[ICE]: `data should not be frozen if we're still attempting to mutate it`

[matthiaskrgr]

auto-reduced (treereduce-rust):

//@compile-flags: --edition=2024
trait Trait {
    fn bar(&self) {}
}

impl Trait for u8 {}

struct S(u8);

mod to_import {
    pub fn check(arg: &u8) -> &u8 {
        arg
    }
}

impl Trait for S {
    reuse Trait::{foo, bar} {
        use to_import::check;

        let _arr = (async move || {});
        check(&self.0)
    }
}

original:

//@ check-pass

#![feature(fn_delegation)]
#![allow(incomplete_features)]

trait Trait {
    fn foo(&self) {}
    fn bar(&self) {}
}

impl Trait for u8 {}

struct S(u8);

mod to_import {
    pub fn check(arg: &u8) -> &u8 { arg }
}

impl Trait for S {
    reuse Trait::{foo, bar} {
        use to_import::check;

        let _arr = (async move |_a: &'static str| {});
        check(&self.0)
    }
}

fn main() {
    let s = S(0);
    s.foo();
    s.b();
}

Version information

rustc 1.97.0-nightly (532f8f144 2026-04-12)
binary: rustc
commit-hash: 532f8f1443be8f605bcc9da7968e3476d3f27d26
commit-date: 2026-04-12
host: x86_64-unknown-linux-gnu
release: 1.97.0-nightly
LLVM version: 22.1.2

Possibly related line of code:

rust/compiler/rustc_middle/src/ty/context.rs

Lines 1382 to 1394 in 532f8f1

	//
	// Any LocalDefId which is used within queries, either as key or result, either:
	// - has been created before the construction of the TyCtxt;
	// - has been created by this call to `create_def`.
	// As a consequence, this LocalDefId is always re-created before it is needed by the incr.
	// comp. engine itself.
	let def_id = self.untracked.definitions.write().create_def(parent, data, disambiguator);
	// This function modifies `self.definitions` using a side-effect.
	// We need to ensure that these side effects are re-run by the incr. comp. engine.
	// Depending on the forever-red node will tell the graph that the calling query
	// needs to be re-evaluated.
	self.dep_graph.read_index(DepNodeIndex::FOREVER_RED_NODE);

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

Program output

error[E0407]: method `foo` is not a member of trait `Trait`
  --> /tmp/icemaker_global_tempdir.WK6W08sXKaoA/rustc_testrunner_tmpdir_reporting.qmoV0N4O8aRu/mvce.rs:16:19
   |
16 |     reuse Trait::{foo, bar} {
   |                   ^^^ not a member of trait `Trait`

error[E0425]: cannot find function `foo` in trait `Trait`
  --> /tmp/icemaker_global_tempdir.WK6W08sXKaoA/rustc_testrunner_tmpdir_reporting.qmoV0N4O8aRu/mvce.rs:16:19
   |
16 |     reuse Trait::{foo, bar} {
   |                   ^^^ not found in `Trait`

error[E0658]: functions delegation is not yet fully implemented
  --> /tmp/icemaker_global_tempdir.WK6W08sXKaoA/rustc_testrunner_tmpdir_reporting.qmoV0N4O8aRu/mvce.rs:16:5
   |
16 | /     reuse Trait::{foo, bar} {
17 | |         use to_import::check;
18 | |
19 | |         let _arr = (async move || {});
20 | |         check(&self.0)
21 | |     }
   | |_____^
   |
   = note: see issue #118212 <https://github.com/rust-lang/rust/issues/118212> for more information
   = help: add `#![feature(fn_delegation)]` to the crate attributes to enable
   = note: this compiler was built on 2026-04-12; consider upgrading it if it is out of date

warning: unnecessary parentheses around assigned value
  --> /tmp/icemaker_global_tempdir.WK6W08sXKaoA/rustc_testrunner_tmpdir_reporting.qmoV0N4O8aRu/mvce.rs:19:20
   |
19 |         let _arr = (async move || {});
   |                    ^                ^
   |
   = note: `#[warn(unused_parens)]` (part of `#[warn(unused)]`) on by default
help: remove these parentheses
   |
19 -         let _arr = (async move || {});
19 +         let _arr = async move || {} ;
   |

error[E0601]: `main` function not found in crate `mvce`
  --> /tmp/icemaker_global_tempdir.WK6W08sXKaoA/rustc_testrunner_tmpdir_reporting.qmoV0N4O8aRu/mvce.rs:22:2
   |
22 | }
   |  ^ consider adding a `main` function to `/tmp/icemaker_global_tempdir.WK6W08sXKaoA/rustc_testrunner_tmpdir_reporting.qmoV0N4O8aRu/mvce.rs`


