ICE: delegation: index out of bounds

[matthiaskrgr]

auto-reduced (treereduce-rust):

mod inner {
    pub trait TraitFoo {
        fn foo(&self) -> u8;
    }
    

    
    
}

trait Trait {
    fn foo<T>();
    
}

impl Trait for u8 {
    reuse inner::TraitFoo::*;
    ;
}

original:

//@ check-pass

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

mod inner {
    pub trait TraitFoo {
        fn foo(&self) -> u8;
    }
    pub trait TraitBar {
        fn bar(&self) -> u8;
    }

    impl TraitFoo for u8 {
        fn foo(&self) -> u8 { 0 }
    }
    impl TraitBar for u8 {
        fn bar(&self) -> u8 { 1 }
    }
}

trait Trait {
    fn foo<T>();
    fn bar(&self) -> u8;
}

impl Trait for u8 {
    reuse inner::TraitFoo::*;
    reuse inner::TraitBar::*;
}

fn main() {
    let u = 0u8;
    u.foo();
    u.bar();
}

Version information

rustc 1.87.0-nightly (ebf0cf75d 2025-03-11)
binary: rustc
commit-hash: ebf0cf75d368c035f4c7e7246d203bd469ee4a51
commit-date: 2025-03-11
host: x86_64-unknown-linux-gnu
release: 1.87.0-nightly
LLVM version: 20.1.0

Possibly related line of code:

rust/compiler/rustc_ast_lowering/src/delegation.rs

Lines 341 to 353 in ebf0cf7

	None,
	);
	let segment = self.arena.alloc(segment);
	self.arena.alloc(hir::Expr {
	hir_id: self.next_id(),
	kind: hir::ExprKind::MethodCall(segment, &args[0], &args[1..], span),
	span,
	})
	} else {
	let path = self.lower_qpath(
	delegation.id,
	&delegation.qself,

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

Program output

error: non-item in item list
  --> /tmp/icemaker_global_tempdir.vsOshhe321Ji/rustc_testrunner_tmpdir_reporting.25P4hBretFbI/mvce.rs:23:5
   |
21 | impl Trait for u8 {
   |                   - item list starts here
22 |     reuse inner::TraitFoo::*;
23 |     ;
   |     ^ non-item starts here
24 | }
   | - item list ends here

error[E0658]: functions delegation is not yet fully implemented
  --> /tmp/icemaker_global_tempdir.vsOshhe321Ji/rustc_testrunner_tmpdir_reporting.25P4hBretFbI/mvce.rs:22:5
   |
22 |     reuse inner::TraitFoo::*;
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = 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   = note: this compiler was built on 2025-03-11; consider upgrading it if it is out of date


