ICE: invalid enum variant tag while decoding `MonoItem`

Code

<extern crate trpl;
use std::time::Duration;
use trpl::Either;

async fn compute_short() -> i32 {
    trpl::sleep(Duration::from_secs(1)).await;
    42
}

async fn compute_long() -> i32 {
    trpl::sleep(Duration::from_secs(3)).await;
    99
}

fn main() {
    trpl::run(async {
        // call a simple async fn and await its result
        let value = compute_short().await;
        println!("compute_short returned: {}", value); // expect 42

        // race compute_long against a timeout of 2 seconds
        match trpl::race(compute_long(), trpl::sleep(Duration::from_secs(2))).await {
            Either::Left(result) => println!("long finished first with {}", result),
            Either::Right(()) => println!("Timed out after 2 seconds"),
        }

        // Bonus race compute_short vs 2s timer => compute_short finished first
        match trpl::race(compute_short(), trpl::sleep(Duration::from_secs(2))).await {
            Either::Left(result) => println!("Short finished first with {}", result),
            Either::Right(()) => println!("Short timed out (unexpected)"),
        }
    });
}
>

Meta

rustc --version --verbose:

<version>

Error output

<admin@MacBook-Pro tiny_async_func % cargo run -q

thread 'rustc' panicked at /rustc/4eb161250e340c8f48f66e2b929ef4a5bed7c181/compiler/rustc_middle/src/mir/mono.rs:49:76:
invalid enum variant tag while decoding `MonoItem`, expected 0..3, actual 3
stack backtrace:
   0:        0x10ffd30c0 - <std::sys::backtrace::BacktraceLock::print::DisplayBacktrace as core::fmt::Display>::fmt::h4b01b56329a0a8fc
   1:        0x10d7db0e4 - core::fmt::write::h5ac783d2f16cae69
   2:        0x10ffc6fa4 - std::io::Write::write_fmt::h740ae8bf4425e1f4
   3:        0x10ffd2f80 - std::sys::backtrace::BacktraceLock::print::h822ac0a1da741b33
   4:        0x10ffd54a4 - std::panicking::default_hook::{{closure}}::h182e34e1427a763f
   5:        0x10ffd52a0 - std::panicking::default_hook::hf3c673ab0d652139
   6:        0x10e245f4c - std[c851462af2470259]::panicking::update_hook::<alloc[5a748f96c383aa6f]::boxed::Box<rustc_driver_impl[b0715d04bd56a776]::install_ice_hook::{closure#1}>>::{closure#0}
   7:        0x10ffd5ecc - std::panicking::rust_panic_with_hook::ha68643d4311595e9
   8:        0x10ffd5a38 - std::panicking::begin_panic_handler::{{closure}}::h0edcf2e5c8ce9186
   9:        0x10ffd355c - std::sys::backtrace::__rust_end_short_backtrace::h25049b070cf1a527
  10:        0x10ffd56fc - _rust_begin_unwind
  11:        0x1126a3ce4 - core::panicking::panic_fmt::ha0b5c5252c2f1706
  12:        0x10f62ed18 - <core[5fb0dbaabce25f6b]::iter::adapters::map::Map<core[5fb0dbaabce25f6b]::ops::range::Range<usize>, <[rustc_span[a77c8e897e30def6]::source_map::Spanned<rustc_middle[d103fa9019da59ee]::mir::mono::MonoItem>] as rustc_middle[d103fa9019da59ee]::ty::codec::RefDecodable<rustc_middle[d103fa9019da59ee]::query::on_disk_cache::CacheDecoder>>::decode::{closure#0}> as core[5fb0dbaabce25f6b]::iter::traits::iterator::Iterator>::next
  13:        0x10f6cab94 - <&[rustc_span[a77c8e897e30def6]::source_map::Spanned<rustc_middle[d103fa9019da59ee]::mir::mono::MonoItem>] as rustc_serialize[c6d4baa930e56201]::serialize::Decodable<rustc_middle[d103fa9019da59ee]::query::on_disk_cache::CacheDecoder>>::decode
  14:        0x10f71db24 - <rustc_query_impl[b1806c3197f49f15]::query_impl::items_of_instance::dynamic_query::{closure#6} as core[5fb0dbaabce25f6b]::ops::function::FnOnce<(rustc_middle[d103fa9019da59ee]::ty::context::TyCtxt, &(rustc_middle[d103fa9019da59ee]::ty::instance::Instance, rustc_middle[d103fa9019da59ee]::mir::mono::CollectionMode), rustc_query_system[760a27df5145ae02]::dep_graph::serialized::SerializedDepNodeIndex, rustc_query_system[760a27df5145ae02]::dep_graph::graph::DepNodeIndex)>>::call_once
