Program Hangs when Using Associated Types in Structs with Trait Bounds

[iamanonymouscs]

I tried this code（reduced）:

//...
trait Trait {
    type AssTrait;
}
//...
struct St<'a, T: Trait> {
    _var1: Box<&'a T>,
    _var2: Box<St<'a, T::AssTrait>>,
}

fn main() {
//...
}

When attempting to compile the provided code, the program hangs indefinitely, resulting in a non-responsive state. This issue occurs when using associated types in structs with trait bounds.

😃I expected to see this happen: The code should compile successfully or produce a clear error message indicating the cause of the issue.

😥Instead, this happened: The compilation process hangs indefinitely, and the program does not execute.

Meta

rustc --version --verbose:

rustc 1.72.0 (5680fa18f 2023-08-23)
binary: rustc
commit-hash: 5680fa18feaa87f3ff04063800aec256c3d4b4be
commit-date: 2023-08-23
host: x86_64-unknown-linux-gnu
release: 1.72.0
LLVM version: 16.0.5

We have attempted to use the BACKTRACE=1command to obtain a backtrace for debugging purposes😢, but no output is generated. It is worth mentioning that the provided code snippet causes a hang issue on both the latest stable and nightly(nightly-x86_64-unknown-linux-gnu unchanged - rustc 1.74.0-nightly (59a8294 2023-08-30)) versions of Rust.

What‘s more,when we change _var2: Box<St<'a, T::AssTrait>> to _var2: Box<St<'a, T>>, the program will produce the correct output

Backtrace

<backtrace>

[fmease]

Hangs in the current version of the next solver, too. Haven't looked for potential duplicates of this issue yet.

@rustbot label -needs-triage T-compiler I-hang A-traits A-associated-items

[fmease]

Inference of implied outlives-bounds might be the culprit, it seems to loop indefinitely on

15ms DEBUG rustc_hir_analysis::outlives::implicit_infer check_explicit_predicates(def_id=DefId(0:5 ~ e[892c]::St2), args=['a/#0, Alias(Projection, AliasTy { args: [T/#1], def_id: DefId(0:4 ~ e[892c]::Trait::AssTrait), .. })]

[fmease]

Potential duplicate of #102966.