parallel rustc: async closures not reproducible

[matthiaskrgr]

I tried this code:

//@ edition:2021
// skip-filecheck

#![allow(unused)]

use std::future::Future;
use std::ops::{AsyncFn, AsyncFnMut, AsyncFnOnce};
use std::pin::pin;
use std::task::*;

pub fn block_on<T>(fut: impl Future<Output = T>) -> T {
    let mut fut = pin!(fut);
    let ctx = &mut Context::from_waker(Waker::noop());

    loop {
        match fut.as_mut().poll(ctx) {
            Poll::Pending => {}
            Poll::Ready(t) => break t,
        }
    }
}

async fn call(f: &impl AsyncFn(i32)) {
    f(0).await;
}

async fn call_mut(f: &mut impl AsyncFnMut(i32)) {
    f(0).await;
}

async fn call_once(f: impl AsyncFnOnce(i32)) {
    f(1).await;
}

async fn call_normal<F: Future<Output = ()>>(f: &impl Fn(i32) -> F) {
    f(1).await;
}

async fn call_normal_mut<F: Future<Output = ()>>(f: &mut impl FnMut(i32) -> F) {
    f(1).await;
}

// EMIT_MIR async_closure_shims.main-{closure#0}-{closure#0}.coroutine_closure_by_move.0.mir
// EMIT_MIR async_closure_shims.main-{closure#0}-{closure#0}-{closure#0}.built.after.mir
// EMIT_MIR async_closure_shims.main-{closure#0}-{closure#0}-{synthetic#0}.built.after.mir
// EMIT_MIR async_closure_shims.main-{closure#0}-{closure#1}.coroutine_closure_by_ref.0.mir
// EMIT_MIR async_closure_shims.main-{closure#0}-{closure#1}.coroutine_closure_by_move.0.mir
// EMIT_MIR async_closure_shims.main-{closure#0}-{closure#1}-{closure#0}.built.after.mir
// EMIT_MIR async_closure_shims.main-{closure#0}-{closure#1}-{synthetic#0}.built.after.mir
pub fn main() {
    block_on(async {
        let b = 2i32;
        let mut async_closure = async move |a: i32| {
            let a = &a;
            let b = &b;
        };
        call(&async_closure).await;
        call_mut(&mut async_closure).await;
        call_once(async_closure).await;

        let b = 2i32;
        let mut async_closure = async |a: i32| {
            let a = &a;
            let b = &b;
        };
        call_normal(&async_closure).await;
        call_normal_mut(&mut async_closure).await;
        call_once(async_closure).await;
    });
}

`rustc async_closure_shims.rs -Zthreads=16 --edition=2021; md5sum async_closure_shims

I expected to see this happen: explanation
same md5 every time

Instead, this happened: explanation
different md5 from time to time

Meta

rustc --version --verbose:

rustc 1.88.0-nightly (25cdf1f67 2025-04-28)
binary: rustc
commit-hash: 25cdf1f67463c9365d8d83778c933ec7480e940b
commit-date: 2025-04-28
host: x86_64-unknown-linux-gnu
release: 1.88.0-nightly
LLVM version: 20.1.2

