calling opaque types and `Copy` closures footgun

[lcnr]

fn item_bound_is_too_weak(b: bool) -> impl FnOnce() -> u32 {
    if b {
        let mut var = item_bound_is_too_weak(false);
        println!("{}", var());
        println!("{}", var());
    }

    let mut state = 0u32;
    move || {
        state += 1;
        state
    }
}

fn main() {
    item_bound_is_too_weak(true);
}

this results in 1 1 because we unnecessarily assume the fn call to be FnOnce which then copies the closure. Should mention in the stabilization report and also... maybe lint when copying closures xd