rust-lang / rust-lang/rust

[ICE]: `failed to resolve delegation to inherent impl`

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@paradoxicalguy is already working on this.

Since Sep 17, 2026.

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

auto-reduced (treereduce-rust):

#![feature(fn_delegation)]

trait Trait {
    fn foo<'a: 'a>(&self) {}
}

struct F;
impl Trait for F {}

struct S(F);
impl S {
    reuse Trait::foo::<> { self.0 }

    reuse Self::foo::<'_> as bar { self.0 }
}

fn main() {}

original:

//@ check-pass
//@ compile-flags: -Z deduplicate-diagnostics=yes

#![feature(fn_delegation)]

trait Trait {
    fn foo<'a: 'a>(&self) {}
}

struct F;
impl Trait for F {}

struct S(F);
impl S {
    reuse Trait::foo::<> { self.0 }

    reuse Self::foo::<'_> as bar { self.0 }
}

fn main() {}

Version information

rustc 1.100.0-nightly (a8a1e6fd9 2026-09-14)
binary: rustc
commit-hash: a8a1e6fd9df2e094d6f09c0d57991508680acc1c
commit-date: 2026-09-14
host: x86_64-unknown-linux-gnu
release: 1.100.0-nightly
LLVM version: 23.1.1

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

Program output 

warning: the feature `fn_delegation` is incomplete and may not be safe to use and/or cause compiler crashes
 --> /tmp/icemaker_global_tempdir.FOTYhPGUBckV/rustc_testrunner_tmpdir_reporting.W5Ae5KwyzJd6/mvce.rs:1:12
  |
1 | #![feature(fn_delegation)]
  |            ^^^^^^^^^^^^^
  |
  = note: see issue #118212 <https://github.com/rust-lang/rust/issues/118212> for more information
  = note: `#[warn(incomplete_features)]` on by default

warning: trait `Trait` is never used
 --> /tmp/icemaker_global_tempdir.FOTYhPGUBckV/rustc_testrunner_tmpdir_reporting.W5Ae5KwyzJd6/mvce.rs:3:7
  |
3 | trait Trait {
  |       ^^^^^
  |
  = note: `#[warn(dead_code)]` (part of `#[warn(unused)]`) on by default

warning: struct `F` is never constructed
 --> /tmp/icemaker_global_tempdir.FOTYhPGUBckV/rustc_testrunner_tmpdir_reporting.W5Ae5KwyzJd6/mvce.rs:7:8
  |
7 | struct F;
  |        ^

warning: struct `S` is never constructed
  --> /tmp/icemaker_global_tempdir.FOTYhPGUBckV/rustc_testrunner_tmpdir_reporting.W5Ae5KwyzJd6/mvce.rs:10:8
   |
10 | struct S(F);
   |        ^

warning: associated items `foo` and `bar` are never used
  --> /tmp/icemaker_global_tempdir.FOTYhPGUBckV/rustc_testrunner_tmpdir_reporting.W5Ae5KwyzJd6/mvce.rs:12:18
   |
11 | impl S {
   | ------ associated items in this implementation
12 |     reuse Trait::foo::<> { self.0 }
   |                  ^^^
13 |
14 |     reuse Self::foo::<'_> as bar { self.0 }
   |                              ^^^