thread 'rustc' panicked at compiler/rustc_ast_lowering/src/delegation.rs:347:59:
index out of bounds: the len is 0 but the index is 0
stack backtrace:
   0:     0x798183825b33 - <std::sys::backtrace::BacktraceLock::print::DisplayBacktrace as core::fmt::Display>::fmt::h7fb80f6afbe41a33
   1:     0x7981840059d8 - core::fmt::write::h000527be0eea6fe8
   2:     0x79818544ffd1 - std::io::Write::write_fmt::h4991a22851475fb5
   3:     0x798183825992 - std::sys::backtrace::BacktraceLock::print::hffad87dde00cbe3e
   4:     0x7981838282b8 - std::panicking::default_hook::{{closure}}::ha46397973bec1239
   5:     0x798183827dd7 - std::panicking::default_hook::h706de72a1550eed4
   6:     0x798182893abf - std[ea0914bf68e3efbf]::panicking::update_hook::<alloc[3dfe61f71ba45cfe]::boxed::Box<rustc_driver_impl[23cd132fe59c5dfc]::install_ice_hook::{closure#1}>>::{closure#0}
   7:     0x798183828b23 - std::panicking::rust_panic_with_hook::h6666776865bd21f5
   8:     0x79818382881a - std::panicking::begin_panic_handler::{{closure}}::hb8595612365d24db
   9:     0x798183825ff9 - std::sys::backtrace::__rust_end_short_backtrace::h7037f177c7ae4ed3
  10:     0x7981838284dd - rust_begin_unwind
  11:     0x798180270ea0 - core::panicking::panic_fmt::hd26142fa9828e2a3
  12:     0x7981819eb70c - core::panicking::panic_bounds_check::hd986af7ff0a8c80a
  13:     0x79818247dda3 - <rustc_ast_lowering[e55d79e744b523df]::LoweringContext>::lower_delegation
  14:     0x7981847b3538 - <rustc_ast_lowering[e55d79e744b523df]::LoweringContext>::lower_assoc_item
  15:     0x7981844c6d96 - <rustc_ast_lowering[e55d79e744b523df]::item::ItemLowerer>::lower_node
  16:     0x7981844c4b54 - rustc_ast_lowering[e55d79e744b523df]::lower_to_hir
  17:     0x79818511f9a2 - rustc_query_impl[977456df771add7]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[977456df771add7]::query_impl::hir_crate::dynamic_query::{closure#2}::{closure#0}, rustc_middle[8b56cfa39295cc9e]::query::erase::Erased<[u8; 8usize]>>
  18:     0x79818511dbc7 - rustc_query_system[f942f53a11d60297]::query::plumbing::try_execute_query::<rustc_query_impl[977456df771add7]::DynamicConfig<rustc_query_system[f942f53a11d60297]::query::caches::SingleCache<rustc_middle[8b56cfa39295cc9e]::query::erase::Erased<[u8; 8usize]>>, false, false, false>, rustc_query_impl[977456df771add7]::plumbing::QueryCtxt, false>
  19:     0x79818511d841 - rustc_query_impl[977456df771add7]::query_impl::hir_crate::get_query_non_incr::__rust_end_short_backtrace
  20:     0x79818449025e - rustc_query_impl[977456df771add7]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[977456df771add7]::query_impl::hir_attrs::dynamic_query::{closure#2}::{closure#0}, rustc_middle[8b56cfa39295cc9e]::query::erase::Erased<[u8; 8usize]>>
  21:     0x79818448f879 - rustc_query_system[f942f53a11d60297]::query::plumbing::try_execute_query::<rustc_query_impl[977456df771add7]::DynamicConfig<rustc_data_structures[a5b0ed65aaf1af1]::vec_cache::VecCache<rustc_hir[9c8f1de648b82ae]::hir_id::OwnerId, rustc_middle[8b56cfa39295cc9e]::query::erase::Erased<[u8; 8usize]>, rustc_query_system[f942f53a11d60297]::dep_graph::graph::DepNodeIndex>, false, false, false>, rustc_query_impl[977456df771add7]::plumbing::QueryCtxt, false>
  22:     0x79818448f5cd - rustc_query_impl[977456df771add7]::query_impl::hir_attrs::get_query_non_incr::__rust_end_short_backtrace
  23:     0x7981844812f5 - <rustc_middle[8b56cfa39295cc9e]::hir::map::Map>::attrs
  24:     0x798185367b00 - rustc_passes[e91b22d0fd073176]::entry::entry_fn
  25:     0x798185367a98 - rustc_query_impl[977456df771add7]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[977456df771add7]::query_impl::entry_fn::dynamic_query::{closure#2}::{closure#0}, rustc_middle[8b56cfa39295cc9e]::query::erase::Erased<[u8; 12usize]>>
  26:     0x798185367a73 - <rustc_query_impl[977456df771add7]::query_impl::entry_fn::dynamic_query::{closure#2} as core[72d9f83855d8cc5b]::ops::function::FnOnce<(rustc_middle[8b56cfa39295cc9e]::ty::context::TyCtxt, ())>>::call_once
