rust-lang / rust-lang/rust

[ICE]: expected type for param #1 in [u8, '?0, '?1, (&'?2 u8,)]

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

auto-reduced (treereduce-rust):

#![feature(unboxed_closures)]
#[rustc_paren_sugar]
trait Tr<'a, 'b, T> {
    fn method() {}
}

fn main() {
    <u8 as Tr(&u8)>::method;
}

original:

#![rustc_paren_sugar]
#![feature(unboxed_closures)]

#[rustc_paren_sugar]
trait Tr<'a, 'b, T> {
    type Output = u8;
    fn method() {}
}

impl<T> Tr<(), Output = &u8> for u8 {}

fn main() {
    <u8 as Tr(&u8)>::method; // Sugared, ICE
    <u8 as Tr<(), Output = &u8>>::method; // Desugared, no ICE
}

Version information

rustc 1.96.0-nightly (a0e206b56 2026-03-13)
binary: rustc
commit-hash: a0e206b5665d7c075cdc0ca5f3637a697e8ff55a
commit-date: 2026-03-13
host: x86_64-unknown-linux-gnu
release: 1.96.0-nightly
LLVM version: 22.1.0

Possibly related line of code:
https://github.com/rust-lang/rust/blob/a0e206b5665d7c075cdc0ca5f3637a697e8ff55a/compiler/rustc_middle/src/ty/print/pretty.rs#L3310-L3322

Command:
/home/matthias/.rustup/toolchains/master/bin/rustc

Program output 

error[E0658]: unboxed_closures are still evolving
 --> /tmp/icemaker_global_tempdir.ZbGwHYjJtpvR/rustc_testrunner_tmpdir_reporting.UM5umR6Iy6zW/mvce.rs:1:1
  |
1 | #[rustc_paren_sugar]
  | ^^^^^^^^^^^^^^^^^^^^
  |
  = note: see issue #29625 <https://github.com/rust-lang/rust/issues/29625> for more information
  = help: add `#![feature(unboxed_closures)]` to the crate attributes to enable
  = note: this compiler was built on 2026-03-13; consider upgrading it if it is out of date

error: the `#[rustc_paren_sugar]` attribute is a temporary means of controlling which traits can use parenthetical notation
 --> /tmp/icemaker_global_tempdir.ZbGwHYjJtpvR/rustc_testrunner_tmpdir_reporting.UM5umR6Iy6zW/mvce.rs:2:1
  |
2 | / trait Tr<'a, 'b, T> {
3 | |     fn method() {}
4 | | }
  | |_^
  |
  = help: add `#![feature(unboxed_closures)]` to the crate attributes to use it

error[E0229]: associated item constraints are not allowed here
 --> /tmp/icemaker_global_tempdir.ZbGwHYjJtpvR/rustc_testrunner_tmpdir_reporting.UM5umR6Iy6zW/mvce.rs:7:12
  |
7 |     <u8 as Tr(&u8)>::method;
  |            ^^^^^^^ associated item constraint not allowed here

error: internal compiler error: /rustc-dev/a0e206b5665d7c075cdc0ca5f3637a697e8ff55a/compiler/rustc_middle/src/ty/print/pretty.rs:3316:50: expected type for param #1 in [u8, '?0, '?1, (&'?2 u8,)]