thread 'rustc' (4146612) panicked at /rustc-dev/532f8f1443be8f605bcc9da7968e3476d3f27d26/compiler/rustc_middle/src/ty/context.rs:1388:49:
data should not be frozen if we're still attempting to mutate it
stack backtrace:
   0:     0x7feb2c42b31b - <<std[30bf32b9e108fde8]::sys::backtrace::BacktraceLock>::print::DisplayBacktrace as core[afc6995f3fa91ad]::fmt::Display>::fmt
   1:     0x7feb2ca20a48 - core[afc6995f3fa91ad]::fmt::write
   2:     0x7feb2c4425e6 - <std[30bf32b9e108fde8]::sys::stdio::unix::Stderr as std[30bf32b9e108fde8]::io::Write>::write_fmt
   3:     0x7feb2c401618 - std[30bf32b9e108fde8]::panicking::default_hook::{closure#0}
   4:     0x7feb2c41e983 - std[30bf32b9e108fde8]::panicking::default_hook
   5:     0x7feb2b3fb5bc - std[30bf32b9e108fde8]::panicking::update_hook::<alloc[ad9adc906e64577e]::boxed::Box<rustc_driver_impl[56c0fec0d7fc84ae]::install_ice_hook::{closure#1}>>::{closure#0}
   6:     0x7feb2c41ec62 - std[30bf32b9e108fde8]::panicking::panic_with_hook
   7:     0x7feb2c4016d8 - std[30bf32b9e108fde8]::panicking::panic_handler::{closure#0}
   8:     0x7feb2c3f5c39 - std[30bf32b9e108fde8]::sys::backtrace::__rust_end_short_backtrace::<std[30bf32b9e108fde8]::panicking::panic_handler::{closure#0}, !>
   9:     0x7feb2c40313d - __rustc[a6530efb79a2fd0]::rust_begin_unwind
  10:     0x7feb28f5d63c - core[afc6995f3fa91ad]::panicking::panic_fmt
  11:     0x7feb2a34d844 - core[afc6995f3fa91ad]::option::expect_failed
  12:     0x7feb2d0b8b9e - <rustc_middle[5ed6dbbca286abee]::ty::context::TyCtxt>::create_def
  13:     0x7feb2bbfa339 - rustc_mir_transform[120329aa3a4b4f36]::coroutine::by_move_body::coroutine_by_move_body_def_id
  14:     0x7feb2becc04c - rustc_query_impl[c84ee57d90a0448f]::query_impl::coroutine_by_move_body_def_id::invoke_provider_fn::__rust_begin_short_backtrace
  15:     0x7feb2ca142ca - rustc_query_impl[c84ee57d90a0448f]::execution::try_execute_query::<rustc_middle[5ed6dbbca286abee]::query::caches::DefIdCache<rustc_middle[5ed6dbbca286abee]::query::erase::ErasedData<[u8; 8usize]>>, false>
  16:     0x7feb2becc0b2 - rustc_query_impl[c84ee57d90a0448f]::query_impl::coroutine_by_move_body_def_id::execute_query_non_incr::__rust_end_short_backtrace
  17:     0x7feb2d59bbfe - rustc_mir_transform[120329aa3a4b4f36]::mir_promoted
  18:     0x7feb2d599b92 - rustc_query_impl[c84ee57d90a0448f]::query_impl::mir_promoted::invoke_provider_fn::__rust_begin_short_backtrace
  19:     0x7feb2d33ba37 - rustc_query_impl[c84ee57d90a0448f]::execution::try_execute_query::<rustc_data_structures[981e95ba1a8afd5c]::vec_cache::VecCache<rustc_span[786217fdcf1835d0]::def_id::LocalDefId, rustc_middle[5ed6dbbca286abee]::query::erase::ErasedData<[u8; 16usize]>, rustc_middle[5ed6dbbca286abee]::dep_graph::graph::DepNodeIndex>, false>
  20:     0x7feb2d33b5cf - rustc_query_impl[c84ee57d90a0448f]::query_impl::mir_promoted::execute_query_non_incr::__rust_end_short_backtrace
  21:     0x7feb2db3e530 - <rustc_borrowck[4b495eaf82a2de1a]::root_cx::BorrowCheckRootCtxt>::do_mir_borrowck
  22:     0x7feb2db383af - rustc_borrowck[4b495eaf82a2de1a]::mir_borrowck
  23:     0x7feb2db381e5 - rustc_query_impl[c84ee57d90a0448f]::query_impl::mir_borrowck::invoke_provider_fn::__rust_begin_short_backtrace
  24:     0x7feb2cdee466 - rustc_query_impl[c84ee57d90a0448f]::execution::try_execute_query::<rustc_data_structures[981e95ba1a8afd5c]::vec_cache::VecCache<rustc_span[786217fdcf1835d0]::def_id::LocalDefId, rustc_middle[5ed6dbbca286abee]::query::erase::ErasedData<[u8; 8usize]>, rustc_middle[5ed6dbbca286abee]::dep_graph::graph::DepNodeIndex>, false>
  25:     0x7feb2cdedb03 - rustc_query_impl[c84ee57d90a0448f]::query_impl::mir_borrowck::execute_query_non_incr::__rust_end_short_backtrace
  26:     0x7feb2d90e768 - <rustc_middle[5ed6dbbca286abee]::ty::context::TyCtxt>::par_hir_body_owners::<rustc_interface[82dfd3b1fd905165]::passes::run_required_analyses::{closure#2}::{closure#0}>::{closure#0}
  27:     0x7feb2d90dbdd - rustc_interface[82dfd3b1fd905165]::passes::analysis
  28:     0x7feb2dbbe0c9 - rustc_query_impl[c84ee57d90a0448f]::execution::try_execute_query::<rustc_middle[5ed6dbbca286abee]::query::caches::SingleCache<rustc_middle[5ed6dbbca286abee]::query::erase::ErasedData<[u8; 0usize]>>, false>
  29:     0x7feb2dbbdd29 - rustc_query_impl[c84ee57d90a0448f]::query_impl::analysis::execute_query_non_incr::__rust_end_short_backtrace
  30:     0x7feb2db94890 - rustc_interface[82dfd3b1fd905165]::interface::run_compiler::<(), rustc_driver_impl[56c0fec0d7fc84ae]::run_compiler::{closure#0}>::{closure#1}
  31:     0x7feb2db7e8be - std[30bf32b9e108fde8]::sys::backtrace::__rust_begin_short_backtrace::<rustc_interface[82dfd3b1fd905165]::util::run_in_thread_with_globals<rustc_interface[82dfd3b1fd905165]::util::run_in_thread_pool_with_globals<rustc_interface[82dfd3b1fd905165]::interface::run_compiler<(), rustc_driver_impl[56c0fec0d7fc84ae]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>
  32:     0x7feb2db7f160 - <std[30bf32b9e108fde8]::thread::lifecycle::spawn_unchecked<rustc_interface[82dfd3b1fd905165]::util::run_in_thread_with_globals<rustc_interface[82dfd3b1fd905165]::util::run_in_thread_pool_with_globals<rustc_interface[82dfd3b1fd905165]::interface::run_compiler<(), rustc_driver_impl[56c0fec0d7fc84ae]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>::{closure#1} as core[afc6995f3fa91ad]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  33:     0x7feb2db7ffec - <std[30bf32b9e108fde8]::sys::thread::unix::Thread>::new::thread_start
  34:     0x7feb276a597a - <unknown>
  35:     0x7feb277292bc - <unknown>
  36:                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: rustc 1.97.0-nightly (532f8f144 2026-04-12) running on x86_64-unknown-linux-gnu

