rust-lang / rust-lang/rust

ICE: Unsize coercion, but `std::pin::Pin<std::boxed::Box<{async block ..` isn't coercible

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-Zvalidate-mir C-bug I-ICE S-bug-has-test S-has-mcve T-compiler
Dominant language
Rust
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Description

auto-reduced (treereduce-rust):

//@compile-flags: --edition=2024 -Zvalidate-mir
use ::core::pin::Pin;

trait Foo {
    fn bar<'me, 'async_trait, T: Send>(x: &'me T) -> Pin<Box<dyn Future<Output = ()> + Send>>
    where
        'me: 'async_trait;
}

impl Foo for () {
    fn bar<'me, 'async_trait, T: Send>(x: &'me T) -> Pin<Box<dyn Future<Output = ()> + Send>>
    where
        'me: 'async_trait,
    {
        Box::pin(async move {
            let x = x;
        })
    }
}

fn main() {}

original:

//@ edition:2018

use ::core::pin::Pin;
use ::core::future::Future;
use ::core::marker::Send;

trait Foo {
    fn bar<'me, 'async_trait, T: Send>(x: &'me T)
        -> Pin<Box<dyn Future<Output = ()> + Send + 'async_trait>>
        where 'me: 'async_trait;
}

impl Foo for () {
    fn bar<'me, 'async_trait, T: Send>(x: &'me T)
        -> Pin<Box<dyn Future<Output = ()> + Send + 'async_trait>>
        where 'me:'async_trait {
            Box::pin( //@ edition:2018
                async move {
                    let x = x;
                }
            )
         }
}

fn main() { }

Version information

rustc 1.91.0-nightly (71289c378 2025-09-04)
binary: rustc
commit-hash: 71289c378d0a406a4f537fe4001282d19362931f
commit-date: 2025-09-04
host: x86_64-unknown-linux-gnu
release: 1.91.0-nightly
LLVM version: 21.1.0

Possibly related line of code:
https://github.com/rust-lang/rust/blob/71289c378d0a406a4f537fe4001282d19362931f/compiler/rustc_mir_transform/src/validate.rs#L74-L86

Command:
/home/matthias/.rustup/toolchains/master/bin/rustc --edition=2024 -Zvalidate-mir

Program output 

warning: unused variable: `x`
  --> /tmp/icemaker_global_tempdir.WalRtMxhm63X/rustc_testrunner_tmpdir_reporting.WqFA43bpg2gM/mvce.rs:15:17
   |
15 |             let x = x;
   |                 ^ help: if this is intentional, prefix it with an underscore: `_x`
   |
   = note: `#[warn(unused_variables)]` (part of `#[warn(unused)]`) on by default

error: internal compiler error: compiler/rustc_mir_transform/src/validate.rs:80:25: broken MIR in Item(DefId(0:10 ~ mvce[5d7d]::{impl#0}::bar)) (after pass CheckForceInline) at bb1[0]:
                                Unsize coercion, but `std::pin::Pin<std::boxed::Box<{async block@/tmp/icemaker_global_tempdir.WalRtMxhm63X/rustc_testrunner_tmpdir_reporting.WqFA43bpg2gM/mvce.rs:14:18: 14:28}>>` isn't coercible to `std::pin::Pin<std::boxed::Box<dyn std::future::Future<Output = ()> + std::marker::Send>>`
  --> /tmp/icemaker_global_tempdir.WalRtMxhm63X/rustc_testrunner_tmpdir_reporting.WqFA43bpg2gM/mvce.rs:14:9
   |
14 | /         Box::pin(async move {
15 | |             let x = x;
16 | |         })
   | |__________^


