`Reborrow` unsoundness due to not checking for `Copy`

This unsoundness was discovered by @coolcatcoder. I'm filing this issue on their behalf.

The following code causes a double free. It should not compile.

#![feature(reborrow)]

#[expect(dead_code)]
// This code correctly gives a compile error if &'a mut () is changed to &'a ()
struct Wrap<'a>(&'a mut (), String);

impl<'a> std::marker::Reborrow for Wrap<'a> {}

fn get(x: Wrap<'_>) -> String {
    x.1
}

fn main() {
    let x = Wrap(&mut (), String::from("a"));
    let _y = get(x);
    let _z = get(x);
}

cc @aapoalas

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Reproducible on the playground with version 1.97.0-nightly (2026-05-08 fb0a5a5a9c892b351f34)

Start by reproducing the provided example with Rust nightly 1.97.0 on the playground, then trace the Reborrow feature implementation and its handling of Copy checks. Done means the example is rejected by the compiler and no longer permits the reported double-free behavior.