Trait resolution fails, and then `rustc` hangs.

[frankmcsherry]

I tried this code:

trait Index {
    type IndexRef;
}

trait Borrow {
    type Borrowed<'a>;
}

trait Test {
    type Ref<'a>;
    type Container: Borrow where for<'a> <Self::Container as Borrow>::Borrowed<'a> : Index<IndexRef = Self::Ref<'a>>;
}

This builds fine if <Self::Container as Borrow>::Borrowed<'a> is replaced by &'a Self::Container. However, the above both errors, and also hangs rustc (still running after many minutes; wedges playground). The error is

binding for associated type Ref references lifetime 'a, which does not appear in the trait input types

I don't understand the error, though I could imagine that it is my fault or a known limitation of GATs. However, the hanging of rustc seems likely to be a bug.

Meta

rustc --version --verbose:

rustc 1.83.0 (90b35a623 2024-11-26)

Compiler output before hang

error[E0582]: binding for associated type `IndexRef` references lifetime `'a`, which does not appear in the trait input types
  --> lib.rs:11:92
   |
11 |     type Container: Borrow where for<'a> <Self::Container as Borrow>::Borrowed<'a> : Index<IndexRef = Self::Ref<'a>>;
   |                                                                                            ^^^^^^^^^^^^^^^^^^^^^^^^

error: aborting due to 1 previous error