rustc hangs with the next solver

[nossie531]

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

When editing the short code described below in VSCode, it takes about 30 seconds for the tooltip contents to appear.

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.

[lcnr]

No worries, we can't (and don't) expect users to know the way rustc or r-a are implemented. rust-lang/trait-system-refactor-initiative#267 should be fixed soon-ish as @jdonszelmann is currently working on a fix

[lcnr]

Yeah, this has been fixed by #155443. Closing