ICE eii: multiple annotations

[matthiaskrgr]

snippet:

#[eii(eii1)]
fn a() {}

#[eii(eii1)]
fn main() {}

Version information

rustc 1.94.0-nightly (3f4dc1e02 2025-12-14)
binary: rustc
commit-hash: 3f4dc1e02d759aa3c3807d4efc1f7f6e293536a5
commit-date: 2025-12-14
host: x86_64-unknown-linux-gnu
release: 1.94.0-nightly
LLVM version: 21.1.5

Possibly related line of code:

rust/compiler/rustc_passes/src/eii.rs

Lines 113 to 125 in 3f4dc1e

	.collect::<Vec<_>>()
	.join(", "),
	});
	}
	if default_impls.len() > 1 {
	panic!("multiple not supported right now");
	}
	let (local_impl, is_default) =
	// note, for a single crate we never need to generate both a default and an explicit implementation.
	// In that case, generating the explicit implementation is enough!
	match (checking_mode, explicit_impls.first(), default_impls.first()) {

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

Program output

error[E0658]: use of unstable library feature `extern_item_impls`
 --> /tmp/icemaker_global_tempdir.H7kAaxdihhDG/rustc_testrunner_tmpdir_reporting.mZz7YiFX9vOj/mvce.rs:1:3
 |
1 | #[eii(eii1)]
 | ^^^
 |
 = note: see issue #125418 <https://github.com/rust-lang/rust/issues/125418> for more information
 = help: add `#![feature(extern_item_impls)]` to the crate attributes to enable
 = note: this compiler was built on 2025-12-14; consider upgrading it if it is out of date

error[E0658]: use of unstable library feature `extern_item_impls`
 --> /tmp/icemaker_global_tempdir.H7kAaxdihhDG/rustc_testrunner_tmpdir_reporting.mZz7YiFX9vOj/mvce.rs:4:3
 |
4 | #[eii(eii1)]
 | ^^^
 |
 = note: see issue #125418 <https://github.com/rust-lang/rust/issues/125418> for more information
 = help: add `#![feature(extern_item_impls)]` to the crate attributes to enable
 = note: this compiler was built on 2025-12-14; consider upgrading it if it is out of date

error[E0428]: the name `eii1` is defined multiple times
 --> /tmp/icemaker_global_tempdir.H7kAaxdihhDG/rustc_testrunner_tmpdir_reporting.mZz7YiFX9vOj/mvce.rs:4:1
 |
1 | #[eii(eii1)]
 | ------------ previous definition of the macro `eii1` here
...
4 | #[eii(eii1)]
 | ^^^^^^^^^^^^ `eii1` redefined here
 |
 = note: `eii1` must be defined only once in the macro namespace of this module

error: the `main` function cannot be declared in an `extern` block
 --> /tmp/icemaker_global_tempdir.H7kAaxdihhDG/rustc_testrunner_tmpdir_reporting.mZz7YiFX9vOj/mvce.rs:5:1
 |
5 | fn main(){}
 | ^^^^^^^^^