thread 'rustc' (1981516) panicked at compiler/rustc_mir_transform/src/validate.rs:80:25:
Box<dyn Any>
stack backtrace:
   0:     0x7f15a0c75fd2 - <std::sys::backtrace::BacktraceLock::print::DisplayBacktrace as core::fmt::Display>::fmt::h74535c69a9a8851c
   1:     0x7f15a140c57c - core::fmt::write::h617ef057c538ffbe
   2:     0x7f15a0c2a251 - std::io::Write::write_fmt::haeef701e34a0f1ce
   3:     0x7f15a0c3b8d2 - std::sys::backtrace::BacktraceLock::print::hc779bb6277bee776
   4:     0x7f15a0c41ba7 - std::panicking::default_hook::{{closure}}::h375a85ea81672879
   5:     0x7f15a0c416b3 - std::panicking::default_hook::h6cf8c92021730d42
   6:     0x7f159fc63691 - std[a85058167e77f399]::panicking::update_hook::<alloc[f3ac3bcb91ff4184]::boxed::Box<rustc_driver_impl[8ae33f5ddd6f22c2]::install_ice_hook::{closure#1}>>::{closure#0}
   7:     0x7f15a0c420af - std::panicking::panic_with_hook::hb1f4786dce1429bc
   8:     0x7f159fca0fe1 - std[a85058167e77f399]::panicking::begin_panic::<rustc_errors[55184e806260cfe4]::ExplicitBug>::{closure#0}
   9:     0x7f159fc95326 - std[a85058167e77f399]::sys::backtrace::__rust_end_short_backtrace::<std[a85058167e77f399]::panicking::begin_panic<rustc_errors[55184e806260cfe4]::ExplicitBug>::{closure#0}, !>
  10:     0x7f159fc9469e - std[a85058167e77f399]::panicking::begin_panic::<rustc_errors[55184e806260cfe4]::ExplicitBug>
  11:     0x7f159fcacc01 - <rustc_errors[55184e806260cfe4]::diagnostic::BugAbort as rustc_errors[55184e806260cfe4]::diagnostic::EmissionGuarantee>::emit_producing_guarantee
  12:     0x7f15a022e5dc - <rustc_errors[55184e806260cfe4]::DiagCtxtHandle>::span_bug::<rustc_span[e82c85b666ab7901]::span_encoding::Span, alloc[f3ac3bcb91ff4184]::string::String>
  13:     0x7f15a0255757 - rustc_middle[5ded7ef5e9c99589]::util::bug::opt_span_bug_fmt::<rustc_span[e82c85b666ab7901]::span_encoding::Span>::{closure#0}
  14:     0x7f15a025594a - rustc_middle[5ded7ef5e9c99589]::ty::context::tls::with_opt::<rustc_middle[5ded7ef5e9c99589]::util::bug::opt_span_bug_fmt<rustc_span[e82c85b666ab7901]::span_encoding::Span>::{closure#0}, !>::{closure#0}
  15:     0x7f15a024407b - rustc_middle[5ded7ef5e9c99589]::ty::context::tls::with_context_opt::<rustc_middle[5ded7ef5e9c99589]::ty::context::tls::with_opt<rustc_middle[5ded7ef5e9c99589]::util::bug::opt_span_bug_fmt<rustc_span[e82c85b666ab7901]::span_encoding::Span>::{closure#0}, !>::{closure#0}, !>
  16:     0x7f159e102f67 - rustc_middle[5ded7ef5e9c99589]::util::bug::span_bug_fmt::<rustc_span[e82c85b666ab7901]::span_encoding::Span>
  17:     0x7f159ebfe16f - <rustc_mir_transform[f939db7989ace53a]::validate::CfgChecker>::fail::<alloc[f3ac3bcb91ff4184]::string::String>
  18:     0x7f15a20fb526 - <rustc_mir_transform[f939db7989ace53a]::validate::Validator as rustc_mir_transform[f939db7989ace53a]::pass_manager::MirPass>::run_pass
  19:     0x7f159ec2f095 - rustc_mir_transform[f939db7989ace53a]::pass_manager::validate_body
  20:     0x7f15a1407136 - rustc_query_impl[9354df03bc15753a]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[9354df03bc15753a]::query_impl::mir_built::dynamic_query::{closure#2}::{closure#0}, rustc_middle[5ded7ef5e9c99589]::query::erase::Erased<[u8; 8usize]>>
  21:     0x7f15a15f8f05 - rustc_query_system[58e4f3e481076de2]::query::plumbing::try_execute_query::<rustc_query_impl[9354df03bc15753a]::DynamicConfig<rustc_data_structures[59c3a3e48a0cb88]::vec_cache::VecCache<rustc_span[e82c85b666ab7901]::def_id::LocalDefId, rustc_middle[5ded7ef5e9c99589]::query::erase::Erased<[u8; 8usize]>, rustc_query_system[58e4f3e481076de2]::dep_graph::graph::DepNodeIndex>, false, false, false>, rustc_query_impl[9354df03bc15753a]::plumbing::QueryCtxt, false>
  22:     0x7f15a15f8a09 - rustc_query_impl[9354df03bc15753a]::query_impl::mir_built::get_query_non_incr::__rust_end_short_backtrace
  23:     0x7f15a2030eda - rustc_mir_build[f65013b02413be2b]::check_unsafety::check_unsafety
  24:     0x7f15a2030ae7 - rustc_query_impl[9354df03bc15753a]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[9354df03bc15753a]::query_impl::check_unsafety::dynamic_query::{closure#2}::{closure#0}, rustc_middle[5ded7ef5e9c99589]::query::erase::Erased<[u8; 0usize]>>
  25:     0x7f15a15f125b - rustc_query_system[58e4f3e481076de2]::query::plumbing::try_execute_query::<rustc_query_impl[9354df03bc15753a]::DynamicConfig<rustc_data_structures[59c3a3e48a0cb88]::vec_cache::VecCache<rustc_span[e82c85b666ab7901]::def_id::LocalDefId, rustc_middle[5ded7ef5e9c99589]::query::erase::Erased<[u8; 0usize]>, rustc_query_system[58e4f3e481076de2]::dep_graph::graph::DepNodeIndex>, false, false, false>, rustc_query_impl[9354df03bc15753a]::plumbing::QueryCtxt, false>
  26:     0x7f15a15f0e8b - rustc_query_impl[9354df03bc15753a]::query_impl::check_unsafety::get_query_non_incr::__rust_end_short_backtrace
  27:     0x7f15a15f0373 - <rustc_middle[5ded7ef5e9c99589]::ty::context::TyCtxt>::par_hir_body_owners::<rustc_interface[b9c81810830046cc]::passes::run_required_analyses::{closure#1}::{closure#0}>::{closure#0}
  28:     0x7f15a15edcdb - rustc_interface[b9c81810830046cc]::passes::analysis
  29:     0x7f15a15ed18d - rustc_query_impl[9354df03bc15753a]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[9354df03bc15753a]::query_impl::analysis::dynamic_query::{closure#2}::{closure#0}, rustc_middle[5ded7ef5e9c99589]::query::erase::Erased<[u8; 0usize]>>
  30:     0x7f15a24b9920 - rustc_query_system[58e4f3e481076de2]::query::plumbing::try_execute_query::<rustc_query_impl[9354df03bc15753a]::DynamicConfig<rustc_query_system[58e4f3e481076de2]::query::caches::SingleCache<rustc_middle[5ded7ef5e9c99589]::query::erase::Erased<[u8; 0usize]>>, false, false, false>, rustc_query_impl[9354df03bc15753a]::plumbing::QueryCtxt, false>
  31:     0x7f15a24b9576 - rustc_query_impl[9354df03bc15753a]::query_impl::analysis::get_query_non_incr::__rust_end_short_backtrace
  32:     0x7f15a27c1576 - rustc_interface[b9c81810830046cc]::passes::create_and_enter_global_ctxt::<core[388173e3ebade246]::option::Option<rustc_interface[b9c81810830046cc]::queries::Linker>, rustc_driver_impl[8ae33f5ddd6f22c2]::run_compiler::{closure#0}::{closure#2}>::{closure#2}::{closure#0}
  33:     0x7f15a26a3f85 - rustc_interface[b9c81810830046cc]::interface::run_compiler::<(), rustc_driver_impl[8ae33f5ddd6f22c2]::run_compiler::{closure#0}>::{closure#1}
  34:     0x7f15a259f403 - std[a85058167e77f399]::sys::backtrace::__rust_begin_short_backtrace::<rustc_interface[b9c81810830046cc]::util::run_in_thread_with_globals<rustc_interface[b9c81810830046cc]::util::run_in_thread_pool_with_globals<rustc_interface[b9c81810830046cc]::interface::run_compiler<(), rustc_driver_impl[8ae33f5ddd6f22c2]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>
  35:     0x7f15a259f0c8 - <<std[a85058167e77f399]::thread::Builder>::spawn_unchecked_<rustc_interface[b9c81810830046cc]::util::run_in_thread_with_globals<rustc_interface[b9c81810830046cc]::util::run_in_thread_pool_with_globals<rustc_interface[b9c81810830046cc]::interface::run_compiler<(), rustc_driver_impl[8ae33f5ddd6f22c2]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>::{closure#1} as core[388173e3ebade246]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  36:     0x7f15a25a55ef - std::sys::pal::unix::thread::Thread::new::thread_start::hf0c6c3cf2c1ee824
  37:     0x7f159bc969cb - <unknown>
  38:     0x7f159bd1aa0c - <unknown>
  39:                0x0 - <unknown>

