Fix unsoundness of Debug implementation for linked_list::IterMut

[steffahn]

Fix #85813, new marker field follows the example of linked_list::Iter.

[rust-highfive]

r? @m-ou-se

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[steffahn]

I could also add some improvement to the implementation so that the actual items of the iterator are printed. Similar to how other collections seem to show the iterator’s elements in their Debug impls, too. The new implementation could look like

impl<T: fmt::Debug> fmt::Debug for IterMut<'_, T> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_tuple("IterMut")
            .field(&*mem::ManuallyDrop::new(LinkedList {
                head: self.head,
                tail: self.tail,
                len: self.len,
                marker: PhantomData,
            }))
            .field(&self.len)
            .finish()
    }
}

(and similar for Iter)
I’m only ~99% certain that this is sound, I would feel better if I could somehow run this implementation through miri, but I don’t know how to feed a custom std lib to miri.

So, if you think that this kind of impl is sound and that it’s a good idea to include it with this PR then I can add the relevant commit (b4dcdb4) to this PR.

[steffahn]

I would feel better if I could somehow run this implementation through miri

Okay, today I learned how that works, thanks @RalfJung. I’m feeling comfortable regarding soundness now, so I’ll add the commit. (It’s appearing further up in the thread because the commit itself is from yesterday, but it was on a different branch until now.)

---

The improved Debug implementation is producing much nicer output compared to the one where only the size is printed; the usage of ManuallyDrop plus a new LinkedList is a bit hacky, but it’s way simpler than the “more proper” approach of e.g. creating a new linked_list::Iter<'_> (in both the Iter and the IterMut case) and wrapping that up into a local wrapper struct that in turn implements Debug analogously to how LinkedList does by using Formatter::debug_list.

But feel free to state if a “more proper” approach is wanted during review, or if it’s better split into a separate PR, in case either is the case.

[m-ou-se]

Thanks!

@bors r+

[bors]

📌 Commit b4dcdb4 has been approved by m-ou-se

[bors]

⌛ Testing commit b4dcdb4 with merge 6a3dce9...

[bors]

☀️ Test successful - checks-actions
Approved by: m-ou-se
Pushing 6a3dce9 to master...