rust-lang / rust-lang/rust

[ICE]: `Can't write value with incompatible type &'{erased} mut std::task::Context<'{erased}> to place with type u8`

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C-bug F-async_drop F-coroutines I-ICE needs-triage T-compiler
Dominant language
Rust
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Description

rustc --edition=2024 -Clink-dead-code=true -Zcodegen-backend=cranelift
This triggers an assert inside cranelift, when I remove async_drop feature its fine, so I assume an async_drop bug.

Code
#![feature(coroutines)]
#![feature(async_drop)]
fn main() {
    async {
        #[coroutine]
        static |x: u8| match x {
            y if yield == y => (),
            _ => (),
        };
    };
}
Meta

rustc --version --verbose:

rustc 1.98.0-nightly (6bdf43094 2026-06-01)
binary: rustc
commit-hash: 6bdf43094fae65d298bc430362f116176cd25a3c
commit-date: 2026-06-01
host: x86_64-unknown-linux-gnu
release: 1.98.0-nightly
LLVM version: 22.1.6
Error output
<output>
Backtrace 

warning: the feature `async_drop` is incomplete and may not be safe to use and/or cause compiler crashes
 --> code.rs:2:12
  |
2 | #![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: unused implementer of `Future` that must be used
  --> code.rs:4:5
   |
 4 | /     async {
 5 | |         #[coroutine]
 6 | |         static |x: u8| match x {
 7 | |             y if yield == y => (),
 8 | |             _ => (),
 9 | |         };
10 | |     };
   | |_____^
   |
   = note: futures do nothing unless you `.await` or poll them
   = note: `#[warn(unused_must_use)]` (part of `#[warn(unused)]`) on by default

warning: unused coroutine that must be used
 --> code.rs:6:9
  |
6 | /         static |x: u8| match x {
7 | |             y if yield == y => (),
8 | |             _ => (),
9 | |         };
  | |_________^
  |
  = note: coroutines are lazy and do nothing unless resumed


thread 'rustc' (231802) panicked at /rustc-dev/6bdf43094fae65d298bc430362f116176cd25a3c/compiler/rustc_codegen_cranelift/src/value_and_place.rs:512:9:
assertion `left == right` failed: Can't write value with incompatible type &'{erased} mut std::task::Context<'{erased}> to place with type u8

[Coroutine(DefId(0:5 ~ code[6157]::main::{closure#0}::{closure#0}), [(), u8, (), (), ()]), (), std::future::ResumeTy, (), (), (&'{erased} mut Coroutine(DefId(0:5 ~ code[6157]::main::{closure#0}::{closure#0}), [(), u8, (), (), ()]),)]
[CPlace { inner: Addr(Pointer { base: Stack(ss0), offset: Offset32(0) }, None), layout: TyAndLayout { ty: Poll<()>, layout: Layout { size: Size(1 bytes), align: AbiAlign { abi: Align(1 bytes) }, backend_repr: Scalar(Initialized { value: Int(I8, false), valid_range: 0..=1 }), fields: Arbitrary { offsets: [Size(0 bytes)], in_memory_order: [0] }, largest_niche: Some(Niche { offset: Size(0 bytes), value: Int(I8, false), valid_range: 0..=1 }), uninhabited: false, variants: Multiple { tag: Initialized { value: Int(I8, false), valid_range: 0..=1 }, tag_encoding: Direct, tag_field: 0, variants: [VariantLayout { size: Size(1 bytes), backend_repr: Scalar(Initialized { value: Int(I8, false), valid_range: 0..=1 }), field_offsets: [Size(1 bytes)], fields_in_memory_order: [0], largest_niche: None, uninhabited: false }, VariantLayout { size: Size(1 bytes), backend_repr: Memory { sized: true }, field_offsets: [], fields_in_memory_order: [], largest_niche: None, uninhabited: false }] }, max_repr_align: None, unadjusted_abi_align: Align(1 bytes), randomization_seed: 11768639066024928700 } } }, CPlace { inner: Addr(Pointer { base: Stack(ss1), offset: Offset32(0) }, None), layout: TyAndLayout { ty: Pin<&mut {async fn body of async_drop_in_place<{static coroutine@code.rs:6:9: 6:23}>()}>, layout: Layout { size: Size(8 bytes), align: AbiAlign { abi: Align(8 bytes) }, backend_repr: Scalar(Initialized { value: Pointer(AddressSpace(0)), valid_range: 1..=18446744073709551615 }), fields: Arbitrary { offsets: [Size(0 bytes)], in_memory_order: [0] }, largest_niche: Some(Niche { offset: Size(0 bytes), value: Pointer(AddressSpace(0)), valid_range: 1..=18446744073709551615 }), uninhabited: false, variants: Single { index: 0 }, max_repr_align: None, unadjusted_abi_align: Align(8 bytes), randomization_seed: 7806131535709625041 } } }, CPlace { inner: Var(_2, var1), layout: TyAndLayout { ty: u8, layout: Layout { size: Size(1 bytes), align: AbiAlign { abi: Align(1 bytes) }, backend_repr: Scalar(Initialized { value: Int(I8, false), valid_range: 0..=255 }), fields: Primitive, largest_niche: None, uninhabited: false, variants: Single { index: 0 }, max_repr_align: None, unadjusted_abi_align: Align(1 bytes), randomization_seed: 71776127651151873 } } }]