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

query stack during panic:
#0 [coroutine_by_move_body_def_id] looking up the coroutine by-move body for `<impl at /tmp/icemaker_global_tempdir.WK6W08sXKaoA/rustc_testrunner_tmpdir_reporting.qmoV0N4O8aRu/mvce.rs:15:1: 15:17>::bar::{closure#0}::{closure#0}`
#1 [mir_promoted] promoting constants in MIR for `<impl at /tmp/icemaker_global_tempdir.WK6W08sXKaoA/rustc_testrunner_tmpdir_reporting.qmoV0N4O8aRu/mvce.rs:15:1: 15:17>::bar::{closure#0}::{closure#0}`
#2 [mir_borrowck] borrow-checking `<impl at /tmp/icemaker_global_tempdir.WK6W08sXKaoA/rustc_testrunner_tmpdir_reporting.qmoV0N4O8aRu/mvce.rs:15:1: 15:17>::bar`
#3 [analysis] running analysis passes on crate `mvce`
end of query stack
error: aborting due to 4 previous errors; 1 warning emitted

Some errors have detailed explanations: E0407, E0425, E0601, E0658.
For more information about an error, try `rustc --explain E0407`.

@rustbot label +F-fn_delegation

[matthiaskrgr]

smaller

#![feature(fn_delegation)]
trait Trait {
    fn bar(&self) {}
}
impl Trait for u8 {}
struct S;
pub fn check() -> u8 {}

impl S {
    reuse Trait::bar {
        use check;
        async || {};
        check()
    }
}

fn main() {}

[matthiaskrgr]

even smaller

#![feature(fn_delegation)]
trait Trait {
    fn bar(self);
}
impl Trait for () {
    reuse Trait::bar {
        async || {};
    }
}

fn main() {}