Compiler hangs during execution · Issue #139197 · rust-lang/rust (original) (raw)

I tried this code:

trait Trait {}

struct W(T);

impl<T, U> Trait for W<(<&Self as IntoIterator>, W)> where W: Trait, W: Trait, { }

fn impls<T: Trait>() {}

fn main() { impls::<W<_>>(); }

I expected to see this happen:

The compilation process should complete successfully, and any errors in the code should be reported with diagnostic messages.

Instead, this happened:

The compiler hangs indefinitely and appears to never terminate.

Meta

rustc --version --verbose:

rustc 1.85.1 (4eb161250 2025-03-15)
binary: rustc
commit-hash: 4eb161250e340c8f48f66e2b929ef4a5bed7c181
commit-date: 2025-03-15
host: x86_64-pc-windows-msvc
release: 1.85.1
LLVM version: 19.1.7

Backtrace

error: expected `::`, found `,`
 --> .\temp.rs:7:48
  |
7 | impl<T, U> Trait for W<(<&Self as IntoIterator>, W<U>)>
  |                                                ^ expected `::`

error: expected `::`, found `,`
 --> .\temp.rs:7:48
  |
7 | impl<T, U> Trait for W<(<&Self as IntoIterator>, W<U>)>
  |                                                ^ expected `::`
  |
help: expressions must be enclosed in braces to be used as const generic arguments
  |
7 | impl<T, U> Trait for W<{ (<&Self as IntoIterator>, W<U>) }>
  |                        +                                 +

error[E0747]: constant provided when a type was expected
 --> .\temp.rs:7:24
  |
7 | impl<T, U> Trait for W<(<&Self as IntoIterator>, W<U>)>
  |                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^