r? @scottmcm

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Punting on pin,
@rustbot reroll

Sure, though this isn't intended to compile anyway...
@bors r+ rollup

📌 Commit 39b08b6 has been approved by joboet

It is now in the queue for this repository.

this isn't intended to compile anyway...

Only because get_pin_mut doesn't exist. It compiles without that line.

Also, I figured the argument to exploit_ref_cell should be mut.

This compiles:

use std::cell::RefCell;
use std::pin::Pin;

trait RefCellPinExt<'a, T> {
    fn get_pin_mut(self) -> Pin<&'a mut T>;
}

impl<'a, T> RefCellPinExt<'a, T> for Pin<&'a mut RefCell<T>> {
    fn get_pin_mut(self) -> Pin<&'a mut T> {
        // SAFETY: That's actually unsafe. Don't do this in real code!
        unsafe { self.map_unchecked_mut(|rc| rc.get_mut()) }
    }
}

fn exploit_ref_cell<T>(mut rc: Pin<&mut RefCell<T>>) {
    // Here we get pinned access to the `T`.
    let _: Pin<&mut T> = rc.as_mut().get_pin_mut();

    // And here we have `&mut T` to the same data.
    let shared: &RefCell<T> = rc.into_ref().get_ref();
    let mut borrow = shared.borrow_mut();
    let content = &mut *borrow;
}

https://play.rust-lang.org/?version=stable&mode=debug&edition=2024&gist=e54a0a6082785b63359650b128f15071

Maybe we better replace it with this version? It's longer, but the compiler can check that it compiles. It's incorrect and unsafe, but having it compile makes sure the example is not bogus.

⚠️ A new commit 2a68b2ec8a6735435fe91af086b0f32042540151 was pushed to the branch, the PR will need to be re-approved.

Updated to code that compiles, LMK WDYT