Reborrow unsoundness due to not checking for Copy

@coolcatcoder

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)

Below is a related unsoundness which I independently discovered. This code creates aliasing &muts, which should not be allowed.

#![feature(reborrow)]

#[expect(dead_code)]
// This works even if the field is &'a (), not &'a mut ()
struct Wrap<'a, T>(&'a (), T);
impl<'a> std::marker::Reborrow for Wrap<'a, &'static mut i32> {}

fn get<'a, 'b>(x: Wrap<'a, &'b mut i32>) -> &'b mut i32 {
    x.1
}

fn main() {
    let mut x = 1;
    let thing = Wrap(&(), &mut x);
    let y = get(thing);
    let z = get(thing);
    *y = 2;
    *z = 3;
    *y = 4;
    *z = 5;
    assert!(std::ptr::eq(y, z));
}

The first snippet in this issue has been fixed by #156784. The second snippet has not been fixed yet.

The second snipped no longer compiles

Rollup merge of rust-lang#164888 - yashaggarwal85d:fix-156380-reborro…

Auto merge of #165000 - JonathanBrouwer:rollup-qAF68v2, r=JonathanBro…

Rollup merge of rust-lang#164888 - yashaggarwal85d:fix-156380-reborro…

Auto merge of #165002 - JonathanBrouwer:rollup-XbLRp0m, r=JonathanBro…

Unrolled build for #164888 in rollup 165002

Rollup merge of #164888 - yashaggarwal85d:fix-156380-reborrow-test, r…

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`Reborrow` unsoundness due to not checking for `Copy` #156380

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Reborrow unsoundness due to not checking for Copy #156380

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theemathas commented on May 10, 2026

theemathas commented on May 24, 2026

Jules-Bertholet commented on Sep 30, 2026

yashaggarwal85d commented on Oct 25, 2026

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`Reborrow` unsoundness due to not checking for `Copy` #156380