTarget
where
    ImplTarget: HasSeveralImpls<A>,
    ImplTarget: HasSeveralImpls<B>,
{}

impl<A, B, C> HasSeveralImpls<(A, B, C)> for ImplTarget
where
    ImplTarget: HasSeveralImpls<A>,
    ImplTarget: HasSeveralImpls<B>,
    ImplTarget: HasSeveralImpls<C>,
{}

impl<A, B, C, D> HasSeveralImpls<(A, B, C, D)> for ImplTarget
where
    ImplTarget: HasSeveralImpls<A>,
    ImplTarget: HasSeveralImpls<B>,
    ImplTarget: HasSeveralImpls<C>,
    ImplTarget: HasSeveralImpls<D>,
{}

[ChayimFriedman2]

This hangs for rustc with the next trait solver as well, so it's not a rust-analyzer bug but a solver bug.

[theemathas]

This seems similar to rust-lang/trait-system-refactor-initiative#267

cc @lcnr

[nossie531]

@ChayimFriedman2

not a rust-analyzer bug but a solver bug

I got mistaken because rust-analyzer’s CPU and memory usage had exploded. Also, since I haven’t enabled next-solver, I never thought that could be the cause (So that explains it—I’ve just realized that rust-analyzer shares code with rustc and enables next-solver internally). In any case, I’m sorry for the misunderstanding.