thread 'rustc' (3711041) panicked at compiler/rustc_passes/src/eii.rs:119:13:
multiple not supported right now
stack backtrace:
 0: 0x7f3299790173 - <<std[e49fa185a119b89b]::sys::backtrace::BacktraceLock>::print::DisplayBacktrace as core[17865207963548b5]::fmt::Display>::fmt
 1: 0x7f3299e10648 - core[17865207963548b5]::fmt::write
 2: 0x7f32997a67c6 - <std[e49fa185a119b89b]::sys::stdio::unix::Stderr as std[e49fa185a119b89b]::io::Write>::write_fmt
 3: 0x7f3299766938 - std[e49fa185a119b89b]::panicking::default_hook::{closure#0}
 4: 0x7f3299784043 - std[e49fa185a119b89b]::panicking::default_hook
 5: 0x7f32987741d7 - std[e49fa185a119b89b]::panicking::update_hook::<alloc[756b9f94e2773738]::boxed::Box<rustc_driver_impl[e1c4671be5271ce8]::install_ice_hook::{closure#1}>>::{closure#0}
 6: 0x7f3299784322 - std[e49fa185a119b89b]::panicking::panic_with_hook
 7: 0x7f3299766a2a - std[e49fa185a119b89b]::panicking::panic_handler::{closure#0}
 8: 0x7f329975dab9 - std[e49fa185a119b89b]::sys::backtrace::__rust_end_short_backtrace::<std[e49fa185a119b89b]::panicking::panic_handler::{closure#0}, !>
 9: 0x7f32997685ad - __rustc[df3bb69314ad135f]::rust_begin_unwind
 10: 0x7f3296a6df0c - core[17865207963548b5]::panicking::panic_fmt
 11: 0x7f329a2d0c4c - rustc_passes[9766a5655bde9e6]::eii::check_externally_implementable_items
 12: 0x7f329a2d022f - rustc_query_impl[462c6a5584cea8d4]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[462c6a5584cea8d4]::query_impl::check_externally_implementable_items::dynamic_query::{closure#2}::{closure#0}, rustc_middle[7055997c00c0bb98]::query::erase::Erased<[u8; 0usize]>>
 13: 0x7f329af725a6 - rustc_query_system[5be0c069f6d5c539]::query::plumbing::try_execute_query::<rustc_query_impl[462c6a5584cea8d4]::DynamicConfig<rustc_query_system[5be0c069f6d5c539]::query::caches::SingleCache<rustc_middle[7055997c00c0bb98]::query::erase::Erased<[u8; 0usize]>>, false, false, false>, rustc_query_impl[462c6a5584cea8d4]::plumbing::QueryCtxt, false>
 14: 0x7f329af7238e - rustc_query_impl[462c6a5584cea8d4]::query_impl::check_externally_implementable_items::get_query_non_incr::__rust_end_short_backtrace
 15: 0x7f3299eec481 - rustc_interface[50af632e35c6bce3]::passes::analysis
 16: 0x7f3299eec2e5 - rustc_query_impl[462c6a5584cea8d4]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[462c6a5584cea8d4]::query_impl::analysis::dynamic_query::{closure#2}::{closure#0}, rustc_middle[7055997c00c0bb98]::query::erase::Erased<[u8; 0usize]>>
 17: 0x7f329af725a6 - rustc_query_system[5be0c069f6d5c539]::query::plumbing::try_execute_query::<rustc_query_impl[462c6a5584cea8d4]::DynamicConfig<rustc_query_system[5be0c069f6d5c539]::query::caches::SingleCache<rustc_middle[7055997c00c0bb98]::query::erase::Erased<[u8; 0usize]>>, false, false, false>, rustc_query_impl[462c6a5584cea8d4]::plumbing::QueryCtxt, false>
 18: 0x7f329af72176 - rustc_query_impl[462c6a5584cea8d4]::query_impl::analysis::get_query_non_incr::__rust_end_short_backtrace
 19: 0x7f329b12cdd4 - <rustc_interface[50af632e35c6bce3]::passes::create_and_enter_global_ctxt<core[17865207963548b5]::option::Option<rustc_interface[50af632e35c6bce3]::queries::Linker>, rustc_driver_impl[e1c4671be5271ce8]::run_compiler::{closure#0}::{closure#2}>::{closure#2} as core[17865207963548b5]::ops::function::FnOnce<(&rustc_session[307f8e7a4263bd4]::session::Session, rustc_middle[7055997c00c0bb98]::ty::context::CurrentGcx, alloc[756b9f94e2773738]::sync::Arc<rustc_data_structures[30161969bbcb553f]::jobserver::Proxy>, &std[e49fa185a119b89b]::sync::once_lock::OnceLock<rustc_middle[7055997c00c0bb98]::ty::context::GlobalCtxt>, &rustc_data_structures[30161969bbcb553f]::sync::worker_local::WorkerLocal<rustc_middle[7055997c00c0bb98]::arena::Arena>, &rustc_data_structures[30161969bbcb553f]::sync::worker_local::WorkerLocal<rustc_hir[5d00f225b65c0257]::Arena>, rustc_driver_impl[e1c4671be5271ce8]::run_compiler::{closure#0}::{closure#2})>>::call_once::{shim:vtable#0}
 20: 0x7f329afc19da - rustc_interface[50af632e35c6bce3]::interface::run_compiler::<(), rustc_driver_impl[e1c4671be5271ce8]::run_compiler::{closure#0}>::{closure#1}
 21: 0x7f329af31d8e - std[e49fa185a119b89b]::sys::backtrace::__rust_begin_short_backtrace::<rustc_interface[50af632e35c6bce3]::util::run_in_thread_with_globals<rustc_interface[50af632e35c6bce3]::util::run_in_thread_pool_with_globals<rustc_interface[50af632e35c6bce3]::interface::run_compiler<(), rustc_driver_impl[e1c4671be5271ce8]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>
 22: 0x7f329af31b60 - <std[e49fa185a119b89b]::thread::lifecycle::spawn_unchecked<rustc_interface[50af632e35c6bce3]::util::run_in_thread_with_globals<rustc_interface[50af632e35c6bce3]::util::run_in_thread_pool_with_globals<rustc_interface[50af632e35c6bce3]::interface::run_compiler<(), rustc_driver_impl[e1c4671be5271ce8]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>::{closure#1} as core[17865207963548b5]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
 23: 0x7f329af374f8 - <std[e49fa185a119b89b]::sys::thread::unix::Thread>::new::thread_start
 24: 0x7f32948969cb - <unknown>
 25: 0x7f329491aa0c - <unknown>
 26: 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 1.94.0-nightly (3f4dc1e02 2025-12-14) running on x86_64-unknown-linux-gnu

query stack during panic:
#0 [check_externally_implementable_items] check externally implementable items
#1 [analysis] running analysis passes on crate `mvce`
end of query stack
error: aborting due to 4 previous errors

Some errors have detailed explanations: E0428, E0658.
For more information about an error, try `rustc --explain E0428`.

[jdonszelmann]

@matthiaskrgr be aware that eiis is a highly experimental feature, potentially not ready for fuzzing. If you find any issues with it, please assign me so I'm aware of them. I'm happy to get the feedback, but the ices likely aren't mistakes, they're just not finished.