"Type mismatch resolving..." only on new trait solver

[fcard]

Standalone Repro: https://rust.godbolt.org/z/8eYbPzeqq

(playground: https://play.rust-lang.org/?version=stable&mode=debug&edition=2024&gist=a806b35a71c5d31d715eebde4651f21e)

I will try to minimize it more if possible later, I already spent a good amount of hours on it for now tho, hehe.

The new trait solver has never worked with my (WIP) crate, it never finished compiling, even after hours of waiting. But when I updated the compiler today turns out that in the last few weeks that got fixed! But now I have 500+ error messages saying "type mismatch resolving ..." or "trait bound ... not satisfied". I am not sure that's a bug or a breaking change, but I thought I'd file an issue anyway.

[compiler-errors]

Can you share a link in Godbolt with -Znext-solver so the reader can see the errors w/o testing locally?

[fcard]

Sure, I edited that in, hope that works

[lcnr]

slightly further minimized, no idea what's going on yet

#![allow(unused)]
struct B;
struct C;
struct D;

pub trait Trait {
    type Output;
}
macro_rules! k {
    ($t:ty) => {
        <$t as Trait>::Output
    };
}

trait CallB<T1, T2> {
    type Output;
    type Return;
}

trait CallC<T1> {
    type Output;
    type Return;
}

trait CallD<T1, T2> {
    type Output;
}

fn foo<X, Y>()
where
    X: Trait,
    Y: Trait,
    D: CallD<k![X], k![Y]>,
    C: CallC<<D as CallD<k![X], k![Y]>>::Output>,
    <C as CallC<<D as CallD<k![X], k![Y]>>::Output>>::Output: Trait,
    B: CallB<
            <C as CallC<<D as CallD<k![X], k![Y]>>::Output>>::Return,
            <C as CallC<<D as CallD<k![X], k![Y]>>::Output>>::Output,
        >,
    <B as CallB<
        <C as CallC<<D as CallD<k![X], k![Y]>>::Output>>::Return,
        <C as CallC<<D as CallD<k![X], k![Y]>>::Output>>::Output,
    >>::Output: Trait<Output = ()>,
{
}

[lcnr]

fixed:tm:

diff --git a/compiler/rustc_type_ir/src/search_graph/mod.rs b/compiler/rustc_type_ir/src/search_graph/mod.rs
index 8f8f019510f..d9906fa0d88 100644
--- a/compiler/rustc_type_ir/src/search_graph/mod.rs
+++ b/compiler/rustc_type_ir/src/search_graph/mod.rs
@@ -686,6 +686,7 @@ pub fn finish_single_candidate(&mut self) -> CandidateHeadUsages {
     }
 
     pub fn ignore_candidate_head_usages(&mut self, usages: CandidateHeadUsages) {
+        return;
         if let Some(usages) = usages.usages {
             let (entry_index, entry) = self.stack.last_mut_with_index().unwrap();
             #[allow(rustc::potential_query_instability)]

something is wrong about the way we ignore cycle head usages :/

[lcnr]

@fcard this should have been fixed by rust-lang/rust#147061, can you please test whether the errors have been fixed? Either by using https://github.com/kennytm/rustup-toolchain-install-master or by waiting until tomorrows nightly?

[fcard]

compiles perfectly now without any errors!

[lcnr]

awesome 👍