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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"`.map_unchecked_mut()` is unsound for self-referential types","articleBody":"Hi! I’ve recently created [a similar crate](https://github.com/GoldsteinE/name-it), but without dynamic dispatch and with automatic size calculation and Send/Sync inference, and found [a problem](https://github.com/GoldsteinE/name-it/issues/11) with using `.map_unchecked_mut()` in `.poll()`.\r\n\r\nThis line:\r\n\r\nhttps://github.com/microsoft/stackfuture/blob/ae58939/src/lib.rs#L207\r\n\r\ncauses a retag of the whole byte array, invalidating all internal references inside of a future. To prevent this, you should directly transmute `Pin\u003c\u0026mut MaybeUninit[u8; _]\u003e` instead of mapping.\r\n\r\nThis is a simple testcase that fails under Miri (adapted from `futures-rs` test suite):\r\n\r\n```rust\r\nuse futures::{\r\n    channel::mpsc,\r\n    executor::block_on,\r\n    sink::SinkExt as _,\r\n    stream::{Stream, StreamExt as _},\r\n};\r\nuse stackfuture::StackFuture;\r\nuse std::thread;\r\n\r\n#[test]\r\nfn stress_drop_sender() {\r\n    const ITER: usize = if cfg!(miri) { 100 } else { 10000 };\r\n\r\n    fn list() -\u003e impl Stream\u003cItem = i32\u003e {\r\n        let (tx, rx) = mpsc::channel(1);\r\n        thread::spawn(move || {\r\n            block_on(send_one_two_three(tx));\r\n        });\r\n        rx\r\n    }\r\n\r\n    for _ in 0..ITER {\r\n        let v: Vec\u003c_\u003e = block_on(list().collect());\r\n        assert_eq!(v, vec![1, 2, 3]);\r\n    }\r\n}\r\n\r\nfn send_one_two_three(mut tx: mpsc::Sender\u003ci32\u003e) -\u003e StackFuture\u003c'static, (), 512\u003e {\r\n    StackFuture::from(async move {\r\n        for i in 1..=3 {\r\n            tx.send(i).await.unwrap();\r\n        }\r\n    })\r\n}\r\n```","author":{"url":"https://github.com/GoldsteinE","@type":"Person","name":"GoldsteinE"},"datePublished":"2022-08-12T06:51:17.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":1},"url":"https://github.com/9/stackfuture/issues/9"}