GAT's arent typechecked, and can cause UB

[DutchGhost]

It looks like Generic Associated Types aren't validated on whether the specified type on the impl-side indeed implements the traits as written down in the definition of the associated type.

The following program shows a use-after-free of a String:

#![feature(generic_associated_types)]
trait UnsafeCopy {
    type Item<'a>: Copy;
    
    fn copy<'a>(item: &Self::Item<'a>) -> Self::Item<'a> {
        *item
    }
}

impl <T> UnsafeCopy for T {
    type Item<'a> = T;
}

fn main() {
    let mut s = String::from("Hello world!");
    
    let copy = String::copy(&s);
    
    // Do we indeed point to the samme memory?
    assert!(s.as_ptr() == copy.as_ptr());
    
    // Any use of `copy` is certeinly UB after this
    drop(s);
    
    // UB UB UB UB UB!!
    println!("{}", copy);
}