Unconstrained lifetime in associated type using trait object leads to unsound lifetime extension and ICE · Issue #130347 · rust-lang/rust (original) (raw)

I was playing around with defining higher ranked types using trait objects and tried the following pattern.

trait A: B<T = T> {}

trait B { type T; }

To my surprise trait objects of the form dyn for<'a> A<&'a T> were allowed. After some experimentation I found that this leads to UB in safe rust.

trait A: B<T = T> {}

trait B { type T; }

struct Erase<T: ?Sized + B>(T::T);

fn main() { let x = { let x = String::from("hello");

    Erase::<dyn for<'a> A<&'a _>>(x.as_str())
};

dbg!(x.0);

}

[src/main.rs:16:5] x.0 = "\0\0\0\0\0"

This is likely related to 25860 somehow, but I haven't seen this flavor before.