rust-lang / rust-lang/rust

[ICE]: Unexpected type {type error} for `Field` projection

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C-bug I-ICE requires-nightly S-bug-has-test S-has-mcve T-compiler
Dominant language
Rust
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Description

auto-reduced (treereduce-rust):

//@compile-flags: --edition=2021 -Zprint-type-sizes
fn bop() {
    fold(run(), |foo: Foo<_>| async { foo.bar })
}
fn fold<Fut, F, U>(_: Foo<U>, f: F) {}
struct Foo<F> {
    bar: (), // Vec<F>
}
fn run() -> Foo<()> {
    todo!()
}

original:

fn bop() {
    fold(run(), |foo: Foo<_>| async { foo.bar })
}
fn fold<Fut, F, U>(_: Foo<U>, f: F)
where
    F: FnMut(Foo<U>) -> Fut,
{
}
struct Foo<F> {
    bar: (), // Vec<F>
}
fn run() -> Foo<()> {
    todo!()
}

fn main() {}

Version information

rustc 1.96.0-nightly (c3bd6289f 2026-04-07)
binary: rustc
commit-hash: c3bd6289f65fa7210c50bdeeb0ed4541caa33d2d
commit-date: 2026-04-07
host: x86_64-unknown-linux-gnu
release: 1.96.0-nightly
LLVM version: 22.1.2

Possibly related line of code:
https://github.com/rust-lang/rust/blob/c3bd6289f65fa7210c50bdeeb0ed4541caa33d2d/compiler/rustc_middle/src/ty/closure.rs#L110-L122

Command:
/home/matthias/.rustup/toolchains/master/bin/rustc --edition=2021 -Zprint-type-sizes

Program output 

error[E0601]: `main` function not found in crate `mvce`
  --> /tmp/icemaker_global_tempdir.pogdqn6QOkB7/rustc_testrunner_tmpdir_reporting.Lry8SoRR7rGu/mvce.rs:10:2
   |
10 | }
   |  ^ consider adding a `main` function to `/tmp/icemaker_global_tempdir.pogdqn6QOkB7/rustc_testrunner_tmpdir_reporting.Lry8SoRR7rGu/mvce.rs`

error[E0392]: type parameter `F` is never used
 --> /tmp/icemaker_global_tempdir.pogdqn6QOkB7/rustc_testrunner_tmpdir_reporting.Lry8SoRR7rGu/mvce.rs:5:12
  |
5 | struct Foo<F> {
  |            ^ unused type parameter
  |
  = help: consider removing `F`, referring to it in a field, or using a marker such as `PhantomData`
  = help: if you intended `F` to be a const parameter, use `const F: /* Type */` instead

error[E0282]: type annotations needed
 --> /tmp/icemaker_global_tempdir.pogdqn6QOkB7/rustc_testrunner_tmpdir_reporting.Lry8SoRR7rGu/mvce.rs:2:5
  |
2 |     fold(run(), |foo: Foo<_>| async { foo.bar })
  |     ^^^^ cannot infer type of the type parameter `Fut` declared on the function `fold`
  |
help: consider specifying the generic arguments
  |
2 |     fold::<Fut, _, U>(run(), |foo: Foo<_>| async { foo.bar })
  |         +++++++++++++

error: internal compiler error: /rustc-dev/c3bd6289f65fa7210c50bdeeb0ed4541caa33d2d/compiler/rustc_middle/src/ty/closure.rs:116:25: Unexpected type {type error} for `Field` projection