  15:        0x10f5af6b0 - rustc_query_system[760a27df5145ae02]::query::plumbing::try_execute_query::<rustc_query_impl[b1806c3197f49f15]::DynamicConfig<rustc_query_system[760a27df5145ae02]::query::caches::DefaultCache<(rustc_middle[d103fa9019da59ee]::ty::instance::Instance, rustc_middle[d103fa9019da59ee]::mir::mono::CollectionMode), rustc_middle[d103fa9019da59ee]::query::erase::Erased<[u8; 32usize]>>, false, false, false>, rustc_query_impl[b1806c3197f49f15]::plumbing::QueryCtxt, true>
  16:        0x10f7ee958 - rustc_query_impl[b1806c3197f49f15]::query_impl::items_of_instance::get_query_incr::__rust_end_short_backtrace
  17:        0x10f2007e8 - rustc_monomorphize[9aae175af317f764]::collector::collect_items_rec::{closure#0}
  18:        0x10f1ff0f0 - rustc_monomorphize[9aae175af317f764]::collector::collect_items_rec
  19:        0x10f1fff20 - rustc_monomorphize[9aae175af317f764]::collector::collect_items_rec
  20:        0x10f1fff20 - rustc_monomorphize[9aae175af317f764]::collector::collect_items_rec
  21:        0x10f1fff20 - rustc_monomorphize[9aae175af317f764]::collector::collect_items_rec
  22:        0x10f1fa404 - rustc_monomorphize[9aae175af317f764]::partitioning::collect_and_partition_mono_items
  23:        0x10f77216c - rustc_query_impl[b1806c3197f49f15]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[b1806c3197f49f15]::query_impl::collect_and_partition_mono_items::dynamic_query::{closure#2}::{closure#0}, rustc_middle[d103fa9019da59ee]::query::erase::Erased<[u8; 24usize]>>
  24:        0x10f538948 - rustc_query_system[760a27df5145ae02]::query::plumbing::try_execute_query::<rustc_query_impl[b1806c3197f49f15]::DynamicConfig<rustc_query_system[760a27df5145ae02]::query::caches::SingleCache<rustc_middle[d103fa9019da59ee]::query::erase::Erased<[u8; 24usize]>>, false, false, false>, rustc_query_impl[b1806c3197f49f15]::plumbing::QueryCtxt, true>
  25:        0x10f7d5f64 - rustc_query_impl[b1806c3197f49f15]::query_impl::collect_and_partition_mono_items::get_query_incr::__rust_end_short_backtrace
  26:        0x10decc2d8 - rustc_codegen_ssa[b14f965e64741b78]::base::codegen_crate::<rustc_codegen_llvm[8693c5e9711c96e6]::LlvmCodegenBackend>
  27:        0x10dfb3164 - <rustc_codegen_llvm[8693c5e9711c96e6]::LlvmCodegenBackend as rustc_codegen_ssa[b14f965e64741b78]::traits::backend::CodegenBackend>::codegen_crate
  28:        0x10eaaa1dc - <rustc_interface[f03bf5bdfd944c31]::queries::Linker>::codegen_and_build_linker
  29:        0x10e239728 - rustc_interface[f03bf5bdfd944c31]::passes::create_and_enter_global_ctxt::<core[5fb0dbaabce25f6b]::option::Option<rustc_interface[f03bf5bdfd944c31]::queries::Linker>, rustc_driver_impl[b0715d04bd56a776]::run_compiler::{closure#0}::{closure#2}>
  30:        0x10e24c480 - rustc_interface[f03bf5bdfd944c31]::interface::run_compiler::<(), rustc_driver_impl[b0715d04bd56a776]::run_compiler::{closure#0}>::{closure#1}
  31:        0x10e23b1dc - std[c851462af2470259]::sys::backtrace::__rust_begin_short_backtrace::<rustc_interface[f03bf5bdfd944c31]::util::run_in_thread_with_globals<rustc_interface[f03bf5bdfd944c31]::util::run_in_thread_pool_with_globals<rustc_interface[f03bf5bdfd944c31]::interface::run_compiler<(), rustc_driver_impl[b0715d04bd56a776]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>
  32:        0x10e24f504 - <<std[c851462af2470259]::thread::Builder>::spawn_unchecked_<rustc_interface[f03bf5bdfd944c31]::util::run_in_thread_with_globals<rustc_interface[f03bf5bdfd944c31]::util::run_in_thread_pool_with_globals<rustc_interface[f03bf5bdfd944c31]::interface::run_compiler<(), rustc_driver_impl[b0715d04bd56a776]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>::{closure#1} as core[5fb0dbaabce25f6b]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  33:        0x10ffe0eb0 - std::sys::pal::unix::thread::Thread::new::thread_start::hdb877a8060df0ba1
  34:        0x199009c08 - __pthread_cond_wait

