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next-solver hang: infinite recursion during hidden type inference for TAIT + unconstrained region #151806
Description
I tried this code:（auto-reduced）
#![feature(type_alias_impl_trait)]
trait UnwrapItemsExt {
type Iter;
}
trait MyTrait<'a> {}
impl<'a> MyTrait<'a> for <() as UnwrapItemsExt>::Iter {}
impl<'a, I> UnwrapItemsExt for I {
type Iter = impl MyTrait<'a>;
fn unwrap_items() -> Self::Iter {
}
}
fn main(){}
| Release channel | Result |
|---|---|
| Current Stable | error |
| Current Nightly (default solver) | error |
Current Nightly (+ -Znext-solver=globally) |
Hang |
Meta
rustc --version --verbose:
rustc 1.95.0-nightly (e96bb7e44 2026-01-27)
binary: rustc
commit-hash: e96bb7e44fbcc23c1e6009e8d0ee8ab208668fb4
commit-date: 2026-01-27
host: x86_64-unknown-linux-gnu
release: 1.95.0-nightly
LLVM version: 21.1.8
Backtrace
error[E0407]: method `unwrap_items` is not a member of trait `UnwrapItemsExt`
--> bug.rs:9:5
|
9 | / fn unwrap_items() -> Self::Iter {
10 | | }
| |_____^ not a member of trait `UnwrapItemsExt`
error[E0658]: `impl Trait` in associated types is unstable
--> bug.rs:8:17
|
8 | type Iter = impl MyTrait<'a>;
| ^^^^^^^^^^^^^^^^
|
= note: see issue #63063 <https://github.com/rust-lang/rust/issues/63063> for more information
= help: add `#![feature(impl_trait_in_assoc_type)]` to the crate attributes to enable
= note: this compiler was built on 2026-01-27; consider upgrading it if it is out of date
error[E0207]: the lifetime parameter `'a` is not constrained by the impl trait, self type, or predicates
--> bug.rs:7:6
|
7 | impl<'a, I> UnwrapItemsExt for I {
| ^^ unconstrained lifetime parameter
hang.
Activity
[-]Hang: Infinite recursion during hidden type inference for TAIT[/-][+]next-solver hang: infinite recursion during hidden type inference for TAIT[/+][-]next-solver hang: infinite recursion during hidden type inference for TAIT[/-][+]next-solver hang: infinite recursion during hidden type inference for TAIT + unconstrained infer var[/+][-]next-solver hang: infinite recursion during hidden type inference for TAIT + unconstrained infer var[/-][+]next-solver hang: infinite recursion during hidden type inference for TAIT + unconstrained region[/+]lcnr commentedon Oct 8, 2026
same root cause as rust-lang/trait-system-refactor-initiative#242