Infinite loop in compiler

[wbrickner]

Minimal example code which causes infinite compile times:

https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=13fec5b370c8046012770d2e2470c934

trait Bound { type Assoc; }

struct Recursive<'a, A: Bound> {
  value: A,
  rest: &'a Option<Recursive<'a, A::Assoc>>
}

I expected to see: the compiler finish executing.

Instead, this happened: the compiler hangs with modest CPU usage, seemingly indefinitely.

As an aside, the lack of a bound on the Assoc type should raise an error (otherwise the declaration is self-contradictory), but we never even get that far. So I have removed the bound in favor of making the example as minimal as possible. To be clear, the version with the correct bound hangs as well.

Meta

I have not done a thorough search over which versions hang.

The bug exists in stable: 1.73.0, 1.74.0, and possibly more versions of stable compiler. The bug exists in beta 1.75.0-beta.3, and nightly 1.76.0-nightly, and possibly more versions of these compilers.

rustc --version --verbose:

rustc 1.73.0 (cc66ad468 2023-10-03)
binary: rustc
commit-hash: cc66ad468955717ab92600c770da8c1601a4ff33
commit-date: 2023-10-03
host: aarch64-apple-darwin
release: 1.73.0
LLVM version: 17.0.2

[veera-sivarajan]

@rustbot assign

[rustbot]

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[veera-sivarajan]

@rustbot claim

[wbrickner]

Bump as this bug still exists in 1.80 stable + beta + nightly compilers