thread 'rustc' panicked at compiler/rustc_resolve/src/macros.rs:749:2

[GlenDC]

Seen in: https://github.com/plabayo/rama/actions/runs/15433552048/job/43435739684

And only there. None of our other CI targets trigger it and locally (despite being on macOS as well)
we do not see it either.

Meta

rustc --version --verbose:

 stable-aarch64-apple-darwin unchanged - rustc 1.87.0 (17067e9ac 2025-05-09)

Error output

Run cargo test --all-features --examples --workspace
    Updating crates.io index
   Compiling proc-macro2 v1.0.95
   Compiling unicode-ident v1.0.18
   Compiling libc v0.2.172
   Compiling cfg-if v1.0.0
   Compiling autocfg v1.4.0
   Compiling smallvec v1.15.0
   Compiling pin-project-lite v0.2.16
   Compiling portable-atomic v1.11.0
   Compiling memchr v2.7.4
   Compiling quote v1.0.40
   Compiling syn v2.0.101

thread 'rustc' panicked at compiler/rustc_resolve/src/macros.rs:749:21:
index out of bounds: the len is 0 but the index is 0
stack backtrace:
   0:        0x112dc04bc - <std::sys::backtrace::BacktraceLock::print::DisplayBacktrace as core::fmt::Display>::fmt::h92dda645f072dcaf
   1:        0x110473bbc - core::fmt::write::hbc92919d8e8f9a96
   2:        0x112db5310 - std::io::Write::write_fmt::hcee3b5dc9ab531be
   3:        0x112dc037c - std::sys::backtrace::BacktraceLock::print::h0f497abce563e5d2
   4:        0x112dc297c - std::panicking::default_hook::{{closure}}::h62595143a6c21f05
   5:        0x112dc2648 - std::panicking::default_hook::hd800536ed1df5085
   6:        0x110f4baf0 - std[dece160403b877c5]::panicking::update_hook::<alloc[b2f5842f192e62d9]::boxed::Box<rustc_driver_impl[10b09d078f964436]::install_ice_hook::{closure#1}>>::{closure#0}
   7:        0x112dc3368 - std::panicking::rust_panic_with_hook::h1882a30575fbb763
   8:        0x112dc2f5c - std::panicking::begin_panic_handler::{{closure}}::h39275ef3005e6337
   9:        0x112dc094c - std::sys::backtrace::__rust_end_short_backtrace::h6ede323c05a76849
  10:        0x112dc2c24 - __rustc[95feac21a9532783]::rust_begin_unwind
  11:        0x115a1c144 - core::panicking::panic_fmt::h529fda7ea817ba4f
  12:        0x115a1c2c4 - core::panicking::panic_bounds_check::he315898ba5b8216d
  13:        0x11270cee4 - <rustc_resolve[c12e87a1ee516b9]::Resolver>::resolve_macro_or_delegation_path
  14:        0x112734ce0 - <rustc_resolve[c12e87a1ee516b9]::late::LateResolutionVisitor>::smart_resolve_path_fragment
  15:        0x1126fcd68 - <rustc_resolve[c12e87a1ee516b9]::late::LateResolutionVisitor>::resolve_expr
  16:        0x1126fcaf4 - <rustc_resolve[c12e87a1ee516b9]::late::LateResolutionVisitor>::resolve_expr
  17:        0x1126fb538 - <rustc_resolve[c12e87a1ee516b9]::late::LateResolutionVisitor>::resolve_expr
  18:        0x1126f91cc - <rustc_resolve[c12e87a1ee516b9]::late::LateResolutionVisitor>::resolve_block
  19:        0x1126dfb5c - <rustc_resolve[c12e87a1ee516b9]::late::LateResolutionVisitor as rustc_ast[d8ab2fd52514a3a0]::visit::Visitor>::visit_fn
  20:        0x1125f8904 - rustc_ast[d8ab2fd52514a3a0]::visit::walk_item_ctxt::<rustc_resolve[c12e87a1ee516b9]::late::LateResolutionVisitor, rustc_ast[d8ab2fd52514a3a0]::ast::AssocItemKind>
  21:        0x1126f3d94 - <rustc_resolve[c12e87a1ee516b9]::late::LateResolutionVisitor>::resolve_implementation::{closure#0}::{closure#0}::{closure#0}::{closure#0}::{closure#0}::{closure#0}::{closure#0}
  22:        0x1126d38c8 - <rustc_resolve[c12e87a1ee516b9]::late::LateResolutionVisitor as rustc_ast[d8ab2fd52514a3a0]::visit::Visitor>::visit_item
  23:        0x1125f9bf0 - rustc_ast[d8ab2fd52514a3a0]::visit::walk_item_ctxt::<rustc_resolve[c12e87a1ee516b9]::late::LateResolutionVisitor, rustc_ast[d8ab2fd52514a3a0]::ast::ItemKind>
  24:        0x1126c948c - <rustc_resolve[c12e87a1ee516b9]::late::LateResolutionVisitor as rustc_ast[d8ab2fd52514a3a0]::visit::Visitor>::visit_item
  25:        0x11271c07c - <rustc_resolve[c12e87a1ee516b9]::Resolver>::resolve_crate::{closure#0}
  26:        0x1127187e8 - <rustc_resolve[c12e87a1ee516b9]::Resolver>::resolve_crate
  27:        0x11180b834 - rustc_interface[36d0dc89b6111b4f]::passes::resolver_for_lowering_raw
  28:        0x112520784 - rustc_query_impl[ddeb1b8cb2654587]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[ddeb1b8cb2654587]::query_impl::resolver_for_lowering_raw::dynamic_query::{closure#2}::{closure#0}, rustc_middle[19a1a29194537e3]::query::erase::Erased<[u8; 16usize]>>
  29:        0x1122ef924 - rustc_query_system[5bcf2b842147579]::query::plumbing::try_execute_query::<rustc_query_impl[ddeb1b8cb2654587]::DynamicConfig<rustc_query_system[5bcf2b842147579]::query::caches::SingleCache<rustc_middle[19a1a29194537e3]::query::erase::Erased<[u8; 16usize]>>, false, false, false>, rustc_query_impl[ddeb1b8cb2654587]::plumbing::QueryCtxt, false>
  30:        0x112528014 - rustc_query_impl[ddeb1b8cb2654587]::query_impl::resolver_for_lowering_raw::get_query_non_incr::__rust_end_short_backtrace
  31:        0x110ef8fa4 - rustc_interface[36d0dc89b6111b4f]::passes::create_and_enter_global_ctxt::<core[7ef867844175194f]::option::Option<rustc_interface[36d0dc89b6111b4f]::queries::Linker>, rustc_driver_impl[10b09d078f964436]::run_compiler::{closure#0}::{closure#2}>
  32:        0x110f4894c - rustc_interface[36d0dc89b6111b4f]::interface::run_compiler::<(), rustc_driver_impl[10b09d078f964436]::run_compiler::{closure#0}>::{closure#1}
  33:        0x110f38dcc - std[dece160403b877c5]::sys::backtrace::__rust_begin_short_backtrace::<rustc_interface[36d0dc89b6111b4f]::util::run_in_thread_with_globals<rustc_interface[36d0dc89b6111b4f]::util::run_in_thread_pool_with_globals<rustc_interface[36d0dc89b6111b4f]::interface::run_compiler<(), rustc_driver_impl[10b09d078f964436]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>
  34:        0x110f4f7f8 - <<std[dece160403b877c5]::thread::Builder>::spawn_unchecked_<rustc_interface[36d0dc89b6111b4f]::util::run_in_thread_with_globals<rustc_interface[36d0dc89b6111b4f]::util::run_in_thread_pool_with_globals<rustc_interface[36d0dc89b6111b4f]::interface::run_compiler<(), rustc_driver_impl[10b09d078f964436]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>::{closure#1} as core[7ef867844175194f]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  35:        0x112dc73fc - std::sys::pal::unix::thread::Thread::new::thread_start::he7774aa0ca446eef
  36:        0x19a8caf94 - __pthread_joiner_wake

