nightly: #[feature(async_drop)] causes stack overflow

[JakubKoralewski]

I tried this code:

https://github.com/JakubKoralewski/repro-rust-asyncdrop-stack-overflow/blob/main/src/main.rs

I expected to see this happen:
no error

Instead, this happened:

    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.03s
     Running `/mnt/vol/.cargo/target/debug/async-drop-stack-overflow-repro`
1889328 (1845 KB)

thread 'main' (3359453) has overflowed its stack
fatal runtime error: stack overflow, aborting
Aborted (core dumped)

Just adding the feature at the top of the file causes the stack overflow. It's not necessary to even implement Drop/AsyncDrop.

Massaging this line :
let mut x = [ 0; 30 * 1024];
1889296 (1845 KB)

into 20*1024 fixes it:
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Meta

rustc --version --verbose:

$ rustc --version --verbose
rustc 1.94.0-nightly (56f24e00c 2025-12-29)
binary: rustc
commit-hash: 56f24e00c62ec2db9a8b407e7aef532caaf71062
commit-date: 2025-12-29
host: aarch64-unknown-linux-gnu
release: 1.94.0-nightly
LLVM version: 21.1.8

Edit

I think causes may be an overstatement. I must have had an already large async fn call tree, and AsyncDrop seems to add a little bit more to the stack size and probably was the straw that broke the camels back.

I can't reproduce this particular behavior in the reproduction, but I've been trying to keep AsyncDrop in my original code, and started calling .boxed(), .boxed_local() on some futures, removing some SmallVecs to Vecs, but even then I couldn't get it to work :/ Even added a new error to the collection:

Caused by:
  process didn't exit successfully: `/mnt/vol/.cargo/target/debug/deps/single_step-8ac7d03d04480459 test_data_run_all_single_step --exact --show-output --nocapture` (signal: 7, SIGBUS: access to undefined memory)

I've seen SIGSEGV before as the first step of the stack overflow, but this is new. (error disappers when I disable AsyncDrop)

[madhav-madhusoodanan]

Try running in release mode.
I think the stack usage would then be optimized.

When I run in release mode, I get the below:

warning: the feature `async_drop` is incomplete and may not be safe to use and/or cause compiler crashes
 --> src/main.rs:1:12
  |
1 | #![feature(async_drop)]
  |            ^^^^^^^^^^
  |
  = note: see issue #126482 <https://github.com/rust-lang/rust/issues/126482> for more information
  = note: `#[warn(incomplete_features)]` on by default

warning: `async-drop-stack-overflow-repro` (bin "async-drop-stack-overflow-repro") generated 1 warning
    Finished `release` profile [optimized] target(s) in 8.03s
     Running `target/release/async-drop-stack-overflow-repro`
1889296 (1845 KB)
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Dropped 0

rustc --version --verbose:

rustc 1.94.0-nightly (56f24e00c 2025-12-29)
binary: rustc
commit-hash: 56f24e00c62ec2db9a8b407e7aef532caaf71062
commit-date: 2025-12-29
host: aarch64-unknown-linux-gnu
release: 1.94.0-nightly
LLVM version: 21.1.8