function u0:6(i64, i64) -> i8 uext system_v {
    ss0 = explicit_slot 1
    ss1 = explicit_slot 8, align = 8

block0(v0: i64, v1: i64):
    nop
    stack_store v0, ss1
}


  left: &'{erased} mut std::task::Context<'{erased}>
 right: u8
stack backtrace:
   0:     0x7fe3f3eb7e69 - <<std[f3c9ac17ed98d4bf]::sys::backtrace::BacktraceLock>::print::DisplayBacktrace as core[38febd7418bd713c]::fmt::Display>::fmt
   1:     0x7fe3f462a4c8 - core[38febd7418bd713c]::fmt::write
   2:     0x7fe3f3ece966 - <std[f3c9ac17ed98d4bf]::sys::stdio::unix::Stderr as std[f3c9ac17ed98d4bf]::io::Write>::write_fmt
   3:     0x7fe3f3e8e09e - std[f3c9ac17ed98d4bf]::panicking::default_hook::{closure#0}
   4:     0x7fe3f3eab7a3 - std[f3c9ac17ed98d4bf]::panicking::default_hook
   5:     0x7fe3f2c97ca1 - std[f3c9ac17ed98d4bf]::panicking::update_hook::<alloc[e58d9d17a66c7a99]::boxed::Box<rustc_driver_impl[8b54d5bb9fab87c3]::install_ice_hook::{closure#1}>>::{closure#0}
   6:     0x7fe3f3eaba82 - std[f3c9ac17ed98d4bf]::panicking::panic_with_hook
   7:     0x7fe3f3e8e162 - std[f3c9ac17ed98d4bf]::panicking::panic_handler::{closure#0}
   8:     0x7fe3f3e82689 - std[f3c9ac17ed98d4bf]::sys::backtrace::__rust_end_short_backtrace::<std[f3c9ac17ed98d4bf]::panicking::panic_handler::{closure#0}, !>
   9:     0x7fe3f3e8fbcd - __rustc[d889b9b15be890d2]::rust_begin_unwind
  10:     0x7fe3f09a1e3c - core[38febd7418bd713c]::panicking::panic_fmt
  11:     0x7fe3f25c0973 - core[38febd7418bd713c]::panicking::assert_failed_inner
  12:     0x7fe3f334b251 - core[38febd7418bd713c]::panicking::assert_failed::<rustc_middle[941705664756e64a]::ty::Ty, rustc_middle[941705664756e64a]::ty::Ty>
  13:     0x7fe3e16a85b1 - rustc_codegen_cranelift[ee78beaea24cf31c]::value_and_place::assert_assignable
  14:     0x7fe3e16b6508 - rustc_codegen_cranelift[ee78beaea24cf31c]::base::codegen_fn_body
  15:     0x7fe3e16e0a29 - rustc_codegen_cranelift[ee78beaea24cf31c]::driver::aot::codegen_cgu_content
  16:     0x7fe3e167c069 - rustc_codegen_cranelift[ee78beaea24cf31c]::driver::aot::run_aot::{closure#3}::{closure#0}::{closure#0}
  17:     0x7fe3e167d8eb - rustc_codegen_cranelift[ee78beaea24cf31c]::driver::aot::run_aot::{closure#3}::{closure#0}
  18:     0x7fe3e16eced9 - <core[38febd7418bd713c]::iter::adapters::filter_map::FilterMap<alloc[e58d9d17a66c7a99]::vec::into_iter::IntoIter<(usize, &rustc_middle[941705664756e64a]::mono::CodegenUnit)>, rustc_data_structures[91b7b8f6cee58889]::sync::parallel::par_map<(usize, &rustc_middle[941705664756e64a]::mono::CodegenUnit), alloc[e58d9d17a66c7a99]::vec::Vec<(usize, &rustc_middle[941705664756e64a]::mono::CodegenUnit)>, rustc_data_structures[91b7b8f6cee58889]::marker::IntoDynSyncSend<rustc_codegen_cranelift[ee78beaea24cf31c]::driver::aot::OngoingModuleCodegen>, alloc[e58d9d17a66c7a99]::vec::Vec<rustc_data_structures[91b7b8f6cee58889]::marker::IntoDynSyncSend<rustc_codegen_cranelift[ee78beaea24cf31c]::driver::aot::OngoingModuleCodegen>>, rustc_codegen_cranelift[ee78beaea24cf31c]::driver::aot::run_aot::{closure#3}::{closure#0}>::{closure#0}::{closure#3}> as core[38febd7418bd713c]::iter::traits::iterator::Iterator>::next
  19:     0x7fe3e16e4f44 - rustc_codegen_cranelift[ee78beaea24cf31c]::driver::aot::run_aot
  20:     0x7fe3e16f6344 - <rustc_codegen_cranelift[ee78beaea24cf31c]::CraneliftCodegenBackend as rustc_codegen_ssa[2461904fbf16def0]::traits::backend::CodegenBackend>::codegen_crate
  21:     0x7fe3f57ba65c - <rustc_interface[e4073cecbcc8453d]::queries::Linker>::codegen_and_build_linker
  22:     0x7fe3f57b4ce9 - rustc_interface[e4073cecbcc8453d]::interface::run_compiler::<(), rustc_driver_impl[8b54d5bb9fab87c3]::run_compiler::{closure#0}>::{closure#1}
  23:     0x7fe3f5761bba - std[f3c9ac17ed98d4bf]::sys::backtrace::__rust_begin_short_backtrace::<rustc_interface[e4073cecbcc8453d]::util::run_in_thread_with_globals<rustc_interface[e4073cecbcc8453d]::util::run_in_thread_pool_with_globals<rustc_interface[e4073cecbcc8453d]::interface::run_compiler<(), rustc_driver_impl[8b54d5bb9fab87c3]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>
  24:     0x7fe3f57622ad - <std[f3c9ac17ed98d4bf]::thread::lifecycle::spawn_unchecked<rustc_interface[e4073cecbcc8453d]::util::run_in_thread_with_globals<rustc_interface[e4073cecbcc8453d]::util::run_in_thread_pool_with_globals<rustc_interface[e4073cecbcc8453d]::interface::run_compiler<(), rustc_driver_impl[8b54d5bb9fab87c3]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>::{closure#1} as core[38febd7418bd713c]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  25:     0x7fe3f57630ac - <std[f3c9ac17ed98d4bf]::sys::thread::unix::Thread>::new::thread_start
  26:     0x7fe3eec981b9 - <unknown>
  27:     0x7fe3eed1d21c - <unknown>
  28:                0x0 - <unknown>