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: rustc 1.87.0 (17067e9ac 2025-05-09) running on aarch64-apple-darwin

note: compiler flags: --crate-type lib -C opt-level=3 -C embed-bitcode=no -C debug-assertions=on -C strip=debuginfo

note: some of the compiler flags provided by cargo are hidden

query stack during panic:
#0 [resolver_for_lowering_raw] getting the resolver for lowering
end of query stack
error: could not compile `syn` (lib)

Caused by:
  process didn't exit successfully: `/Users/runner/.rustup/toolchains/stable-aarch64-apple-darwin/bin/rustc --crate-name syn --edition=2021 /Users/runner/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/syn-2.0.101/src/lib.rs --error-format=json --json=diagnostic-rendered-ansi,artifacts,future-incompat --crate-type lib --emit=dep-info,metadata,link -C opt-level=3 -C embed-bitcode=no -C debug-assertions=on --cfg 'feature="clone-impls"' --cfg 'feature="default"' --cfg 'feature="derive"' --cfg 'feature="extra-traits"' --cfg 'feature="fold"' --cfg 'feature="full"' --cfg 'feature="parsing"' --cfg 'feature="printing"' --cfg 'feature="proc-macro"' --cfg 'feature="visit"' --cfg 'feature="visit-mut"' --check-cfg 'cfg(docsrs,test)' --check-cfg 'cfg(feature, values("clone-impls", "default", "derive", "extra-traits", "fold", "full", "parsing", "printing", "proc-macro", "test", "visit", "visit-mut"))' -C metadata=98b74af06e92695d -C extra-filename=-2f3ac4538c18e747 --out-dir /Users/runner/work/rama/rama/target/debug/deps -C strip=debuginfo -L dependency=/Users/runner/work/rama/rama/target/debug/deps --extern proc_macro2=/Users/runner/work/rama/rama/target/debug/deps/libproc_macro2-0304af4502360ec7.rmeta --extern quote=/Users/runner/work/rama/rama/target/debug/deps/libquote-a8a650f1e3baf9ce.rmeta --extern unicode_ident=/Users/runner/work/rama/rama/target/debug/deps/libunicode_ident-a75bc21b0a010fdc.rmeta --cap-lints allow` (signal: 11, SIGSEGV: invalid memory reference)
warning: build failed, waiting for other jobs to finish...

Backtrace

<backtrace>

[workingjubilee]

how odd.

[workingjubilee]

@GlenDC Does this repro if you change the runner to use nightly?

[GlenDC]

@GlenDC Does this repro if you change the runner to use nightly?

Not sure, it's not even that consistent, it only happened once that I know so far, for a PR of a contributor. But a retry of the failed job made it succeed immediately, so it might be a hard one to track down.

[workingjubilee]

oh.

incr comp...?

[GlenDC]

oh.

incr comp...?

Yes that must be it. But it's also the first time that I see this happening, despite 1.87 already out for a while, so must be an edge case

[lqd]

This looks to be in a release build where incremental compilation is disabled.