warning: 5 warnings emitted

note: no errors encountered even though delayed bugs were created

note: those delayed bugs will now be shown as internal compiler errors

error: internal compiler error: failed to resolve delegation to inherent impl
  --> /tmp/icemaker_global_tempdir.FOTYhPGUBckV/rustc_testrunner_tmpdir_reporting.W5Ae5KwyzJd6/mvce.rs:14:17
   |
14 |     reuse Self::foo::<'_> as bar { self.0 }
   |                 ^^^
   |
note: delayed at /rustc-dev/a8a1e6fd9df2e094d6f09c0d57991508680acc1c/compiler/rustc_ast_lowering/src/delegation/resolution.rs:83:68
         0: <rustc_errors::DiagCtxtInner>::emit_diagnostic
         1: <rustc_errors::DiagCtxtHandle>::emit_diagnostic
         2: <rustc_errors::diagnostic::Diag>::emit
         3: <rustc_errors::DiagCtxtHandle>::span_delayed_bug::<rustc_span::span_encoding::Span, &str>
         4: rustc_ast_lowering::delegation::resolution::resolve_type_relative_delegations
         5: rustc_query_impl::query_vtables::resolve_type_relative_delegations::invoke_provider_fn::__rust_begin_short_backtrace
         6: rustc_query_impl::execution::try_execute_query::<rustc_middle::query::caches::SingleCache<rustc_middle::query::erase::ErasedData<[u8; 8]>>, false>
         7: rustc_query_impl::query_vtables::resolve_type_relative_delegations::execute_query_non_incr::__rust_end_short_backtrace
         8: rustc_ast_lowering::lower_to_hir
         9: rustc_query_impl::query_vtables::lower_to_hir::invoke_provider_fn::__rust_begin_short_backtrace
        10: rustc_query_impl::execution::try_execute_query::<rustc_data_structures::vec_cache::VecCache<rustc_span::def_id::LocalDefId, rustc_middle::query::erase::ErasedData<[u8; 16]>, rustc_middle::dep_graph::graph::DepNodeIndex>, false>
        11: rustc_query_impl::query_vtables::lower_to_hir::execute_query_non_incr::__rust_end_short_backtrace
        12: rustc_query_impl::query_vtables::hir_owner::invoke_provider_fn::__rust_begin_short_backtrace
        13: rustc_query_impl::execution::try_execute_query::<rustc_data_structures::vec_cache::VecCache<rustc_span::def_id::LocalDefId, rustc_middle::query::erase::ErasedData<[u8; 32]>, rustc_middle::dep_graph::graph::DepNodeIndex>, false>
        14: rustc_query_impl::query_vtables::hir_owner::execute_query_non_incr::__rust_end_short_backtrace
        15: <rustc_middle::ty::context::TyCtxt>::expect_hir_owner_nodes
        16: rustc_middle::hir::map::hir_crate_items
        17: rustc_query_impl::query_vtables::hir_crate_items::invoke_provider_fn::__rust_begin_short_backtrace
        18: rustc_query_impl::execution::try_execute_query::<rustc_middle::query::caches::SingleCache<rustc_middle::query::erase::ErasedData<[u8; 8]>>, false>
        19: rustc_query_impl::query_vtables::hir_crate_items::execute_query_non_incr::__rust_end_short_backtrace
        20: rustc_interface::passes::analysis
        21: rustc_query_impl::execution::try_execute_query::<rustc_middle::query::caches::SingleCache<rustc_middle::query::erase::ErasedData<[u8; 0]>>, false>
        22: rustc_query_impl::query_vtables::analysis::execute_query_non_incr::__rust_end_short_backtrace
        23: rustc_interface::interface::run_compiler::<(), rustc_driver_impl::run_compiler::{closure#0}>::{closure#2}
        24: std::sys::backtrace::__rust_begin_short_backtrace::<rustc_interface::util::run_in_thread_with_globals<rustc_interface::util::run_in_thread_pool_with_globals<rustc_interface::interface::run_compiler<(), rustc_driver_impl::run_compiler::{closure#0}>::{closure#2}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>
        25: <std::thread::lifecycle::spawn_unchecked<rustc_interface::util::run_in_thread_with_globals<rustc_interface::util::run_in_thread_pool_with_globals<rustc_interface::interface::run_compiler<(), rustc_driver_impl::run_compiler::{closure#0}>::{closure#2}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>::{closure#1} as core::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
        26: <std::sys::thread::unix::Thread>::new::thread_start
        27: <unknown>
        28: <unknown>
      
  --> /tmp/icemaker_global_tempdir.FOTYhPGUBckV/rustc_testrunner_tmpdir_reporting.W5Ae5KwyzJd6/mvce.rs:14:17
   |
14 |     reuse Self::foo::<'_> as bar { self.0 }
   |                 ^^^