Backtrace

--- /proc/self/fd/11	2025-04-29 06:44:18.100415395 +0200
+++ /proc/self/fd/12	2025-04-29 06:44:18.097415424 +0200
@@ -49,8 +49,8 @@
 │000002f0│ 78 38 36 2d 36 34 2e 73 ┊ 6f 2e 32 00 04 00 00 00 │x86-64.s┊o.2⋄•⋄⋄⋄│
 │00000300│ 10 00 00 00 01 00 00 00 ┊ 47 4e 55 00 00 00 00 00 │•⋄⋄⋄•⋄⋄⋄┊GNU⋄⋄⋄⋄⋄│
 │00000310│ 04 00 00 00 04 00 00 00 ┊ 00 00 00 00 04 00 00 00 │•⋄⋄⋄•⋄⋄⋄┊⋄⋄⋄⋄•⋄⋄⋄│
-│00000320│ 14 00 00 00 03 00 00 00 ┊ 47 4e 55 00 67 24 f6 cd │•⋄⋄⋄•⋄⋄⋄┊GNU⋄g$××│
-│00000330│ 69 dd 6a eb ff 37 f1 29 ┊ 4d ef 35 87 ab c4 b5 26 │i×j××7×)┊M×5××××&│
+│00000320│ 14 00 00 00 03 00 00 00 ┊ 47 4e 55 00 d5 f4 67 b4 │•⋄⋄⋄•⋄⋄⋄┊GNU⋄××g×│
+│00000330│ 2f 28 18 02 c7 8a a5 0a ┊ 1d 7b 09 7b 4e 5c fa e1 │/(••×××_┊•{_{N\××│
 │00000340│ 00 00 00 00 00 00 00 00 ┊ 00 00 00 00 00 00 00 00 │⋄⋄⋄⋄⋄⋄⋄⋄┊⋄⋄⋄⋄⋄⋄⋄⋄│
 │00000350│ 00 00 00 00 00 00 00 00 ┊ 01 00 00 00 12 00 00 00 │⋄⋄⋄⋄⋄⋄⋄⋄┊•⋄⋄⋄•⋄⋄⋄│
 │00000360│ 00 00 00 00 00 00 00 00 ┊ 00 00 00 00 00 00 00 00 │⋄⋄⋄⋄⋄⋄⋄⋄┊⋄⋄⋄⋄⋄⋄⋄⋄│
@@ -3787,9 +3787,9 @@
 │0000ed20│ e0 83 00 00 0e 00 00 00 ┊ 00 00 00 00 18 00 00 00 │××⋄⋄•⋄⋄⋄┊⋄⋄⋄⋄•⋄⋄⋄│
 │0000ed30│ 18 03 00 00 dc 83 00 00 ┊ 53 00 00 00 00 44 0e 20 │••⋄⋄××⋄⋄┊S⋄⋄⋄⋄D• │
 │0000ed40│ 02 41 0e 08 41 0e 20 00 ┊ 18 00 00 00 34 03 00 00 │•A••A• ⋄┊•⋄⋄⋄4•⋄⋄│
-│0000ed50│ 20 84 00 00 52 00 00 00 ┊ 00 44 0e 20 02 40 0e 08 │ ×⋄⋄R⋄⋄⋄┊⋄D• •@••│
+│0000ed50│ 20 84 00 00 53 00 00 00 ┊ 00 44 0e 20 02 41 0e 08 │ ×⋄⋄S⋄⋄⋄┊⋄D• •A••│
 │0000ed60│ 41 0e 20 00 18 00 00 00 ┊ 50 03 00 00 64 84 00 00 │A• ⋄•⋄⋄⋄┊P•⋄⋄d×⋄⋄│
-│0000ed70│ 53 00 00 00 00 44 0e 20 ┊ 02 41 0e 08 41 0e 20 00 │S⋄⋄⋄⋄D• ┊•A••A• ⋄│
+│0000ed70│ 52 00 00 00 00 44 0e 20 ┊ 02 40 0e 08 41 0e 20 00 │R⋄⋄⋄⋄D• ┊•@••A• ⋄│
 │0000ed80│ 14 00 00 00 6c 03 00 00 ┊ a8 84 00 00 17 00 00 00 │•⋄⋄⋄l•⋄⋄┊××⋄⋄•⋄⋄⋄│
 │0000ed90│ 00 41 0e 10 55 0e 08 00 ┊ 10 00 00 00 84 03 00 00 │⋄A••U••⋄┊•⋄⋄⋄ו⋄⋄│
 │0000eda0│ b0 84 00 00 05 00 00 00 ┊ 00 00 00 00 10 00 00 00 │××⋄⋄•⋄⋄⋄┊⋄⋄⋄⋄•⋄⋄⋄│
@@ -5441,7 +5441,7 @@
 │00015480│ 0c 1f 04 00 48 8b 7c 24 ┊ 28 48 83 c7 04 e8 5e f5 │_••⋄H×|$┊(H×ו×^×│
