ICE: `polymorphization does not support coroutines from async closures`

[matthiaskrgr]

auto-reduced (treereduce-rust):

#![feature(async_closure, noop_waker, async_fn_traits)]

use std::future::Future;
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_once(f: impl async FnOnce(DropMe)) {
    f(DropMe("world")).await;
}

struct DropMe(&'static str);

pub fn future() {
    block_on(async {
        let async_closure = async move |a: DropMe| {};
        call_once(async_closure).await;
    });
}

original:

#![feature(async_closure, noop_waker, async_fn_traits)]

use std::future::Future;
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_once(f: impl async FnOnce(DropMe)) {
    f(DropMe("world")).await;
}

#[derive(Debug)]
struct DropMe(&'static str);

impl Drop for DropMe {
    fn drop(&mut self) {
        println!("{}", self.0);
    }
}

pub fn future() {
    block_on(async {
        let b = DropMe("hello");
        let async_closure = async move |a: DropMe| {
            println!("{a:?} {b:?}");
        };
        call_once(async_closure).await;
    });
}

Version information

rustc 1.79.0-nightly (63f70b3d1 2024-04-16)
binary: rustc
commit-hash: 63f70b3d104e20289a1a0df82747066c3d85b9a1
commit-date: 2024-04-16
host: x86_64-unknown-linux-gnu
release: 1.79.0-nightly
LLVM version: 18.1.3

Command: /home/matthias/.rustup/toolchains/master/bin/rustc -Zpolymorphize=on --edition=2018 --crate-type=lib