thread 'rustc' (71058) panicked at /rustc-dev/a0e206b5665d7c075cdc0ca5f3637a697e8ff55a/compiler/rustc_middle/src/ty/print/pretty.rs:3316:50:
Box<dyn Any>
stack backtrace:
   0:     0x7fdb9173012b - <<std[f332ca7ac2aab280]::sys::backtrace::BacktraceLock>::print::DisplayBacktrace as core[caaf70a19b46f696]::fmt::Display>::fmt
   1:     0x7fdb91e1ccc8 - core[caaf70a19b46f696]::fmt::write
   2:     0x7fdb91747136 - <std[f332ca7ac2aab280]::sys::stdio::unix::Stderr as std[f332ca7ac2aab280]::io::Write>::write_fmt
   3:     0x7fdb91706218 - std[f332ca7ac2aab280]::panicking::default_hook::{closure#0}
   4:     0x7fdb917235a3 - std[f332ca7ac2aab280]::panicking::default_hook
   5:     0x7fdb9071f9bc - std[f332ca7ac2aab280]::panicking::update_hook::<alloc[4f779a1cfcd17fd2]::boxed::Box<rustc_driver_impl[12052c784fa38642]::install_ice_hook::{closure#1}>>::{closure#0}
   6:     0x7fdb91723882 - std[f332ca7ac2aab280]::panicking::panic_with_hook
   7:     0x7fdb9074f051 - std[f332ca7ac2aab280]::panicking::begin_panic::<rustc_errors[85a4c2af6d2ff10c]::ExplicitBug>::{closure#0}
   8:     0x7fdb90747d36 - std[f332ca7ac2aab280]::sys::backtrace::__rust_end_short_backtrace::<std[f332ca7ac2aab280]::panicking::begin_panic<rustc_errors[85a4c2af6d2ff10c]::ExplicitBug>::{closure#0}, !>
   9:     0x7fdb90747a9b - std[f332ca7ac2aab280]::panicking::begin_panic::<rustc_errors[85a4c2af6d2ff10c]::ExplicitBug>
  10:     0x7fdb9075a6c1 - <rustc_errors[85a4c2af6d2ff10c]::diagnostic::BugAbort as rustc_errors[85a4c2af6d2ff10c]::diagnostic::EmissionGuarantee>::emit_producing_guarantee
  11:     0x7fdb90d37e49 - rustc_middle[1620aa76878d0fde]::util::bug::opt_span_bug_fmt::<rustc_span[5c41c3a85b90e419]::span_encoding::Span>::{closure#0}
  12:     0x7fdb90d37fd2 - rustc_middle[1620aa76878d0fde]::ty::context::tls::with_opt::<rustc_middle[1620aa76878d0fde]::util::bug::opt_span_bug_fmt<rustc_span[5c41c3a85b90e419]::span_encoding::Span>::{closure#0}, !>::{closure#0}
  13:     0x7fdb90d285ab - rustc_middle[1620aa76878d0fde]::ty::context::tls::with_context_opt::<rustc_middle[1620aa76878d0fde]::ty::context::tls::with_opt<rustc_middle[1620aa76878d0fde]::util::bug::opt_span_bug_fmt<rustc_span[5c41c3a85b90e419]::span_encoding::Span>::{closure#0}, !>::{closure#0}, !>
  14:     0x7fdb8effcbe4 - rustc_middle[1620aa76878d0fde]::util::bug::bug_fmt
  15:     0x7fdb90d88da3 - <rustc_middle[1620aa76878d0fde]::ty::print::pretty::TraitRefPrintSugared as rustc_middle[1620aa76878d0fde]::ty::print::Print<rustc_middle[1620aa76878d0fde]::ty::print::pretty::FmtPrinter>>::print
  16:     0x7fdb90d8aa4a - <rustc_middle[1620aa76878d0fde]::ty::print::pretty::TraitRefPrintSugared as core[caaf70a19b46f696]::fmt::Display>::fmt
  17:     0x7fdb914ffb39 - <rustc_trait_selection[9f54bb3d479049cd]::error_reporting::TypeErrCtxt>::on_unimplemented_components
  18:     0x7fdb914fed5d - <rustc_trait_selection[9f54bb3d479049cd]::error_reporting::TypeErrCtxt>::on_unimplemented_note
  19:     0x7fdb915046e7 - <rustc_trait_selection[9f54bb3d479049cd]::error_reporting::TypeErrCtxt>::report_selection_error
  20:     0x7fdb9000b3a6 - <rustc_trait_selection[9f54bb3d479049cd]::error_reporting::TypeErrCtxt>::report_fulfillment_errors
  21:     0x7fdb9309fe7c - rustc_hir_typeck[e7143c23707bf68d]::typeck_with_inspect::{closure#0}
  22:     0x7fdb93095eb0 - rustc_query_impl[c3886bc306851868]::query_impl::typeck::invoke_provider_fn::__rust_begin_short_backtrace