 │00015490│ ff ff e9 7a ff ff ff 48 ┊ 89 c1 89 d0 48 89 4c 24 │×××z×××H┊××××H×L$│
 │000154a0│ 58 89 44 24 60 eb dd 48 ┊ 8b 7c 24 10 48 8b 74 24 │X×D$`××H┊×|$•H×t$│
-│000154b0│ 50 e8 ba 0c 00 00 88 44 ┊ 24 0f eb 00 8a 44 24 0f │P××_⋄⋄×D┊$•×⋄×D$•│
+│000154b0│ 50 e8 1a 0d 00 00 88 44 ┊ 24 0f eb 00 8a 44 24 0f │Pו_⋄⋄×D┊$•×⋄×D$•│
 │000154c0│ 24 01 0f b6 c0 48 a9 01 ┊ 00 00 00 74 19 c6 44 24 │$••××Hו┊⋄⋄⋄t•×D$│
 │000154d0│ 37 01 48 8b 44 24 28 c6 ┊ 40 10 03 8a 44 24 37 24 │7•H×D$(×┊@••×D$7$│
 │000154e0│ 01 48 83 c4 68 c3 48 8b ┊ 7c 24 28 48 83 c7 04 e8 │•H××h×H×┊|$(H×ו×│
@@ -5465,7 +5465,7 @@
 │00015600│ 00 ff 15 89 1d 04 00 48 ┊ 8b 7c 24 20 48 83 c7 08 │⋄וו•⋄H┊×|$ H×ו│
 │00015610│ e8 eb f3 ff ff e9 78 ff ┊ ff ff 48 89 c1 89 d0 48 │××××××x×┊××H××××H│
 │00015620│ 89 4c 24 58 89 44 24 60 ┊ eb dd 48 8b 7c 24 08 48 │×L$X×D$`┊××H×|$•H│
-│00015630│ 8b 74 24 50 e8 97 0b 00 ┊ 00 88 44 24 07 eb 00 8a │×t$P×ו⋄┊⋄×D$•×⋄×│
+│00015630│ 8b 74 24 50 e8 37 0b 00 ┊ 00 88 44 24 07 eb 00 8a │×t$P×7•⋄┊⋄×D$•×⋄×│
 │00015640│ 44 24 07 24 01 0f b6 c0 ┊ 48 a9 01 00 00 00 74 19 │D$•$••××┊Hו⋄⋄⋄t•│
 │00015650│ c6 44 24 2f 01 48 8b 44 ┊ 24 20 c6 40 18 03 8a 44 │×D$/•H×D┊$ ×@••×D│
 │00015660│ 24 2f 24 01 48 83 c4 68 ┊ c3 48 8b 7c 24 20 48 83 │$/$•H××h┊×H×|$ H×│
@@ -5490,7 +5490,7 @@
 │00015790│ 00 ff 15 f9 1b 04 00 48 ┊ 8b 7c 24 20 48 83 c7 08 │⋄וו•⋄H┊×|$ H×ו│
 │000157a0│ e8 5b f2 ff ff e9 78 ff ┊ ff ff 48 89 c1 89 d0 48 │×[××××x×┊××H××××H│
 │000157b0│ 89 4c 24 58 89 44 24 60 ┊ eb dd 48 8b 7c 24 08 48 │×L$X×D$`┊××H×|$•H│
-│000157c0│ 8b 74 24 50 e8 07 0a 00 ┊ 00 88 44 24 07 eb 00 8a │×t$Pו_⋄┊⋄×D$•×⋄×│
+│000157c0│ 8b 74 24 50 e8 a7 09 00 ┊ 00 88 44 24 07 eb 00 8a │×t$P××_⋄┊⋄×D$•×⋄×│
 │000157d0│ 44 24 07 24 01 0f b6 c0 ┊ 48 a9 01 00 00 00 74 19 │D$•$••××┊Hו⋄⋄⋄t•│
 │000157e0│ c6 44 24 2f 01 48 8b 44 ┊ 24 20 c6 40 18 03 8a 44 │×D$/•H×D┊$ ×@••×D│
 │000157f0│ 24 2f 24 01 48 83 c4 68 ┊ c3 48 8b 7c 24 20 48 83 │$/$•H××h┊×H×|$ H×│
@@ -5515,7 +5515,7 @@
 │00015920│ 00 ff 15 69 1a 04 00 48 ┊ 8b 7c 24 20 48 83 c7 08 │⋄וi••⋄H┊×|$ H×ו│