error: internal compiler error: `Res::Err` but no error emitted
  --> /tmp/icemaker_global_tempdir.FOTYhPGUBckV/rustc_testrunner_tmpdir_reporting.W5Ae5KwyzJd6/mvce.rs:14:36
   |
14 |     reuse Self::foo::<'_> as bar { self.0 }
   |                                    ^^^^
   |
note: delayed at /rustc-dev/a8a1e6fd9df2e094d6f09c0d57991508680acc1c/compiler/rustc_hir_typeck/src/expr.rs:598:32
         0: <rustc_errors::DiagCtxtInner>::emit_diagnostic
         1: <rustc_errors::DiagCtxtHandle>::emit_diagnostic
         2: <rustc_errors::diagnostic::Diag>::emit
         3: <rustc_errors::DiagCtxtHandle>::span_delayed_bug::<rustc_span::span_encoding::Span, &str>
         4: <rustc_hir_typeck::fn_ctxt::FnCtxt>::check_expr_path
         5: <rustc_hir_typeck::fn_ctxt::FnCtxt>::check_expr_with_expectation_and_args
         6: <rustc_hir_typeck::fn_ctxt::FnCtxt>::check_expr_field
         7: <rustc_hir_typeck::fn_ctxt::FnCtxt>::check_expr_with_expectation_and_args
         8: <rustc_hir_typeck::fn_ctxt::FnCtxt>::check_expr_block
         9: <rustc_hir_typeck::fn_ctxt::FnCtxt>::check_expr_with_expectation_and_args
        10: <rustc_hir_typeck::fn_ctxt::FnCtxt>::check_expr_call
        11: <rustc_hir_typeck::fn_ctxt::FnCtxt>::check_expr_with_expectation_and_args
        12: <rustc_hir_typeck::fn_ctxt::FnCtxt>::check_expr_block
        13: <rustc_hir_typeck::fn_ctxt::FnCtxt>::check_expr_with_expectation_and_args
        14: rustc_hir_typeck::check::check_fn
        15: rustc_hir_typeck::typeck_with_inspect::{closure#0}
        16: rustc_query_impl::query_vtables::typeck_root::invoke_provider_fn::__rust_begin_short_backtrace
        17: rustc_query_impl::execution::try_execute_query::<rustc_data_structures::vec_cache::VecCache<rustc_span::def_id::LocalDefId, rustc_middle::query::erase::ErasedData<[u8; 8]>, rustc_middle::dep_graph::graph::DepNodeIndex>, false>
        18: rustc_query_impl::query_vtables::typeck_root::execute_query_non_incr::__rust_end_short_backtrace
        19: <rustc_middle::ty::context::TyCtxt>::par_hir_body_owners::<rustc_hir_analysis::check_crate::{closure#1}>::{closure#0}
        20: rustc_hir_analysis::check_crate
        21: rustc_interface::passes::analysis
        22: rustc_query_impl::execution::try_execute_query::<rustc_middle::query::caches::SingleCache<rustc_middle::query::erase::ErasedData<[u8; 0]>>, false>
        23: rustc_query_impl::query_vtables::analysis::execute_query_non_incr::__rust_end_short_backtrace
        24: rustc_interface::interface::run_compiler::<(), rustc_driver_impl::run_compiler::{closure#0}>::{closure#2}
        25: std::sys::backtrace::__rust_begin_short_backtrace::<rustc_interface::util::run_in_thread_with_globals<rustc_interface::util::run_in_thread_pool_with_globals<rustc_interface::interface::run_compiler<(), rustc_driver_impl::run_compiler::{closure#0}>::{closure#2}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>
        26: <std::thread::lifecycle::spawn_unchecked<rustc_interface::util::run_in_thread_with_globals<rustc_interface::util::run_in_thread_pool_with_globals<rustc_interface::interface::run_compiler<(), rustc_driver_impl::run_compiler::{closure#0}>::{closure#2}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>::{closure#1} as core::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
        27: <std::sys::thread::unix::Thread>::new::thread_start
        28: <unknown>
        29: <unknown>
      
  --> /tmp/icemaker_global_tempdir.FOTYhPGUBckV/rustc_testrunner_tmpdir_reporting.W5Ae5KwyzJd6/mvce.rs:14:36
   |
14 |     reuse Self::foo::<'_> as bar { self.0 }
   |                                    ^^^^

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 1.100.0-nightly (a8a1e6fd9 2026-09-14) running on x86_64-unknown-linux-gnu

query stack during panic:
end of query stack

@rustbot label +F-fn_delegation

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