thread 'rustc' (2407364) panicked at /rustc-dev/c3bd6289f65fa7210c50bdeeb0ed4541caa33d2d/compiler/rustc_middle/src/ty/closure.rs:116:25:
Box<dyn Any>
stack backtrace:
   0:     0x7efec4bde69b - <<std[12a3dcb3f67af163]::sys::backtrace::BacktraceLock>::print::DisplayBacktrace as core[84cabc7109d7a8ea]::fmt::Display>::fmt
   1:     0x7efec521e548 - core[84cabc7109d7a8ea]::fmt::write
   2:     0x7efec4bf5966 - <std[12a3dcb3f67af163]::sys::stdio::unix::Stderr as std[12a3dcb3f67af163]::io::Write>::write_fmt
   3:     0x7efec4bb4998 - std[12a3dcb3f67af163]::panicking::default_hook::{closure#0}
   4:     0x7efec4bd1d03 - std[12a3dcb3f67af163]::panicking::default_hook
   5:     0x7efec3bd7a8c - std[12a3dcb3f67af163]::panicking::update_hook::<alloc[d34ce8513ce20d35]::boxed::Box<rustc_driver_impl[68e4f83b29a5fde8]::install_ice_hook::{closure#1}>>::{closure#0}
   6:     0x7efec4bd1fe2 - std[12a3dcb3f67af163]::panicking::panic_with_hook
   7:     0x7efec3c07aa1 - std[12a3dcb3f67af163]::panicking::begin_panic::<rustc_errors[426e3927eadecb44]::ExplicitBug>::{closure#0}
   8:     0x7efec3c00506 - std[12a3dcb3f67af163]::sys::backtrace::__rust_end_short_backtrace::<std[12a3dcb3f67af163]::panicking::begin_panic<rustc_errors[426e3927eadecb44]::ExplicitBug>::{closure#0}, !>
   9:     0x7efec3bfe308 - std[12a3dcb3f67af163]::panicking::begin_panic::<rustc_errors[426e3927eadecb44]::ExplicitBug>
  10:     0x7efec3c130e1 - <rustc_errors[426e3927eadecb44]::diagnostic::BugAbort as rustc_errors[426e3927eadecb44]::diagnostic::EmissionGuarantee>::emit_producing_guarantee
  11:     0x7efec41ef919 - rustc_middle[be228d222be6ee40]::util::bug::opt_span_bug_fmt::<rustc_span[2f98085627d930bd]::span_encoding::Span>::{closure#0}
  12:     0x7efec41efaa2 - rustc_middle[be228d222be6ee40]::ty::context::tls::with_opt::<rustc_middle[be228d222be6ee40]::util::bug::opt_span_bug_fmt<rustc_span[2f98085627d930bd]::span_encoding::Span>::{closure#0}, !>::{closure#0}
  13:     0x7efec41e053b - rustc_middle[be228d222be6ee40]::ty::context::tls::with_context_opt::<rustc_middle[be228d222be6ee40]::ty::context::tls::with_opt<rustc_middle[be228d222be6ee40]::util::bug::opt_span_bug_fmt<rustc_span[2f98085627d930bd]::span_encoding::Span>::{closure#0}, !>::{closure#0}, !>
  14:     0x7efec1310634 - rustc_middle[be228d222be6ee40]::util::bug::bug_fmt
  15:     0x7efec5aab1c9 - <rustc_middle[be228d222be6ee40]::ty::closure::CapturedPlace>::to_symbol
  16:     0x7efec61c6430 - rustc_query_impl[fd3cc5250c15dd02]::query_impl::closure_saved_names_of_captured_variables::invoke_provider_fn::__rust_begin_short_backtrace
  17:     0x7efec525fdca - rustc_query_impl[fd3cc5250c15dd02]::execution::try_execute_query::<rustc_middle[be228d222be6ee40]::query::caches::DefIdCache<rustc_middle[be228d222be6ee40]::query::erase::ErasedData<[u8; 8usize]>>, false>
  18:     0x7efec5b3338e - rustc_query_impl[fd3cc5250c15dd02]::query_impl::closure_saved_names_of_captured_variables::execute_query_non_incr::__rust_end_short_backtrace
  19:     0x7efec5bcbd63 - rustc_ty_utils[ffead04fb038f19b]::layout::layout_of
  20:     0x7efec5bcb8d1 - rustc_query_impl[fd3cc5250c15dd02]::query_impl::layout_of::invoke_provider_fn::__rust_begin_short_backtrace
  21:     0x7efec5bca7c7 - rustc_query_impl[fd3cc5250c15dd02]::execution::try_execute_query::<rustc_middle[be228d222be6ee40]::query::caches::DefaultCache<rustc_middle[be228d222be6ee40]::ty::PseudoCanonicalInput<rustc_middle[be228d222be6ee40]::ty::Ty>, rustc_middle[be228d222be6ee40]::query::erase::ErasedData<[u8; 16usize]>>, false>
  22:     0x7efec5bca41e - rustc_query_impl[fd3cc5250c15dd02]::query_impl::layout_of::execute_query_non_incr::__rust_end_short_backtrace
  23:     0x7efec52bb8fb - <rustc_middle[be228d222be6ee40]::ty::context::TyCtxt>::par_hir_body_owners::<rustc_interface[8f905f4a8999be7]::passes::run_required_analyses::{closure#2}::{closure#0}>::{closure#0}
  24:     0x7efec52ba1fc - rustc_interface[8f905f4a8999be7]::passes::analysis
  25:     0x7efec6447a49 - rustc_query_impl[fd3cc5250c15dd02]::execution::try_execute_query::<rustc_middle[be228d222be6ee40]::query::caches::SingleCache<rustc_middle[be228d222be6ee40]::query::erase::ErasedData<[u8; 0usize]>>, false>
  26:     0x7efec6447729 - rustc_query_impl[fd3cc5250c15dd02]::query_impl::analysis::execute_query_non_incr::__rust_end_short_backtrace
  27:     0x7efec645e41e - rustc_interface[8f905f4a8999be7]::interface::run_compiler::<(), rustc_driver_impl[68e4f83b29a5fde8]::run_compiler::{closure#0}>::{closure#1}
  28:     0x7efec6415dfe - std[12a3dcb3f67af163]::sys::backtrace::__rust_begin_short_backtrace::<rustc_interface[8f905f4a8999be7]::util::run_in_thread_with_globals<rustc_interface[8f905f4a8999be7]::util::run_in_thread_pool_with_globals<rustc_interface[8f905f4a8999be7]::interface::run_compiler<(), rustc_driver_impl[68e4f83b29a5fde8]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>
  29:     0x7efec64166a0 - <std[12a3dcb3f67af163]::thread::lifecycle::spawn_unchecked<rustc_interface[8f905f4a8999be7]::util::run_in_thread_with_globals<rustc_interface[8f905f4a8999be7]::util::run_in_thread_pool_with_globals<rustc_interface[8f905f4a8999be7]::interface::run_compiler<(), rustc_driver_impl[68e4f83b29a5fde8]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>::{closure#1} as core[84cabc7109d7a8ea]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  30:     0x7efec64175ac - <std[12a3dcb3f67af163]::sys::thread::unix::Thread>::new::thread_start
  31:     0x7efebfea097a - <unknown>
  32:     0x7efebff242bc - <unknown>
  33:                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 1.96.0-nightly (c3bd6289f 2026-04-07) running on x86_64-unknown-linux-gnu