 │00015930│ e8 cb f0 ff ff e9 78 ff ┊ ff ff 48 89 c1 89 d0 48 │××××××x×┊××H××××H│
 │00015940│ 89 4c 24 58 89 44 24 60 ┊ eb dd 48 8b 7c 24 08 48 │×L$X×D$`┊××H×|$•H│
-│00015950│ 8b 74 24 50 e8 77 08 00 ┊ 00 88 44 24 07 eb 00 8a │×t$P×w•⋄┊⋄×D$•×⋄×│
+│00015950│ 8b 74 24 50 e8 17 08 00 ┊ 00 88 44 24 07 eb 00 8a │×t$Pו•⋄┊⋄×D$•×⋄×│
 │00015960│ 44 24 07 24 01 0f b6 c0 ┊ 48 a9 01 00 00 00 74 19 │D$•$••××┊Hו⋄⋄⋄t•│
 │00015970│ c6 44 24 2f 01 48 8b 44 ┊ 24 20 c6 40 18 03 8a 44 │×D$/•H×D┊$ ×@••×D│
 │00015980│ 24 2f 24 01 48 83 c4 68 ┊ c3 48 8b 7c 24 20 48 83 │$/$•H××h┊×H×|$ H×│
@@ -5646,17 +5646,17 @@
 │00016150│ 01 48 83 c4 18 c3 48 8d ┊ 3d 2b ea 03 00 ff 15 25 │•H×ו×H×┊=+ו⋄ו%│
 │00016160│ 12 04 00 cc cc cc cc cc ┊ cc cc cc cc cc cc cc cc │••⋄×××××┊××××××××│
 │00016170│ 48 83 ec 18 48 89 7c 24 ┊ 08 48 8b 44 24 08 0f b6 │H×וH×|$┊•H×D$••×│
-│00016180│ 40 08 a9 01 00 00 00 74 ┊ 08 31 c0 a8 01 75 fa eb │@•×•⋄⋄⋄t┊•1×וu××│
-│00016190│ 24 48 8b 44 24 08 8b 00 ┊ 89 44 24 14 c6 44 24 13 │$H×D$•×⋄┊×D$•×D$•│
-│000161a0│ 00 48 8b 44 24 08 c6 40 ┊ 08 01 8a 44 24 13 24 01 │⋄H×D$•×@┊••×D$•$•│
-│000161b0│ 48 83 c4 18 c3 48 8d 3d ┊ cc e9 03 00 ff 15 c6 11 │H×ו×H×=┊×ו⋄וו│
-│000161c0│ 04 00 cc cc cc cc cc cc ┊ cc cc cc cc cc cc cc cc │•⋄××××××┊××××××××│
+│00016180│ 40 0c a9 01 00 00 00 74 ┊ 08 31 c0 a8 01 75 fa eb │@_ו⋄⋄⋄t┊•1×וu××│
+│00016190│ 25 48 8b 44 24 08 8b 40 ┊ 08 89 44 24 14 c6 44 24 │%H×D$•×@┊•×D$•×D$│
+│000161a0│ 13 00 48 8b 44 24 08 c6 ┊ 40 0c 01 8a 44 24 13 24 │•⋄H×D$•×┊@_•×D$•$│
+│000161b0│ 01 48 83 c4 18 c3 48 8d ┊ 3d e3 e9 03 00 ff 15 c5 │•H×ו×H×┊=×ו⋄ו×│
+│000161c0│ 11 04 00 cc cc cc cc cc ┊ cc cc cc cc cc cc cc cc │••⋄×××××┊××××××××│
 │000161d0│ 48 83 ec 18 48 89 7c 24 ┊ 08 48 8b 44 24 08 0f b6 │H×וH×|$┊•H×D$••×│
-│000161e0│ 40 0c a9 01 00 00 00 74 ┊ 08 31 c0 a8 01 75 fa eb │@_ו⋄⋄⋄t┊•1×וu××│
-│000161f0│ 25 48 8b 44 24 08 8b 40 ┊ 08 89 44 24 14 c6 44 24 │%H×D$•×@┊•×D$•×D$│
-│00016200│ 13 00 48 8b 44 24 08 c6 ┊ 40 0c 01 8a 44 24 13 24 │•⋄H×D$•×┊@_•×D$•$│