Program output

``` warning: unused variable: `a` --> /tmp/icemaker_global_tempdir.QzD0Jl0Ydl5n/rustc_testrunner_tmpdir_reporting.WyC59e3kD9rD/mvce.rs:27:41 | 27 | let async_closure = async move |a: DropMe| {}; | ^ help: if this is intentional, prefix it with an underscore: `_a` | = note: `#[warn(unused_variables)]` on by default warning: field `0` is never read --> /tmp/icemaker_global_tempdir.QzD0Jl0Ydl5n/rustc_testrunner_tmpdir_reporting.WyC59e3kD9rD/mvce.rs:23:15 | 23 | struct DropMe(&'static str); | ------ ^^^^^^^^^^^^ | | | field in this struct | = note: `#[warn(dead_code)]` on by default help: consider changing the field to be of unit type to suppress this warning while preserving the field numbering, or remove the field | 23 | struct DropMe(()); | ~~ thread 'rustc' panicked at compiler/rustc_middle/src/ty/instance.rs:828:13: assertion `left == right` failed: polymorphization does not support coroutines from async closures left: i32 right: () stack backtrace: 0: 0x74ce26e32cd5 - std::backtrace_rs::backtrace::libunwind::trace::hdd748c7838285883 at /rustc/63f70b3d104e20289a1a0df82747066c3d85b9a1/library/std/src/../../backtrace/src/backtrace/libunwind.rs:105:5 1: 0x74ce26e32cd5 - std::backtrace_rs::backtrace::trace_unsynchronized::ha1462979ee6a2e4a at /rustc/63f70b3d104e20289a1a0df82747066c3d85b9a1/library/std/src/../../backtrace/src/backtrace/mod.rs:66:5 2: 0x74ce26e32cd5 - std::sys_common::backtrace::_print_fmt::h423f6c0147b1e726 at /rustc/63f70b3d104e20289a1a0df82747066c3d85b9a1/library/std/src/sys_common/backtrace.rs:68:5 3: 0x74ce26e32cd5 - ::fmt::h8781c340849a7502 at /rustc/63f70b3d104e20289a1a0df82747066c3d85b9a1/library/std/src/sys_common/backtrace.rs:44:22 4: 0x74ce26e81f9b - core::fmt::rt::Argument::fmt::he5b1af0e8d850256 at /rustc/63f70b3d104e20289a1a0df82747066c3d85b9a1/library/core/src/fmt/rt.rs:165:63 5: 0x74ce26e81f9b - core::fmt::write::hcd3c5ccee382395a at /rustc/63f70b3d104e20289a1a0df82747066c3d85b9a1/library/core/src/fmt/mod.rs:1157:21 6: 0x74ce26e2785f - std::io::Write::write_fmt::h7d1e4c46a9034f24 at /rustc/63f70b3d104e20289a1a0df82747066c3d85b9a1/library/std/src/io/mod.rs:1832:15 7: 0x74ce26e32aae - std::sys_common::backtrace::_print::he7fe8e3f6f78aca7 at /rustc/63f70b3d104e20289a1a0df82747066c3d85b9a1/library/std/src/sys_common/backtrace.rs:47:5 8: 0x74ce26e32aae - std::sys_common::backtrace::print::h3ca58da6c9bbfe9e at /rustc/63f70b3d104e20289a1a0df82747066c3d85b9a1/library/std/src/sys_common/backtrace.rs:34:9 9: 0x74ce26e35429 - std::panicking::default_hook::{{closure}}::h04ae4afc91fb7ed6 10: 0x74ce26e3516d - std::panicking::default_hook::hb7ac4e3868494960 at /rustc/63f70b3d104e20289a1a0df82747066c3d85b9a1/library/std/src/panicking.rs:291:9 11: 0x74ce2367b1cb - std[cc9b189ccd9d9748]::panicking::update_hook::>::{closure#0} 12: 0x74ce26e35b2c - as core::ops::function::Fn>::call::h1bda734d4b3a4644 at /rustc/63f70b3d104e20289a1a0df82747066c3d85b9a1/library/alloc/src/boxed.rs:2032:9 13: 0x74ce26e35b2c - std::panicking::rust_panic_with_hook::hd0146bfa2503919c at /rustc/63f70b3d104e20289a1a0df82747066c3d85b9a1/library/std/src/panicking.rs:792:13 14: 0x74ce26e358d6 - std::panicking::begin_panic_handler::{{closure}}::hd32f7e647243a109 at /rustc/63f70b3d104e20289a1a0df82747066c3d85b9a1/library/std/src/panicking.rs:657:13 15: 0x74ce26e33199 - std::sys_common::backtrace::__rust_end_short_backtrace::h14d8021dc65165b8 at /rustc/63f70b3d104e20289a1a0df82747066c3d85b9a1/library/std/src/sys_common/backtrace.rs:171:18 16: 0x74ce26e35607 - rust_begin_unwind at /rustc/63f70b3d104e20289a1a0df82747066c3d85b9a1/library/std/src/panicking.rs:645:5 17: 0x74ce26e7e446 - core::panicking::panic_fmt::h787b219e21ce34f0 at /rustc/63f70b3d104e20289a1a0df82747066c3d85b9a1/library/core/src/panicking.rs:72:14 18: 0x74ce26e7ea0f - core::panicking::assert_failed_inner::h8c6b5f6736a358ad at /rustc/63f70b3d104e20289a1a0df82747066c3d85b9a1/library/core/src/panicking.rs:397:23 19: 0x74ce23ac7c43 - core[8ca2cc90ba3a0179]::panicking::assert_failed:: 20: 0x74ce23b4b8d0 - rustc_middle[11164a6ce117b708]::ty::instance::polymorphize 21: 0x74ce2228d3cb - rustc_monomorphize[1cf83bd9d05aea12]::collector::collect_items_rec::{closure#0} 22: 0x74ce2584a2f9 - rustc_monomorphize[1cf83bd9d05aea12]::collector::collect_items_rec 23: 0x74ce2584aca6 - rustc_monomorphize[1cf83bd9d05aea12]::collector::collect_items_rec 24: 0x74ce2584aca6 - rustc_monomorphize[1cf83bd9d05aea12]::collector::collect_items_rec 25: 0x74ce2584aca6 - rustc_monomorphize[1cf83bd9d05aea12]::collector::collect_items_rec 26: 0x74ce25844dec - rustc_monomorphize[1cf83bd9d05aea12]::partitioning::collect_and_partition_mono_items 27: 0x74ce258443e8 - rustc_query_impl[2063281436f076b]::plumbing::__rust_begin_short_backtrace::> 28: 0x74ce258443cd - >::call_once 29: 0x74ce25840f54 - rustc_query_system[8bdce35ed35c4821]::query::plumbing::try_execute_query::>, false, false, false>, rustc_query_impl[2063281436f076b]::plumbing::QueryCtxt, false> 30: 0x74ce25840c4b - rustc_query_impl[2063281436f076b]::query_impl::collect_and_partition_mono_items::get_query_non_incr::__rust_end_short_backtrace 31: 0x74ce254df8cd - rustc_codegen_ssa[c0308cd1d1075584]::back::symbol_export::exported_symbols_provider_local 32: 0x74ce24c72f25 - rustc_query_impl[2063281436f076b]::plumbing::__rust_begin_short_backtrace::> 33: 0x74ce24c72ef5 - >::call_once 34: 0x74ce25707bd0 - rustc_query_system[8bdce35ed35c4821]::query::plumbing::try_execute_query::>, false, false, false>, rustc_query_impl[2063281436f076b]::plumbing::QueryCtxt, false> 35: 0x74ce257077a5 - rustc_query_impl[2063281436f076b]::query_impl::exported_symbols::get_query_non_incr::__rust_end_short_backtrace 36: 0x74ce257075ec - rustc_middle[11164a6ce117b708]::query::plumbing::query_get_at::>> 37: 0x74ce2512f54f - ::encode_crate_root 38: 0x74ce258e6ff3 - rustc_metadata[5a02b8786d893b0b]::rmeta::encoder::encode_metadata 39: 0x74ce258f0a36 - rustc_metadata[5a02b8786d893b0b]::fs::encode_and_write_metadata 40: 0x74ce258efc18 - rustc_interface[757df98036431907]::passes::start_codegen 41: 0x74ce258ef358 - ::codegen_and_build_linker 42: 0x74ce25609845 - rustc_interface[757df98036431907]::interface::run_compiler::, rustc_driver_impl[e492dcc29030f304]::run_compiler::{closure#0}>::{closure#0} 43: 0x74ce257c391d - std[cc9b189ccd9d9748]::sys_common::backtrace::__rust_begin_short_backtrace::, rustc_driver_impl[e492dcc29030f304]::run_compiler::{closure#0}>::{closure#0}, core[8ca2cc90ba3a0179]::result::Result<(), rustc_span[1f43cfd63a717c3a]::ErrorGuaranteed>>::{closure#0}, core[8ca2cc90ba3a0179]::result::Result<(), rustc_span[1f43cfd63a717c3a]::ErrorGuaranteed>>::{closure#0}::{closure#0}, core[8ca2cc90ba3a0179]::result::Result<(), rustc_span[1f43cfd63a717c3a]::ErrorGuaranteed>> 44: 0x74ce257c372a - <::spawn_unchecked_, rustc_driver_impl[e492dcc29030f304]::run_compiler::{closure#0}>::{closure#0}, core[8ca2cc90ba3a0179]::result::Result<(), rustc_span[1f43cfd63a717c3a]::ErrorGuaranteed>>::{closure#0}, core[8ca2cc90ba3a0179]::result::Result<(), rustc_span[1f43cfd63a717c3a]::ErrorGuaranteed>>::{closure#0}::{closure#0}, core[8ca2cc90ba3a0179]::result::Result<(), rustc_span[1f43cfd63a717c3a]::ErrorGuaranteed>>::{closure#2} as core[8ca2cc90ba3a0179]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0} 45: 0x74ce26e3fa1b - as core::ops::function::FnOnce>::call_once::hf04c15b7e1b40818 at /rustc/63f70b3d104e20289a1a0df82747066c3d85b9a1/library/alloc/src/boxed.rs:2018:9 46: 0x74ce26e3fa1b - as core::ops::function::FnOnce>::call_once::h6efe135dbcc64c78 at /rustc/63f70b3d104e20289a1a0df82747066c3d85b9a1/library/alloc/src/boxed.rs:2018:9 47: 0x74ce26e3fa1b - std::sys::pal::unix::thread::Thread::new::thread_start::h767b00cfb36de6f0 at /rustc/63f70b3d104e20289a1a0df82747066c3d85b9a1/library/std/src/sys/pal/unix/thread.rs:108:17 48: 0x74ce26bdd55a - 49: 0x74ce26c5aa3c - 50: 0x0 - error: the compiler unexpectedly panicked. this is a bug. note: we would appreciate a bug report: https://github.com/rust-lang/rust/issues/new?labels=C-bug%2C+I-ICE%2C+T-compiler&template=ice.md note: please make sure that you have updated to the latest nightly note: rustc 1.79.0-nightly (63f70b3d1 2024-04-16) running on x86_64-unknown-linux-gnu note: compiler flags: -Z polymorphize=on --crate-type lib -Z dump-mir-dir=dir query stack during panic: #0 [collect_and_partition_mono_items] collect_and_partition_mono_items #1 [exported_symbols] collecting exported symbols for crate `0` end of query stack warning: 2 warnings emitted ```

[matthiaskrgr]

bit smaller:

#![feature(async_closure, noop_waker, async_fn_traits)]

use std::future::Future;
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) {
            _ => (),
        }
    }
}

async fn call_once(f: impl async FnOnce(&'static str)) {
    f("world").await;
}

pub fn future() {
    block_on(async {
        let async_closure = async move |_a: &'static str| {};
        call_once(async_closure).await;
    });
}

[matthiaskrgr]

bisects to #120746 cc @compiler-errors

[compiler-errors]

unsurprising -- prolly won't fix this b/c polymorphization remains unstable and also kind of busted

[matthiaskrgr]

Ah lol yes this was the pr that added the assertion xD