  27:     0x7981853673e4 - rustc_query_system[f942f53a11d60297]::query::plumbing::try_execute_query::<rustc_query_impl[977456df771add7]::DynamicConfig<rustc_query_system[f942f53a11d60297]::query::caches::SingleCache<rustc_middle[8b56cfa39295cc9e]::query::erase::Erased<[u8; 12usize]>>, false, false, false>, rustc_query_impl[977456df771add7]::plumbing::QueryCtxt, false>
  28:     0x79818536718f - rustc_query_impl[977456df771add7]::query_impl::entry_fn::get_query_non_incr::__rust_end_short_backtrace
  29:     0x798184452be9 - rustc_interface[15b2602f89c3a5c0]::passes::run_required_analyses
  30:     0x798184ffa6f8 - rustc_interface[15b2602f89c3a5c0]::passes::analysis
  31:     0x798184ffa6d7 - rustc_query_impl[977456df771add7]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[977456df771add7]::query_impl::analysis::dynamic_query::{closure#2}::{closure#0}, rustc_middle[8b56cfa39295cc9e]::query::erase::Erased<[u8; 0usize]>>
  32:     0x79818511c981 - rustc_query_system[f942f53a11d60297]::query::plumbing::try_execute_query::<rustc_query_impl[977456df771add7]::DynamicConfig<rustc_query_system[f942f53a11d60297]::query::caches::SingleCache<rustc_middle[8b56cfa39295cc9e]::query::erase::Erased<[u8; 0usize]>>, false, false, false>, rustc_query_impl[977456df771add7]::plumbing::QueryCtxt, false>
  33:     0x79818511c674 - rustc_query_impl[977456df771add7]::query_impl::analysis::get_query_non_incr::__rust_end_short_backtrace
  34:     0x79818535bfbc - rustc_interface[15b2602f89c3a5c0]::passes::create_and_enter_global_ctxt::<core[72d9f83855d8cc5b]::option::Option<rustc_interface[15b2602f89c3a5c0]::queries::Linker>, rustc_driver_impl[23cd132fe59c5dfc]::run_compiler::{closure#0}::{closure#2}>::{closure#2}::{closure#0}
  35:     0x798185174c97 - rustc_interface[15b2602f89c3a5c0]::interface::run_compiler::<(), rustc_driver_impl[23cd132fe59c5dfc]::run_compiler::{closure#0}>::{closure#1}
  36:     0x798184fd97c8 - std[ea0914bf68e3efbf]::sys::backtrace::__rust_begin_short_backtrace::<rustc_interface[15b2602f89c3a5c0]::util::run_in_thread_with_globals<rustc_interface[15b2602f89c3a5c0]::util::run_in_thread_pool_with_globals<rustc_interface[15b2602f89c3a5c0]::interface::run_compiler<(), rustc_driver_impl[23cd132fe59c5dfc]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>
  37:     0x798184fda0b4 - <<std[ea0914bf68e3efbf]::thread::Builder>::spawn_unchecked_<rustc_interface[15b2602f89c3a5c0]::util::run_in_thread_with_globals<rustc_interface[15b2602f89c3a5c0]::util::run_in_thread_pool_with_globals<rustc_interface[15b2602f89c3a5c0]::interface::run_compiler<(), rustc_driver_impl[23cd132fe59c5dfc]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>::{closure#1} as core[72d9f83855d8cc5b]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  38:     0x798184fdb4b7 - std::sys::pal::unix::thread::Thread::new::thread_start::ha1530b93ff336c49
  39:     0x79817eea370a - <unknown>
  40:     0x79817ef27aac - <unknown>
  41:                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 1.87.0-nightly (ebf0cf75d 2025-03-11) running on x86_64-unknown-linux-gnu

query stack during panic:
#0 [hir_crate] getting the crate HIR
#1 [hir_attrs] getting HIR owner attributes in ``
#2 [entry_fn] looking up the entry function of a crate
#3 [analysis] running analysis passes on this crate
end of query stack
error: aborting due to 2 previous errors

For more information about this error, try `rustc --explain E0658`.

@rustbot label +F-fn_delegation

[matthiaskrgr]

smaller:

mod a {
    pub trait b {
        fn c(self);
    }
}
trait d {
    fn c();
    
}
impl d for e {
    reuse a::b::*;
}

[petrochenkov]

Likely fixed by #138278.

Further minimized and cleaned up from non-relevant diagnostics:

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

trait HasSelf {
    fn method(self);
}
trait NoSelf {
    fn method();
}
impl NoSelf for u8 {
    reuse HasSelf::method;
}

[matthiaskrgr]

Likely fixed

ebf0cf7 already includes #138278