-│00016210│ 01 48 83 c4 18 c3 48 8d ┊ 3d 83 e9 03 00 ff 15 65 │•H×ו×H×┊=×ו⋄וe│
-│00016220│ 11 04 00 cc cc cc cc cc ┊ cc cc cc cc cc cc cc cc │••⋄×××××┊××××××××│
+│000161e0│ 40 08 a9 01 00 00 00 74 ┊ 08 31 c0 a8 01 75 fa eb │@•×•⋄⋄⋄t┊•1×וu××│
+│000161f0│ 24 48 8b 44 24 08 8b 00 ┊ 89 44 24 14 c6 44 24 13 │$H×D$•×⋄┊×D$•×D$•│
+│00016200│ 00 48 8b 44 24 08 c6 40 ┊ 08 01 8a 44 24 13 24 01 │⋄H×D$•×@┊••×D$•$•│
+│00016210│ 48 83 c4 18 c3 48 8d 3d ┊ 6c e9 03 00 ff 15 66 11 │H×ו×H×=┊lו⋄וf•│
+│00016220│ 04 00 cc cc cc cc cc cc ┊ cc cc cc cc cc cc cc cc │•⋄××××××┊××××××××│
 │00016230│ 50 48 89 f2 48 63 f7 48 ┊ 8d 3d 82 f7 ff ff 31 c9 │PH××Hc×H┊×=××××1×│
 │00016240│ e8 cb e5 ff ff 59 c3 cc ┊ cc cc cc cc cc cc cc cc │×××××Y××┊××××××××│
 │00016250│ e9 9b fa 01 00 cc cc cc ┊ cc cc cc cc cc cc cc cc │××ו⋄×××┊××××××××│
@@ -226718,12 +226718,12 @@
 │00376e60│ 5f 16 00 00 02 00 0f 00 ┊ 90 63 01 00 00 00 00 00 │_•⋄⋄•⋄•⋄┊×c•⋄⋄⋄⋄⋄│
 │00376e70│ 0b 00 00 00 00 00 00 00 ┊ 96 16 00 00 02 00 0f 00 │•⋄⋄⋄⋄⋄⋄⋄┊ו⋄⋄•⋄•⋄│
 │00376e80│ a0 63 01 00 00 00 00 00 ┊ 75 01 00 00 00 00 00 00 │×c•⋄⋄⋄⋄⋄┊u•⋄⋄⋄⋄⋄⋄│
-│00376e90│ eb 16 00 00 02 00 0f 00 ┊ 70 71 01 00 00 00 00 00 │ו⋄⋄•⋄•⋄┊pq•⋄⋄⋄⋄⋄│
+│00376e90│ eb 16 00 00 02 00 0f 00 ┊ d0 71 01 00 00 00 00 00 │ו⋄⋄•⋄•⋄┊×q•⋄⋄⋄⋄⋄│
 │00376ea0│ 52 00 00 00 00 00 00 00 ┊ 79 17 00 00 00 00 0b 00 │R⋄⋄⋄⋄⋄⋄⋄┊y•⋄⋄⋄⋄•⋄│
 │00376eb0│ 70 5b 00 00 00 00 00 00 ┊ 00 00 00 00 00 00 00 00 │p[⋄⋄⋄⋄⋄⋄┊⋄⋄⋄⋄⋄⋄⋄⋄│
 │00376ec0│ 8c 17 00 00 02 00 0f 00 ┊ 20 65 01 00 00 00 00 00 │ו⋄⋄•⋄•⋄┊ e•⋄⋄⋄⋄⋄│
 │00376ed0│ 78 01 00 00 00 00 00 00 ┊ e1 17 00 00 02 00 0f 00 │x•⋄⋄⋄⋄⋄⋄┊ו⋄⋄•⋄•⋄│
-│00376ee0│ d0 71 01 00 00 00 00 00 ┊ 53 00 00 00 00 00 00 00 │×q•⋄⋄⋄⋄⋄┊S⋄⋄⋄⋄⋄⋄⋄│
+│00376ee0│ 70 71 01 00 00 00 00 00 ┊ 53 00 00 00 00 00 00 00 │pq•⋄⋄⋄⋄⋄┊S⋄⋄⋄⋄⋄⋄⋄│
 │00376ef0│ 6f 18 00 00 00 00 0b 00 ┊ a4 5b 00 00 00 00 00 00 │o•⋄⋄⋄⋄•⋄┊×[⋄⋄⋄⋄⋄⋄│
 │00376f00│ 00 00 00 00 00 00 00 00 ┊ 82 18 00 00 02 00 0f 00 │⋄⋄⋄⋄⋄⋄⋄⋄┊ו⋄⋄•⋄•⋄│
 │00376f10│ a0 66 01 00 00 00 00 00 ┊ 0b 00 00 00 00 00 00 00 │×f•⋄⋄⋄⋄⋄┊•⋄⋄⋄⋄⋄⋄⋄│