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: please attach the file at `/home/matthias/vcs/github/CRED/rustc-ice-2026-06-03T05_41_54-231792.txt` to your bug report

note: rustc 1.98.0-nightly (6bdf43094 2026-06-01) running on x86_64-unknown-linux-gnu

note: compiler flags: -C link-dead-code=true -Z codegen-backend=cranelift

query stack during panic:
end of query stack
fn std::future::async_drop_in_place::{closure#0}(_1: std::pin::Pin<&mut {async fn body of std::future::async_drop_in_place<{static coroutine@code.rs:6:9: 6:23}>()}>, _2: u8) -> std::task::Poll<()> {
    let mut _0: std::task::Poll<()>;
    scope 1 {
    }

    bb0: {
        _0 = std::task::Poll::<()>::Ready(const ());
        return;
    }
}

warning: 3 warnings emitted

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  3. Fork the repository and make your change on a branch.
  4. Open a pull request that references the issue number.

Research direction

Reproduce the issue with the provided Rust program and compiler flags, then inspect compiler/rustc_codegen_cranelift/src/value_and_place.rs around the assert_assignable failure at line 512. Trace the Cranelift handling of the async_drop coroutine and its incompatible assignment. Done means this reproducer no longer causes the compiler to panic.

Written by the indexing model from the issue text.

Assessment

Tech stack
rust
Domain
compilers
Issue type
Bug
Difficulty
4/5
Estimated time
3-5 days
Activity status
Quiet
Clarity
Mostly clear
Newbie friendliness
45/100

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