a94d41a3b4
A recursive coroutine is lowered again once its frame is known; the second lowering returned the cached first one, made with the provisional 8-byte layout, so a boxed recursive future (24 bytes) was written into an 8-byte slot. The relowering now bypasses that cache. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
19 lines
1.1 KiB
Rust
19 lines
1.1 KiB
Rust
// A recursive boxed async fn: the future built for `Box::pin(boxed(n - 1))` is of the type being
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// lowered. Its slot must have the final size (24 bytes: state, flags and saved locals), not the
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// layout so far: the callee wrote 24 bytes into an 8-byte slot (coro_frame lowers the body again;
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// the first lowering is not what the build takes). Found at the baseline tier with the quick
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// register allocator (MAKEPAD_RUN_O1=always), where the frame order exposed the overwrite.
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use std::future::Future;
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use std::pin::Pin;
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use std::sync::Arc;
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use std::task::{Context, Poll, Wake, Waker};
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struct Noop; impl Wake for Noop { fn wake(self: Arc<Self>) {} }
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fn block_on<F: Future>(f: F) -> F::Output {
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let mut f = Box::pin(f); let w = Waker::from(Arc::new(Noop)); let mut cx = Context::from_waker(&w);
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loop { if let Poll::Ready(v) = f.as_mut().poll(&mut cx) { return v; } }
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}
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async fn boxed(n: u32) -> u32 { if n == 0 { 0 } else { n + Box::pin(boxed(n - 1)).await } }
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#[test]
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fn run() { assert_eq!(std::mem::size_of_val(&boxed(3)), 24);
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assert_eq!(block_on(boxed(2)), 3);
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assert_eq!(block_on(boxed(12)), 78); }
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