Any::downcast_ref fails on closures across crates

[willcrichton]

Minimal example: https://github.com/willcrichton/issue-34758

If I have a create a value of type Box<Any> which is an upcasted version of a closure (e.g. Fn()) in one crate and then downcast it with the appropriate type in another crate, it fails. For example, if I have the following library in one crate:

use std::any::Any;

pub fn alloc<T: Any>(t: T) -> Box<Any> {
    Box::new(t)
}

pub fn unwrap<T: Any>(x: Box<Any>) {
    x.downcast_ref::<T>().unwrap();
}

pub fn unwrap_i32(x: Box<Any>) {
    x.downcast_ref::<i32>().unwrap();
}

pub fn unwrap_closure(x: Box<Any>) {
    x.downcast_ref::<Box<Fn()>>().unwrap();
}

And then a binary that uses the library in another crate:

extern crate lib;

mod lib_incrate {
    use std::any::Any;

    pub fn alloc<T: Any>(t: T) -> Box<Any> {
        Box::new(t)
    }

    pub fn unwrap_closure(x: Box<Any>) {
        x.downcast_ref::<Box<Fn()>>().unwrap();
    }
}

fn main() {
    let x: i32 = 1i32;
    lib::unwrap::<i32>(lib::alloc(x));

    let x: i32 = 1i32;
    lib::unwrap_i32(lib::alloc(x));

    let x: Box<Fn()> = Box::new(||{});
    lib::unwrap::<Box<Fn()>>(lib::alloc(x));

    let x: Box<Fn()> = Box::new(||{});
    lib_incrate::unwrap_closure(lib_incrate::alloc(x));

    println!("Fails here?");
    let x: Box<Fn()> = Box::new(||{});
    lib::unwrap_closure(lib::alloc(x));
}

Then it fails where noted (i.e. at the unwrap).

[willcrichton]

Never mind, just a duplicate of 33703.