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jrmuizel avatar jrmuizel commented on September 18, 2024

@RalfJung

from linked-list.

jrmuizel avatar jrmuizel commented on September 18, 2024

Here's a simpler test case:

        let mut m = LinkedList::new();
        m.push_back(2);
        m.push_back(3);
        assert_eq!(m.pop_front(), Some(2));

which fails with:

error[E0080]: constant evaluation error: no item to reborrow for Unique from tag 105256 found in borrow stack
    --> /Users/jrmuizel/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/src/libcore/mem.rs:1329:34
     |
1329 |         ManuallyDrop::into_inner(self.value)
     |                                  ^^^^^^^^^^ no item to reborrow for Unique from tag 105256 found in borrow stack
     |
     = note: inside call to `std::mem::MaybeUninit::<std::option::Option<std::boxed::Box<Node<i32>>>>::assume_init` at /Users/jrmuizel/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/src/libcore/ptr.rs:576:5
     = note: inside call to `std::ptr::read::<std::option::Option<std::boxed::Box<Node<i32>>>>` at /Users/jrmuizel/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/src/libcore/ptr.rs:359:17
     = note: inside call to `std::ptr::swap_nonoverlapping_one::<std::option::Option<std::boxed::Box<Node<i32>>>>` at /Users/jrmuizel/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/src/libcore/mem.rs:651:9
     = note: inside call to `std::mem::swap::<std::option::Option<std::boxed::Box<Node<i32>>>>` at /Users/jrmuizel/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/src/libcore/mem.rs:709:5
     = note: inside call to `std::mem::replace::<std::option::Option<std::boxed::Box<Node<i32>>>>` at /Users/jrmuizel/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/src/libcore/option.rs:850:9
note: inside call to `std::option::Option::<std::boxed::Box<Node<i32>>>::take` at src/lib.rs:204:9
    --> src/lib.rs:204:9
     |
204  |         self.head.take().map(|mut head| {
     |         ^^^^^^^^^^^^^^^^
note: inside call to `LinkedList::<i32>::pop_front` at src/lib.rs:877:20
    --> src/lib.rs:877:20
     |
877  |         assert_eq!(m.pop_front(), Some(2));
     |                    ^^^^^^^^^^^^^
note: inside call to `tests::test_basic` at src/lib.rs:873:5
    --> src/lib.rs:873:5
     |
873  | /     fn test_basic() {
874  | |         let mut m = LinkedList::new();
875  | |         m.push_back(2);
876  | |         m.push_back(3);
...    |
903  | |         assert_eq!(n.pop_front(), Some(1));*/
904  | |     }
     | |_____^

from linked-list.

jrmuizel avatar jrmuizel commented on September 18, 2024

A further simplification:

        let ptr: *mut Node<_>;
        let mut head;
        {
            let mut node = Box::new(Node::new(2));
            // unconditionally make the new node the new tail
            ptr = &mut *node;
            head = Some(node);
        }
        {
            let mut node = Box::new(Node::new(3));
            let tail = unsafe { &mut *(ptr as *mut Node<_>) };
            node.prev = Raw::some(tail);
            tail.next = Some(node);
        }

        head.take();

from linked-list.

jrmuizel avatar jrmuizel commented on September 18, 2024

And here's the essence of what's going on:

        let mut node = Box::new(Node::new(2));
        let mut n2 = Box::new(Node::new(3));
        let ptr: *mut Node<_> = &mut *node;

        let tail = unsafe { &mut *(ptr as *mut Node<_>) };

        let mut head = Some(node);
        tail.next = Some(n2);

If we swap the order of let mut head = Some(node); and tail.next = Some(n2); everything is fine.

It's not clear to me why miri's is complaining about this:

error[E0080]: constant evaluation error: no item to reborrow for Unique from tag 105226 found in borrow stack
   --> /Users/jrmuizel/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/src/libcore/ptr.rs:193:1
    |
193 | / unsafe fn real_drop_in_place<T: ?Sized>(to_drop: &mut T) {
194 | |     // Code here does not matter - this is replaced by the
195 | |     // real drop glue by the compiler.
196 | |     real_drop_in_place(to_drop)
197 | | }
    | |_^ no item to reborrow for Unique from tag 105226 found in borrow stack
    |
note: inside call to `std::ptr::real_drop_in_place::<std::option::Option<std::boxed::Box<Node<i32>>>> - shim(Some(std::option::Option<std::boxed::Box<Node<i32>>>))` at src/lib.rs:888:9
   --> src/lib.rs:888:9
    |
888 |         tail.next = Some(n2);
    |         ^^^^^^^^^
note: inside call to `tests::test_basic` at src/lib.rs:880:5

from linked-list.

RalfJung avatar RalfJung commented on September 18, 2024

Can you make this into a self-contained example for the playground? That'd make analysis much easier for me.

from linked-list.

jrmuizel avatar jrmuizel commented on September 18, 2024

Yep. Will do.

from linked-list.

jrmuizel avatar jrmuizel commented on September 18, 2024

@RalfJung https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=f4e643db9511b26a96692b5a6869c853

from linked-list.

RalfJung avatar RalfJung commented on September 18, 2024

node is a unique pointer (a Box). When you use it, you assert that there are no aliases that point to the same memory. If any aliases exist, they become invalid at this point.
In your program, tail is such an alias -- and hence becomes invalid.

from linked-list.

jrmuizel avatar jrmuizel commented on September 18, 2024

It seems like the use of Box in this crate basically needs to go. It looks like this happened in std's LinkedList in rust-lang/rust#34608

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