  23:     0x7fdb91f05066 - rustc_query_impl[c3886bc306851868]::execution::try_execute_query::<rustc_data_structures[672205533455fe64]::vec_cache::VecCache<rustc_span[5c41c3a85b90e419]::def_id::LocalDefId, rustc_middle[1620aa76878d0fde]::query::erase::ErasedData<[u8; 8usize]>, rustc_middle[1620aa76878d0fde]::dep_graph::graph::DepNodeIndex>, false>
  24:     0x7fdb91f04a03 - rustc_query_impl[c3886bc306851868]::query_impl::typeck::execute_query_non_incr::__rust_end_short_backtrace
  25:     0x7fdb92884f3e - <rustc_middle[1620aa76878d0fde]::ty::context::TyCtxt>::par_hir_body_owners::<rustc_hir_analysis[4c40a9e8caa59823]::check_crate::{closure#2}>::{closure#0}
  26:     0x7fdb92884993 - rustc_hir_analysis[4c40a9e8caa59823]::check_crate
  27:     0x7fdb91f06413 - rustc_interface[593877ef20606c3d]::passes::analysis
  28:     0x7fdb93071309 - rustc_query_impl[c3886bc306851868]::execution::try_execute_query::<rustc_middle[1620aa76878d0fde]::query::caches::SingleCache<rustc_middle[1620aa76878d0fde]::query::erase::ErasedData<[u8; 0usize]>>, false>
  29:     0x7fdb93070f75 - rustc_query_impl[c3886bc306851868]::query_impl::analysis::execute_query_non_incr::__rust_end_short_backtrace
  30:     0x7fdb93019a08 - rustc_interface[593877ef20606c3d]::interface::run_compiler::<(), rustc_driver_impl[12052c784fa38642]::run_compiler::{closure#0}>::{closure#1}
  31:     0x7fdb9304533e - std[f332ca7ac2aab280]::sys::backtrace::__rust_begin_short_backtrace::<rustc_interface[593877ef20606c3d]::util::run_in_thread_with_globals<rustc_interface[593877ef20606c3d]::util::run_in_thread_pool_with_globals<rustc_interface[593877ef20606c3d]::interface::run_compiler<(), rustc_driver_impl[12052c784fa38642]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>
  32:     0x7fdb93045be0 - <std[f332ca7ac2aab280]::thread::lifecycle::spawn_unchecked<rustc_interface[593877ef20606c3d]::util::run_in_thread_with_globals<rustc_interface[593877ef20606c3d]::util::run_in_thread_pool_with_globals<rustc_interface[593877ef20606c3d]::interface::run_compiler<(), rustc_driver_impl[12052c784fa38642]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>::{closure#1} as core[caaf70a19b46f696]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  33:     0x7fdb93046a6c - <std[f332ca7ac2aab280]::sys::thread::unix::Thread>::new::thread_start
  34:     0x7fdb8caa598b - <unknown>
  35:     0x7fdb8cb29a0c - <unknown>
  36:                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 (a0e206b56 2026-03-13) running on x86_64-unknown-linux-gnu

query stack during panic:
#0 [typeck] type-checking `main`
#1 [analysis] running analysis passes on crate `mvce`
end of query stack
error: aborting due to 4 previous errors

Some errors have detailed explanations: E0229, E0658.
For more information about an error, try `rustc --explain E0229`.

@rustbot label +F-unboxed_closures

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 the reduced example in /tmp/im/a.rs and reproduce the ICE using the nightly rustc version reported. Read compiler/rustc_middle/src/ty/print/pretty.rs around lines 3310-3322, especially the TraitRefPrintSugared path. Done means the example no longer panics in rustc while still producing appropriate diagnostics.

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
Clearly specified
Newbie friendliness
38/100

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