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: rustc 1.85.1 (4eb161250 2025-03-15) running on aarch64-apple-darwin

note: compiler flags: --crate-type bin -C embed-bitcode=no -C debuginfo=2 -C split-debuginfo=unpacked -C incremental=[REDACTED]

note: some of the compiler flags provided by cargo are hidden

query stack during panic:
panicked at /rustc/4eb161250e340c8f48f66e2b929ef4a5bed7c181/compiler/rustc_query_system/src/dep_graph/graph.rs:1404:5:

thread panicked while processing panic. aborting.
error: could not compile `tiny_async_func` (bin "tiny_async_func")

Caused by:
  process didn't exit successfully: `/Users/admin/.rustup/toolchains/stable-aarch64-apple-darwin/bin/rustc --crate-name tiny_async_func --edition=2024 src/main.rs --error-format=json --json=diagnostic-rendered-ansi,artifacts,future-incompat --diagnostic-width=157 --crate-type bin --emit=dep-info,link -C embed-bitcode=no -C debuginfo=2 -C split-debuginfo=unpacked --check-cfg 'cfg(docsrs,test)' --check-cfg 'cfg(feature, values())' -C metadata=a8e01b0c0de3b26e -C extra-filename=-6db4b02ad1f9fb35 --out-dir /Users/admin/Documents/rust/tiny_async_func/target/debug/deps -C incremental=/Users/admin/Documents/rust/tiny_async_func/target/debug/incremental -L dependency=/Users/admin/Documents/rust/tiny_async_func/target/debug/deps --extern trpl=/Users/admin/Documents/rust/tiny_async_func/target/debug/deps/libtrpl-c2f9304df3cb6e09.rlib -L native=/Users/admin/Documents/rust/tiny_async_func/target/debug/build/ring-e0e614109a72a43d/out` (signal: 6, SIGABRT: process abort signal)
>

<backtrace>

Start by reproducing the ICE with the provided async example using cargo run -q, then inspect compiler/rustc_middle/src/mir/mono.rs:49 and the query stack around items_of_instance. The issue is resolved when this input no longer causes rustc to panic while compiling.