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 1.91.0-nightly (71289c378 2025-09-04) running on x86_64-unknown-linux-gnu

note: compiler flags: -Z validate-mir -Z dump-mir-dir=dir

query stack during panic:
#0 [mir_built] building MIR for `<impl at /tmp/icemaker_global_tempdir.WalRtMxhm63X/rustc_testrunner_tmpdir_reporting.WqFA43bpg2gM/mvce.rs:9:1: 9:16>::bar`
#1 [check_unsafety] unsafety-checking `<impl at /tmp/icemaker_global_tempdir.WalRtMxhm63X/rustc_testrunner_tmpdir_reporting.WqFA43bpg2gM/mvce.rs:9:1: 9:16>::bar`
#2 [analysis] running analysis passes on this crate
end of query stack
error: aborting due to 1 previous error; 1 warning emitted

Contributor guide

Open the contributing guide

First steps

  1. Read the whole issue, then the project's contributing guide.
  2. Comment on the issue to say you are picking it up — it saves two people doing the same work.
  3. Fork the repository and make your change on a branch.
  4. Open a pull request that references the issue number.

Research direction

Start by compiling the reduced Rust program with --edition=2024 -Zvalidate-mir and reproduce the ICE. Then inspect compiler/rustc_mir_transform/src/validate.rs around lines 74-86 and the reported Unsize coercion validation failure. Done means the reproducer no longer triggers an internal compiler error.

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
Stale
Clarity
Mostly clear
Newbie friendliness
42/100

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