note: compiler flags: -Z print-type-sizes -Z dump-mir-dir=dir

query stack during panic:
#0 [closure_saved_names_of_captured_variables] computing debuginfo for closure `bop::{closure#0}::{closure#0}`
#1 [layout_of] computing layout of `{async block@/tmp/icemaker_global_tempdir.pogdqn6QOkB7/rustc_testrunner_tmpdir_reporting.Lry8SoRR7rGu/mvce.rs:2:31: 2:36}`
#2 [analysis] running analysis passes on crate `mvce`
end of query stack
warning: unused variable: `f`
 --> /tmp/icemaker_global_tempdir.pogdqn6QOkB7/rustc_testrunner_tmpdir_reporting.Lry8SoRR7rGu/mvce.rs:4:31
  |
4 | fn fold<Fut, F, U>(_: Foo<U>, f: F) {}
  |                               ^ help: if this is intentional, prefix it with an underscore: `_f`
  |
  = note: `#[warn(unused_variables)]` (part of `#[warn(unused)]`) on by default

error: aborting due to 4 previous errors; 1 warning emitted

Some errors have detailed explanations: E0282, E0392, E0601.
For more information about an error, try `rustc --explain E0282`.

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 with compiler/rustc_middle/src/ty/closure.rs around lines 110-122 and reproduce the ICE using the reduced Rust program with -Zprint-type-sizes. Trace how the closure_saved_names_of_captured_variables query handles the Field projection, then verify the reduced case no longer panics and is covered by an appropriate compiler regression test.

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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