auto trait leakage can be used to leak arbitrary types

When leaking the hidden type of opaques when proving auto traits, we can leak their hidden type to the caller by relying on type inference. See https://github.com/lcnr/random-rust-snippets/issues/13 for minimized examples.

Note that this allows us to access foreign closures during typeck/borrowck. This very easily results in ICE, as this code expects all encountered closures to be local:

// crate dep
struct WaddupGamers<T, U>(Option<T>, U);
impl<T: Leak<Assoc = U>, U> Unpin for WaddupGamers<T, U> {}
pub trait Leak {
    type Assoc;
}
impl<T> Leak for T {
    type Assoc = T;
}
pub fn define<T>() -> impl Sized {
    WaddupGamers(None::<T>, || ())
}

// root
#![feature(type_alias_impl_trait)]
#![allow(unused)]
#![crate_type = "rlib"]

use dep::*;
fn require_auto<T: Unpin>(x: T) -> T { x }
type NameMe<T> = impl Sized;
fn leak<T>() -> NameMe<T>
where 
    T: Leak<Assoc = NameMe<T>>,
{
    // Proving `impl Sized: Unpin` constrains `NameMe<T>` to
    // the closure of `define`.
    let opaque = require_auto(define::<T>());
    let closure;
    loop {}
    return closure; // This constrains this infer var to that closure
}

results in

thread 'rustc' panicked at compiler/rustc_hir_typeck/src/coercion.rs:1202:62:
DefId::expect_local: `DefId(20:20 ~ dep[6a51]::define::{closure#0})` isn't local

https://github.com/rust-lang/rust/blob/fcc1615e47100b376d0a6166faccdd4a8253c314/compiler/rustc_hir_typeck/src/coercion.rs#L1202

There are a lot of such uses, this was simply the first one i've triggered.